typeof
void- and Function-Pointers
asm Operands
enum Types
gcov: a Test Coverage Program
protoize
Richard M. Stallman
Last updated 28 July 1999
for gcc-2.95
(DOC-0464-00)
Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1998, 1999 Free Software Foundation, Inc.
For GCC Version 2.95
Published by the Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307, USA
Last printed April, 1998.
Printed copies are available for $50 each.
ISBN 1-882114-37-X
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" and "Funding for Free Software" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License" and "Funding for Free Software", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.
The C, C++, and Objective C, and Fortran versions of the compiler are integrated; this is why we use the name "GNU Compiler Collection". GCC can compile programs written in C, C++, Objective C, or Fortran. The Fortran compiler is described in a separate manual.
"GCC" is a common shorthand term for the GNU Compiler Collection. This is both the most general name for the compiler, and the name used when the emphasis is on compiling C programs (as the abbreviation formerly stood for "GNU C Compiler").
When referring to C++ compilation, it is usual to call the compiler "G++". Since there is only one compiler, it is also accurate to call it "GCC" no matter what the language context; however, the term "G++" is more useful when the emphasis is on compiling C++ programs.
We use the name "GCC" to refer to the compilation system as a whole, and more specifically to the language-independent part of the compiler. For example, we refer to the optimization options as affecting the behavior of "GCC" or sometimes just "the compiler".
Front ends for other languages, such as Ada 9X, Fortran, Modula-3, and Pascal, are under development. These front-ends, like that for C++, are built in subdirectories of GCC and link to it. The result is an integrated compiler that can compile programs written in C, C++, Objective C, or any of the languages for which you have installed front ends.
In this manual, we only discuss the options for the C, Objective-C, and C++ compilers and those of the GCC core. Consult the documentation of the other front ends for the options to use when compiling programs written in other languages.
G++ is a compiler, not merely a preprocessor. G++ builds object code directly from your C++ program source. There is no intermediate C version of the program. (By contrast, for example, some other implementations use a program that generates a C program from your C++ source.) Avoiding an intermediate C representation of the program means that you get better object code, and better debugging information. The GNU debugger, GDB, works with this information in the object code to give you comprehensive C++ source-level editing capabilities (see section `C and C++' in Debugging with GDB).
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the `-c' option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
See section Compiling C++ Programs, for a summary of special options for compiling C++ programs.
The gcc program accepts options and file names as operands. Many
options have multiletter names; therefore multiple single-letter options
may not be grouped: `-dr' is very different from `-d
-r'.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify `-L' more than once, the directories are searched in the order specified.
Many options have long names starting with `-f' or with `-W'---for example, `-fforce-mem', `-fstrength-reduce', `-Wformat' and so on. Most of these have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. This manual documents only one of these two forms, whichever one is not the default.
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
-c -S -E -o file -pipe -v --help -x language
-ansi -fstd -fallow-single-precision -fcond-mismatch -fno-asm -fno-builtin -ffreestanding -fhosted -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings -traditional -traditional-cpp -trigraphs
-fno-access-control -fcheck-new -fconserve-space -fdollars-in-identifiers -fno-elide-constructors -fexternal-templates -ffor-scope -fno-for-scope -fno-gnu-keywords -fguiding-decls -fhandle-signatures -fhonor-std -fhuge-objects -fno-implicit-templates -finit-priority -fno-implement-inlines -fname-mangling-version-n -fno-default-inline -foperator-names -fno-optional-diags -fpermissive -frepo -fstrict-prototype -fsquangle -ftemplate-depth-n -fthis-is-variable -fvtable-thunks -nostdinc++ -Wctor-dtor-privacy -Wno-deprecated -Weffc++ -Wno-non-template-friend -Wnon-virtual-dtor -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wreorder -Wsign-promo -Wsynth
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wbad-function-cast -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Werror -Wformat -Wid-clash-len -Wimplicit -Wimplicit-int -Wimplicit-function-declaration -Wimport -Werror-implicit-function-declaration -Winline -Wlarger-than-len -Wlong-long -Wmain -Wmissing-declarations -Wmissing-noreturn -Wmissing-prototypes -Wmultichar -Wnested-externs -Wno-import -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wshadow -Wsign-compare -Wstrict-prototypes -Wswitch -Wtraditional -Wtrigraphs -Wundef -Wuninitialized -Wunused -Wwrite-strings -Wunknown-pragmas
-a -ax -dletters -fdump-unnumbered -fpretend-float -fprofile-arcs -ftest-coverage -g -glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2 -ggdb -gstabs -gstabs+ -gxcoff -gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name -print-prog-name=program -print-search-dirs -save-temps
-fbranch-probabilities -foptimize-register-moves -fcaller-saves -fcse-follow-jumps -fcse-skip-blocks -fdelayed-branch -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem -fdata-sections -ffunction-sections -fgcse -finline-functions -finline-limit-n -fkeep-inline-functions -fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline -fno-peephole -fomit-frame-pointer -fregmove -frerun-cse-after-loop -frerun-loop-opt -fschedule-insns -fschedule-insns2 -fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops -fmove-all-movables -freduce-all-givs -fstrict-aliasing -O -O0 -O1 -O2 -O3 -Os
-Aquestion(answer) -C -dD -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir -isystem-c++ dir -M -MD -MM -MMD -MG -nostdinc -P -trigraphs -undef -Umacro -Wp,option
-Wa,option
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -s -static -shared -symbolic -Wl,option -Xlinker option -u symbol
-Bprefix -Idir -I- -Ldir -specs=file
-b machine -V version
M680x0 Options -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float -malign-int VAX Options -mg -mgnu -munix SPARC Options -mcpu=cpu type -mtune=cpu type -mcmodel=code model -malign-jumps=num -malign-loops=num -malign-functions=num -m32 -m64 -mapp-regs -mbroken-saverestore -mcypress -mepilogue -mflat -mfpu -mhard-float -mhard-quad-float -mimpure-text -mlive-g0 -mno-app-regs -mno-epilogue -mno-flat -mno-fpu -mno-impure-text -mno-stack-bias -mno-unaligned-doubles -msoft-float -msoft-quad-float -msparclite -mstack-bias -msupersparc -munaligned-doubles -mv8 Convex Options -mc1 -mc2 -mc32 -mc34 -mc38 -margcount -mnoargcount -mlong32 -mlong64 -mvolatile-cache -mvolatile-nocache AMD29K Options -m29000 -m29050 -mbw -mnbw -mdw -mndw -mlarge -mnormal -msmall -mkernel-registers -mno-reuse-arg-regs -mno-stack-check -mno-storem-bug -mreuse-arg-regs -msoft-float -mstack-check -mstorem-bug -muser-registers ARM Options -mapcs-frame -mno-apcs-frame -mapcs-26 -mapcs-32 -mapcs-stack-check -mno-apcs-stack-check -mapcs-float -mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-little-endian -mshort-load-bytes -mno-short-load-bytes -mshort-load-words -mno-short-load-words -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu= -march= -mfpe= -mstructure-size-boundary= -mbsd -mxopen -mno-symrename -mabort-on-noreturn -mno-sched-prolog Thumb Options -mtpcs-frame -mno-tpcs-frame -mtpcs-leaf-frame -mno-tpcs-leaf-frame -mlittle-endian -mbig-endian -mthumb-interwork -mno-thumb-interwork -mstructure-size-boundary= MN10200 Options -mrelax MN10300 Options -mmult-bug -mno-mult-bug -mrelax M32R/D Options -mcode-model=model type -msdata=sdata type -G num M88K Options -m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division -mhandle-large-shift -midentify-revision -mno-check-zero-division -mno-ocs-debug-info -mno-ocs-frame-position -mno-optimize-arg-area -mno-serialize-volatile -mno-underscores -mocs-debug-info -mocs-frame-position -moptimize-arg-area -mserialize-volatile -mshort-data-num -msvr3 -msvr4 -mtrap-large-shift -muse-div-instruction -mversion-03.00 -mwarn-passed-structs RS/6000 and PowerPC Options -mcpu=cpu type -mtune=cpu type -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mno-powerpc -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mnew-mnemonics -mno-new-mnemonics -mfull-toc -mminimal-toc -mno-fop-in-toc -mno-sum-in-toc -maix64 -maix32 -mxl-call -mno-xl-call -mthreads -mpe -msoft-float -mhard-float -mmultiple -mno-multiple -mstring -mno-string -mupdate -mno-update -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mcall-aix -mcall-sysv -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -G num RT Options -mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return -min-line-mul -mminimum-fp-blocks -mnohc-struct-return MIPS Options -mabicalls -mcpu=cpu type -membedded-data -membedded-pic -mfp32 -mfp64 -mgas -mgp32 -mgp64 -mgpopt -mhalf-pic -mhard-float -mint64 -mips1 -mips2 -mips3 -mips4 -mlong64 -mlong32 -mlong-calls -mmemcpy -mmips-as -mmips-tfile -mno-abicalls -mno-embedded-data -mno-embedded-pic -mno-gpopt -mno-long-calls -mno-memcpy -mno-mips-tfile -mno-rnames -mno-stats -mrnames -msoft-float -m4650 -msingle-float -mmad -mstats -EL -EB -G num -nocpp -mabi=32 -mabi=n32 -mabi=64 -mabi=eabi i386 Options -mcpu=cpu type -march=cpu type -mieee-fp -mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-double -mreg-alloc=list -mregparm=num -malign-jumps=num -malign-loops=num -malign-functions=num -mpreferred-stack-boundary=num HPPA Options -march=architecture type -mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas -mjump-in-delay -mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu type -mspace -mspace-regs Intel 960 Options -mcpu type -masm-compat -mclean-linkage -mcode-align -mcomplex-addr -mleaf-procedures -mic-compat -mic2.0-compat -mic3.0-compat -mintel-asm -mno-clean-linkage -mno-code-align -mno-complex-addr -mno-leaf-procedures -mno-old-align -mno-strict-align -mno-tail-call -mnumerics -mold-align -msoft-float -mstrict-align -mtail-call DEC Alpha Options -mfp-regs -mno-fp-regs -mno-soft-float -msoft-float -malpha-as -mgas -mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu type -mbwx -mno-bwx -mcix -mno-cix -mmax -mno-max -mmemory-latency=time Clipper Options -mc300 -mc400 H8/300 Options -mrelax -mh -ms -mint32 -malign-300 SH Options -m1 -m2 -m3 -m3e -mb -ml -mdalign -mrelax System V Options -Qy -Qn -YP,paths -Ym,dir ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text section -mdata=data section -mrodata=readonly data section TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpreserve-float V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mv850 -mbig-switch NS32K Options -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb -mbitfield -mnobitfield -mhimem -mnohimem
-fcall-saved-reg -fcall-used-reg -fexceptions -ffixed-reg -finhibit-size-directive -fcheck-memory-usage -fprefix-function-name -fno-common -fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC -freg-struct-return -fshared-data -fshort-enums -fshort-double -fvolatile -fvolatile-global -fvolatile-static -fverbose-asm -fpack-struct -fstack-check -fargument-alias -fargument-noalias -fargument-noalias-global -fleading-underscore
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
file.i
file.ii
file.m
file.h
file.cc
file.cxx
file.cpp
file.C
file.s
file.S
other
You can specify the input language explicitly with the `-x' option:
-x language
c objective-c c++ c-header cpp-output c++-cpp-output assembler assembler-with-cpp
-x none
If you only want some of the stages of compilation, you can use
`-x' (or filename suffixes) to tell gcc where to start, and
one of the options `-c', `-S', or `-E' to say where
gcc is to stop. Note that some combinations (for example,
`-x cpp-output -E' instruct gcc to do nothing at all.
-c
-S
-E
-o file
-v
-pipe
--help
gcc. If the -v option is also specified
then --help will also be passed on to the various processes
invoked by gcc, so that they can display the command line options
they accept. If the -W option is also specified then command
line options which have no documentation associated with them will also
be displayed.
C++ source files conventionally use one of the suffixes `.C',
`.cc', `.cpp', `.c++', `.cp', or `.cxx';
preprocessed C++ files use the suffix `.ii'. GCC recognizes
files with these names and compiles them as C++ programs even if you
call the compiler the same way as for compiling C programs (usually with
the name gcc).
However, C++ programs often require class libraries as well as a
compiler that understands the C++ language--and under some
circumstances, you might want to compile programs from standard input,
or otherwise without a suffix that flags them as C++ programs.
g++ is a program that calls GCC with the default language
set to C++, and automatically specifies linking against the C++
library. On many systems, the script g++ is also
installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See section Options Controlling C Dialect, for explanations of options for languages related to C. See section Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.
The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts:
-ansi
asm and typeof keywords, and
predefined macros such as unix and vax that identify the
type of system you are using. It also enables the undesirable and
rarely used ANSI trigraph feature. For the C compiler,
it disables recognition of C++ style `//' comments as well as
the inline keyword. For the C++ compiler,
`-foperator-names' is enabled as well.
The alternate keywords __asm__, __extension__,
__inline__ and __typeof__ continue to work despite
`-ansi'. You would not want to use them in an ANSI C program, of
course, but it is useful to put them in header files that might be included
in compilations done with `-ansi'. Alternate predefined macros
such as __unix__ and __vax__ are also available, with or
without `-ansi'.
The `-ansi' option does not cause non-ANSI programs to be
rejected gratuitously. For that, `-pedantic' is required in
addition to `-ansi'. See section Options to Request or Suppress Warnings.
The macro __STRICT_ANSI__ is predefined when the `-ansi'
option is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ANSI standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
The functions alloca, abort, exit, and
_exit are not builtin functions when `-ansi' is used.
-fstd=
__restrict__ even
when -fstd=c9x is not specified.
-fno-asm
asm, inline or typeof as a
keyword, so that code can use these words as identifiers. You can use
the keywords __asm__, __inline__ and __typeof__
instead. `-ansi' implies `-fno-asm'.
In C++, this switch only affects the typeof keyword, since
asm and inline are standard keywords. You may want to
use the `-fno-gnu-keywords' flag instead, as it also disables the
other, C++-specific, extension keywords such as headof.
-fno-builtin
abort,
abs, alloca, cos, exit, fabs,
ffs, labs, memcmp, memcpy, sin,
sqrt, strcmp, strcpy, and strlen.
GCC normally generates special code to handle certain builtin functions
more efficiently; for instance, calls to alloca may become single
instructions that adjust the stack directly, and calls to memcpy
may become inline copy loops. The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.
The `-ansi' option prevents alloca and ffs from being
builtin functions, since these functions do not have an ANSI standard
meaning.
-fhosted
main has a return
type of int. Examples are nearly everything except a kernel.
This is equivalent to `-fno-freestanding'.
-ffreestanding
main. The most obvious example is an OS kernel.
This is equivalent to `-fno-hosted'.
-trigraphs
-traditional
extern declarations take effect globally even if they
are written inside of a function definition. This includes implicit
declarations of functions.
typeof, inline, signed, const
and volatile are not recognized. (You can still use the
alternative keywords such as __typeof__, __inline__, and
so on.)
unsigned short and unsigned char promote
to unsigned int.
register are preserved by
longjmp. Ordinarily, GNU C follows ANSI C: automatic variables
not declared volatile may be clobbered.
-traditional-cpp
__STDC__ is not defined when you use
`-traditional', but __GNUC__ is (since the GNU extensions
which __GNUC__ indicates are not affected by
`-traditional'). If you need to write header files that work
differently depending on whether `-traditional' is in use, by
testing both of these predefined macros you can distinguish four
situations: GNU C, traditional GNU C, other ANSI C compilers, and other
old C compilers. The predefined macro __STDC_VERSION__ is also
not defined when you use `-traditional'. See section `Standard Predefined Macros' in The C Preprocessor,
for more discussion of these and other predefined macros.
-fcond-mismatch
-funsigned-char
char be unsigned, like unsigned char.
Each kind of machine has a default for what char should
be. It is either like unsigned char by default or like
signed char by default.
Ideally, a portable program should always use signed char or
unsigned char when it depends on the signedness of an object.
But many programs have been written to use plain char and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you
make such a program work with the opposite default.
The type char is always a distinct type from each of
signed char or unsigned char, even though its behavior
is always just like one of those two.
-fsigned-char
char be signed, like signed char.
Note that this is equivalent to `-fno-unsigned-char', which is
the negative form of `-funsigned-char'. Likewise, the option
`-fno-signed-char' is equivalent to `-funsigned-char'.
You may wish to use `-fno-builtin' as well as `-traditional'
if your program uses names that are normally GNU C builtin functions for
other purposes of its own.
You cannot use `-traditional' if you include any header files that
rely on ANSI C features. Some vendors are starting to ship systems with
ANSI C header files and you cannot use `-traditional' on such
systems to compile files that include any system headers.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
signed or unsigned. By
default, such a bitfield is signed, because this is consistent: the
basic integer types such as int are signed types.
However, when `-traditional' is used, bitfields are all unsigned
no matter what.
-fwritable-strings
-fallow-single-precision
This section describes the command-line options that are only meaningful
for C++ programs; but you can also use most of the GNU compiler options
regardless of what language your program is in. For example, you
might compile a file firstClass.C like this:
g++ -g -frepo -O -c firstClass.C
In this example, only `-frepo' is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fno-access-control
-fcheck-new
operator new is non-null
before attempting to modify the storage allocated. The current Working
Paper requires that operator new never return a null pointer, so
this check is normally unnecessary.
An alternative to using this option is to specify that your
operator new does not throw any exceptions; if you declare it
`throw()', g++ will check the return value. See also `new
(nothrow)'.
-fconserve-space
main() has
completed, you may have an object that is being destroyed twice because
two definitions were merged.
This option is no longer useful on most targets, now that support has
been added for putting variables into BSS without making them common.
-fdollars-in-identifiers
-fno-elide-constructors
-fexternal-templates
-falt-external-templates
-ffor-scope
-fno-for-scope
-fno-gnu-keywords
classof, headof, signature,
sigof or typeof as a keyword, so that code can use these
words as identifiers. You can use the keywords __classof__,
__headof__, __signature__, __sigof__, and
__typeof__ instead. `-ansi' implies
`-fno-gnu-keywords'.
-fguiding-decls
-fhandle-signatures
signature and sigof keywords for specifying
abstract types. The default (`-fno-handle-signatures') is not to
recognize them. See section Type Abstraction using Signatures.
-fhonor-std
namespace std as a namespace, instead of ignoring
it. For compatibility with earlier versions of g++, the compiler will,
by default, ignore namespace-declarations,
using-declarations, using-directives, and
namespace-names, if they involve std.
-fhuge-objects
-fno-implicit-templates
-fno-implicit-inline-templates
-finit-priority
-fno-implement-inlines
-fname-mangling-version-n
template <class T, class U> void foo(T t);Like all options that change the ABI, all C++ code, including libgcc must be built with the same setting of this option.
-foperator-names
and, bitand,
bitor, compl, not, or and xor as
synonyms for the symbols they refer to. `-ansi' implies
`-foperator-names'.
-fno-optional-diags
-fpermissive
-frepo
-fno-rtti
-fstrict-prototype
foo can take any combination of arguments, as
in C. `-pedantic' implies `-fstrict-prototype' unless
overridden with `-fno-strict-prototype'.
Specifying this option will also suppress implicit declarations of
functions.
This flag no longer affects declarations with C++ linkage.
-fsquangle
-fno-squangle
-ftemplate-depth-n
-fthis-is-variable
this. The incorporation of user-defined
free store management into C++ has made assignment to `this' an
anachronism. Therefore, by default it is invalid to assign to
this within a class member function; that is, GNU C++ treats
`this' in a member function of class X as a non-lvalue of
type `X *'. However, for backwards compatibility, you can make it
valid with `-fthis-is-variable'.
-fvtable-thunks=thunks-version
-nostdinc++
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
-Wctor-dtor-privacy (C++ only)
-Wnon-virtual-dtor (C++ only)
-Wreorder (C++ only)
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for `i'
and `j' will be rearranged to match the declaration order of the
members.
The following `-W...' options are not affected by `-Wall'.
-Weffc++ (C++ only)
-Wno-deprecated (C++ only)
-Wno-non-template-friend (C++ only)
-Wold-style-cast (C++ only)
-Woverloaded-virtual (C++ only)
-Wno-pmf-conversions (C++ only)
-Wsign-promo (C++ only)
-Wsynth (C++ only)
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, g++ will synthesize a default `A& operator =
(const A&);', while cfront will use the user-defined `operator ='.
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default.
These options control the amount and kinds of warnings produced by GCC:
-fsyntax-only
-pedantic
__extension__. However, only system header files should use
these escape routes; application programs should avoid them.
See section Alternate Keywords.
This option is not intended to be useful; it exists only to satisfy
pedants who would otherwise claim that GCC fails to support the ANSI
standard.
Some users try to use `-pedantic' to check programs for strict ANSI
C conformance. They soon find that it does not do quite what they want:
it finds some non-ANSI practices, but not all--only those for which
ANSI C requires a diagnostic.
A feature to report any failure to conform to ANSI C might be useful in
some instances, but would require considerable additional work and would
be quite different from `-pedantic'. We don't have plans to
support such a feature in the near future.
-pedantic-errors
-w
-Wno-import
-Wchar-subscripts
char. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
-Wcomment
-Wformat
printf and scanf, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified.
-Wimplicit-int
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
-Wimplicit
-Wmain
-Wmultichar
-Wparentheses
if statement an else branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every else branch belongs to the innermost possible if
statement, which in this example is if (b). This is often not
what the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for this
confusion, GNU C will issue a warning when this flag is specified.
To eliminate the warning, add explicit braces around the innermost
if statement so there is no way the else could belong to
the enclosing if. The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wreturn-type
int. Also warn about any return statement with no
return-value in a function whose return-type is not void.
-Wswitch
switch statement has an index of enumeral type
and lacks a case for one or more of the named codes of that
enumeration. (The presence of a default label prevents this
warning.) case labels outside the enumeration range also
provoke warnings when this option is used.
-Wtrigraphs
-Wunused
-Wuninitialized
volatile, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.
These warnings are made optional because GCC is not smart
enough to see all the reasons why the code might be correct
despite appearing to have an error. Here is one example of how
this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of y is always 1, 2 or 3, then x is
always initialized, but GCC doesn't know this. Here is
another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because save_y is used only if it is set.
Some spurious warnings can be avoided if you declare all the functions
you use that never return as noreturn. See section Declaring Attributes of Functions.
-Wunknown-pragmas
-Wall
The following `-W...' options are not implied by `-Wall'. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.
-W
longjmp. These warnings as well are possible only in
optimizing compilation.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because longjmp cannot
in fact be called at the place which would cause a problem.
foo (a)
{
if (a > 0)
return a;
}
static are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
x.h:
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
x.h would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
-Wtraditional
switch statement has an operand of type long.
static function declaration follows a static one.
This construct is not accepted by some traditional C compilers.
-Wundef
-Wshadow
-Wid-clash-len
-Wlarger-than-len
-Wpointer-arith
void. GNU C assigns these types a size of 1, for
convenience in calculations with void * pointers and pointers
to functions.
-Wbad-function-cast
int malloc() is cast to anything *.
-Wcast-qual
const char * is cast
to an ordinary char *.
-Wcast-align
char * is cast to
an int * on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
const char[length] so that
copying the address of one into a non-const char *
pointer will get a warning. These warnings will help you find at
compile time code that can try to write into a string constant, but
only if you have been very careful about using const in
declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make `-Wall' request these warnings.
-Wconversion
x = -1 if x is unsigned. But do not warn about explicit
casts like (unsigned) -1.
-Wsign-compare
-Waggregate-return
-Wstrict-prototypes
-Wmissing-prototypes
-Wmissing-declarations
-Wmissing-noreturn
noreturn.
Note these are only possible candidates, not absolute ones. Care should
be taken to manually verify functions actually do not ever return before
adding the noreturn attribute, otherwise subtle code generation
bugs could be introduced.
-Wredundant-decls
-Wnested-externs
extern declaration is encountered within an function.
-Winline
-Wlong-long
-Werror
GCC has various special options that are used for debugging either your program or GCC:
-g
-ggdb
-gstabs
-gstabs+
-gcoff
-gxcoff
-gxcoff+
-gdwarf
-gdwarf+
-gdwarf-2
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gdwarflevel
-gdwarf-2level
-p
prof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-pg
gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-a
tcov. Note,
however, that the format of the data is not what tcov expects.
Eventually GNU gprof should be extended to process this data.
-Q
-ax
__bb_jumps__
__bb_hidecall__
__bb_showret__
__bb_trace__
PATH. On systems without the `popen'
function, the file will be named `bbtrace' and will not be
compressed. Profiling for even a few seconds on these systems
will produce a very large file. Note: __bb_hidecall__ and
__bb_showret__ will not affect the sequence written to
`bbtrace.gz'.
foo consists of basic blocks
1 and 2 and is called twice from block 3 of function main. After
the calls, block 3 transfers control to block 4 of main.
With __bb_trace__ and main contained in file `bb.in',
the following sequence of blocks is written to file `bbtrace.gz':
0 3 1 2 1 2 4. The return from block 2 to block 3 is not shown, because
the return is to a point inside the block and not to the top. The
block address 0 always indicates, that control is transferred
to the trace from somewhere outside the observed functions. With
`-foo' added to `bb.in', the blocks of function
foo are removed from the trace, so only 0 3 4 remains.
With __bb_jumps__ and main contained in file `bb.in',
jump frequencies will be written to file `bb.out'. The
frequencies are obtained by constructing a trace of blocks
and incrementing a counter for every neighbouring pair of blocks
in the trace. The trace 0 3 1 2 1 2 4 displays the following
frequencies:
Jump from block 0x0 to block 0x3 executed 1 time(s) Jump from block 0x3 to block 0x1 executed 1 time(s) Jump from block 0x1 to block 0x2 executed 2 time(s) Jump from block 0x2 to block 0x1 executed 1 time(s) Jump from block 0x2 to block 0x4 executed 1 time(s)With
__bb_hidecall__, control transfer due to call instructions
is removed from the trace, that is the trace is cut into three parts: 0
3 4, 0 1 2 and 0 1 2. With __bb_showret__, control transfer due
to return instructions is added to the trace. The trace becomes: 0 3 1
2 3 1 2 3 4. Note, that this trace is not the same, as the sequence
written to `bbtrace.gz'. It is solely used for counting jump
frequencies.
-fprofile-arcs
gcov does not have
execution counts for all branches, it must start with the execution
counts for the instrumented branches, and then iterate over the program
flow graph until the entire graph has been solved. Hence, gcov
runs a little more slowly than a program which uses information from
`-a'.
`-fprofile-arcs' also makes it possible to estimate branch
probabilities, and to calculate basic block execution counts. In
general, basic block execution counts do not give enough information to
estimate all branch probabilities. When the compiled program exits, it
saves the arc execution counts to a file called
`sourcename.da'. Use the compiler option
`-fbranch-probabilities' (see section Options That Control Optimization) when recompiling, to optimize using estimated
branch probabilities.
-ftest-coverage
gcov code-coverage utility
(see section gcov: a Test Coverage Program).
The data file names begin with the name of your source file:
sourcename.bb
gcov uses to
associate basic block execution counts with line numbers.
sourcename.bbg
gcov
to reconstruct the program flow graph, so that it can compute all basic
block and arc execution counts from the information in the
sourcename.da file (this last file is the output from
`-fprofile-arcs').
-Q
-dletters
-fdump-unnumbered
-fpretend-float
-save-temps
-print-file-name=library
-print-prog-name=program
-print-libgcc-file-name
gcc -nostdlib files... `gcc -print-libgcc-file-name`
-print-search-dirs
GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing '/'.
See section Environment Variables Affecting GCC.
These options control various sorts of optimizations:
-O
-O1
register in registers. The resulting compiled code is a little
worse than produced by PCC without `-O'.
With `-O', the compiler tries to reduce code size and execution
time.
When you specify `-O', the compiler turns on `-fthread-jumps'
and `-fdefer-pop' on all machines. The compiler turns on
`-fdelayed-branch' on machines that have delay slots, and
`-fomit-frame-pointer' on machines that can support debugging even
without a frame pointer. On some machines the compiler also turns
on other flags.
-O2
-O3
-O0
-Os
Options of the form `-fflag' specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing `no-' or adding it.
-ffloat-store
double is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use `-ffloat-store' for such programs, after modifying
them to store all pertinent intermediate computations into variables.
-fno-default-inline
-fno-defer-pop
-fforce-mem
-fforce-addr
-fomit-frame-pointer
FRAME_POINTER_REQUIRED controls
whether a target machine supports this flag. See section `Register Usage' in Using and Porting GCC.
-fno-inline
inline keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded inline.
-finline-functions
static, then the function is normally not output as
assembler code in its own right.
-finline-limit-n
-fkeep-inline-functions
static, nevertheless output a separate run-time
callable version of the function. This switch does not affect
extern inline functions.
-fkeep-static-consts
static const when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to
check if the variable was referenced, regardless of whether or not
optimization is turned on, use the `-fno-keep-static-consts' option.
-fno-function-cse
-ffast-math
sqrt
function are non-negative numbers and that no floating-point values
are NaNs.
This option should never be turned on by any `-O' option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ANSI rules/specifications for
math functions.
The following options control specific optimizations. The `-O2' option turns on all of these optimizations except `-funroll-loops' `-funroll-all-loops', and `-fstrict-aliasing'. On most machines, the `-O' option turns on the `-fthread-jumps' and `-fdelayed-branch' options, but specific machines may handle it differently.
You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
-fstrength-reduce
-fthread-jumps
-fcse-follow-jumps
if statement with an
else clause, CSE will follow the jump when the condition
tested is false.
-fcse-skip-blocks
if statement with no else clause,
`-fcse-skip-blocks' causes CSE to follow the jump around the
body of the if.
-frerun-cse-after-loop
-frerun-loop-opt
-fgcse
-fexpensive-optimizations
-foptimize-register-moves
-fregmove
-fregmove and -foptimize-register-moves are the same
optimization.
-fdelayed-branch
-fschedule-insns
-fschedule-insns2
-ffunction-sections
-fdata-sections
gprof on all systems if you
specify this option and you may have problems with debugging if
you specify both this option and `-g'.
-fcaller-saves
-funroll-loops
-funroll-all-loops
-fmove-all-movables
-freduce-all-givs
gcc@gcc.gnu.org and fortran@gnu.org)
know how use of these options affects
the performance of your production code.
We're very interested in code that runs slower
when these options are enabled.
-fno-peephole
-fbranch-probabilities
-fstrict-aliasing
unsigned int can alias an int, but not a
void* or a double. A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most
recently written to (called "type-punning") is common. Even with
`-fstrict-aliasing', type-punning is allowed, provided the memory
is accessed through the union type. So, the code above will work as
expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the `-E' option, nothing is done except preprocessing. Some of these options make sense only together with `-E' because they cause the preprocessor output to be unsuitable for actual compilation.
-include file
-imacros file
-idirafter dir
-iprefix prefix
-iwithprefix dir
-iwithprefixbefore dir
-isystem dir
-nostdinc
-undef
-E
-C
-P
-M
make
describing the dependencies of each object file. For each source file,
the preprocessor outputs one make-rule whose target is the object
file name for that source file and whose dependencies are all the
#include header files it uses. This rule may be a single line or
may be continued with `\'-newline if it is long. The list of rules
is printed on standard output instead of the preprocessed C program.
`-M' implies `-E'.
Another way to specify output of a make rule is by setting
the environment variable DEPENDENCIES_OUTPUT (see section Environment Variables Affecting GCC).
-MM
-MD
md to merge multiple dependency
files into a single dependency file suitable for using with the `make'
command.
-MMD
-MG
-H
-Aquestion(answer)
-Dmacro
-Dmacro=defn
-Umacro
-dM
-dD
-dN
-trigraphs
-Wp,option
You can pass options to the assembler.
-Wa,option
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
object-file-name
-c
-S
-E
-llibrary
-lobjc
-nostartfiles
-nostdlib
or -nodefaultlibs is used.
-nodefaultlibs
-nostartfiles
is used. The compiler may generate calls to memcmp, memset, and memcpy
for System V (and ANSI C) environments or to bcopy and bzero for
BSD environments. These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
-nostdlib
collect2.)
-s
-static
-shared
-symbolic
-Xlinker option
-Wl,option
-u symbol
These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
-I-
-Ldir
-Bprefix
GCC_EXEC_PREFIX. See section Environment Variables Affecting GCC.
-specs=file
By default, GCC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GCC, for different target machines, can be installed side by side. Then you specify which one to use with the `-b' option.
In addition, older and newer versions of GCC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another.
-b machine
-V version
The `-b' and `-V' options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GCC, for a given target machine, is normally kept in the directory `/usr/local/lib/gcc-lib/machine/version'.
Thus, sites can customize the effect of `-b' or `-V' either by changing the names of these directories or adding alternate names (or symbolic links). If in directory `/usr/local/lib/gcc-lib/' the file `80386' is a link to the file `i386v', then `-b 80386' becomes an alias for `-b i386v'.
In one respect, the `-b' or `-V' do not completely change
to a different compiler: the top-level driver program gcc
that you originally invoked continues to run and invoke the other
executables (preprocessor, compiler per se, assembler and linker)
that do the real work. However, since no real work is done in the
driver program, it usually does not matter that the driver program
in use is not the one for the specified target and version.
The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine and compiler version.
The driver program executable does control one significant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names.
For example, if the driver for version 2.0 is installed as ogcc
and that for version 2.1 is installed as gcc, then the command
gcc will use version 2.1 by default, while ogcc will use
2.0 by default. However, you can choose either version with either
command with the `-V' option.
Earlier we discussed the standard option `-b' which chooses among different installed compilers for completely different target machines, such as Vax vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with `-m', to choose among various hardware models or configurations--for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.
These are the `-m' options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.
-m68000
-mc68000
-m68020
-mc68020
-m68881
-m68030
-m68040
-m68060
-mcpu32
-m5200
-m68020-40
-m68020-60
-mfpa
-msoft-float
-mshort
int to be 16 bits wide, like short int.
-mnobitfield
-mbitfield
-mrtd
rtd
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
The rtd instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
-malign-int
-mno-align-int
int, long, long long,
float, double, and long double variables on a 32-bit
boundary (`-malign-int') or a 16-bit boundary (`-mno-align-int').
Aligning variables on 32-bit boundaries produces code that runs somewhat
faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the `-malign-int' switch, GCC will
align structures containing the above types differently than
most published application binary interface specifications for the m68k.
These `-m' options are defined for the Vax:
-munix
aobleq and so on)
that the Unix assembler for the Vax cannot handle across long
ranges.
-mgnu
-mg
These `-m' switches are supported on the SPARC:
-mno-app-regs
-mapp-regs
-mfpu
-mhard-float
-mno-fpu
-msoft-float
-mhard-quad-float
-msoft-quad-float
-mno-epilogue
-mepilogue
-mno-flat
-mflat
-mno-unaligned-doubles
-munaligned-doubles
-mv8
-msparclite
ffs) instructions which
exist in SPARClite but not in SPARC v7.
These options are deprecated and will be deleted in a future GCC release.
They have been replaced with `-mcpu=xxx'.
-mcypress
-msupersparc
-mcpu=cpu_type
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc
-mtune=cpu_type
-malign-loops=num
-malign-jumps=num
-malign-functions=num
These `-m' switches are supported in addition to the above on the SPARCLET processor.
-mlittle-endian
-mlive-g0
%g0 as a normal register.
GCC will continue to clobber it as necessary but will not assume
it always reads as 0.
-mbroken-saverestore
save and
restore instructions. Early versions of the SPARCLET processor do
not correctly handle save and restore instructions used with
arguments. They correctly handle them used without arguments. A save
instruction used without arguments increments the current window pointer
but does not allocate a new stack frame. It is assumed that the window
overflow trap handler will properly handle this case as will interrupt
handlers.
These `-m' switches are supported in addition to the above on SPARC V9 processors in 64 bit environments.
-mlittle-endian
-m32
-m64
-mcmodel=medlow
-mcmodel=medmid
-mcmodel=medany
-mcmodel=embmedany
-mstack-bias
-mno-stack-bias
These `-m' options are defined for Convex:
-mc1
__convex__c1__ is defined.
-mc2
__convex_c2__ is defined.
-mc32
__convex_c32__ is defined.
-mc34
__convex_c34__ is defined.
-mc38
__convex_c38__ is defined.
-margcount
-mnoargcount
-mvolatile-cache
-mvolatile-nocache
-mlong32
-mlong64
These `-m' options are defined for the AMD Am29000:
-mdw
DW bit is set, i.e., that byte and
halfword operations are directly supported by the hardware. This is the
default.
-mndw
DW bit is not set.
-mbw
-mnbw
-msmall
call instruction to be used instead
of a const, consth, calli sequence.
-mnormal
call instructions only when
calling functions in the same file and calli instructions
otherwise. This works if each file occupies less than 256 KB but allows
the entire executable to be larger than 256 KB. This is the default.
-mlarge
calli instructions. Specify this option if you expect
a single file to compile into more than 256 KB of code.
-m29050
-m29000
-mkernel-registers
gr64-gr95 instead of to
registers gr96-gr127. This option can be used when compiling
kernel code that wants a set of global registers disjoint from that used
by user-mode code.
Note that when this option is used, register names in `-f' flags
must use the normal, user-mode, names.
-muser-registers
gr96-gr127. This is the
default.
-mstack-check
-mno-stack-check
__msp_check after each stack
adjustment. This is often used for kernel code.
-mstorem-bug
-mno-storem-bug
-mno-reuse-arg-regs
-mreuse-arg-regs
-mno-impure-text
-mimpure-text
-msoft-float
-mno-multm
These `-m' options are defined for Advanced RISC Machines (ARM) architectures:
-mapcs-frame
-mapcs
-mapcs-26
-mapcs-32
-mapcs-stack-check
-mapcs-float
-mapcs-reentrant
-mthumb-interwork
-mno-sched-prolog
-mhard-float
-msoft-float
-mlittle-endian
-mbig-endian
-mwords-little-endian
-mshort-load-bytes
-mno-short-load-bytes
-mshort-load-words
-mno-short-load-words
-mbsd
-mxopen
-mno-symrename
-mcpu=<name>
-mtune=<name>
-march=<name>
-mfpe=<number>
-mfp=<number>
-mstructure-size-boundary=<n>
-mabort-on-noreturn
-mthumb-interwork
-mtpcs-frame
-mtpcs-leaf-frame
-mlittle-endian
-mbig-endian
-mstructure-size-boundary=<n>
These `-m' options are defined for Matsushita MN10200 architectures:
-mrelax
These `-m' options are defined for Matsushita MN10300 architectures:
-mmult-bug
-mno-mult-bug
-mrelax
These `-m' options are defined for Mitsubishi M32R/D architectures:
-mcode-model=small
ld24 instruction), and assume all subroutines
are reachable with the bl instruction.
This is the default.
The addressability of a particular object can be set with the
model attribute.
-mcode-model=medium
seth/add3 instructions to load their addresses), and
assume all subroutines are reachable with the bl instruction.
-mcode-model=large
seth/add3 instructions to load their addresses), and
assume subroutines may not be reachable with the bl instruction
(the compiler will generate the much slower seth/add3/jl
instruction sequence).
-msdata=none
section attribute has been specified).
This is the default.
The small data area consists of sections `.sdata' and `.sbss'.
Objects may be explicitly put in the small data area with the
section attribute using one of these sections.
-msdata=sdata
-msdata=use
-G num
These `-m' options are defined for Motorola 88k architectures:
-m88000
-m88100
-m88110
-mbig-pic
-midentify-revision
ident directive in the assembler output recording the
source file name, compiler name and version, timestamp, and compilation
flags used.
-mno-underscores
-mocs-debug-info
-mno-ocs-debug-info
-mocs-frame-position
-mno-ocs-frame-position
-moptimize-arg-area
-mno-optimize-arg-area
-mshort-data-num
r0,
which allows loading a value using a single instruction (rather than the
usual two). You control which data references are affected by
specifying num with this option. For example, if you specify
`-mshort-data-512', then the data references affected are those
involving displacements of less than 512 bytes.
`-mshort-data-num' is not effective for num greater
than 64k.
-mserialize-volatile
-mno-serialize-volatile
-msvr4
-msvr3
-mversion-03.00
-mno-check-zero-division
-mcheck-zero-division
-muse-div-instruction
-mtrap-large-shift
-mhandle-large-shift
-mwarn-passed-structs
These `-m' options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mnew-mnemonics
-mold-mnemonics
-mcpu=cpu_type
-mtune=cpu_type
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
-maix64
-maix32
long type, and the infrastructure needed to support them.
Specifying `-maix64' implies `-mpowerpc64' and
`-mpowerpc', while `-maix32' disables the 64-bit ABI and
implies `-mno-powerpc64'. GCC defaults to `-maix32'.
-mxl-call
-mno-xl-call
-mthreads
-mpe
-msoft-float
-mhard-float
-mmultiple
-mno-multiple
-mstring
-mno-string
-mupdate
-mno-update
-mfused-madd
-mno-fused-madd
-mno-bit-align
-mbit-align
unsigned bitfields of length 1 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using `-mno-bit-align',
the structure would be aligned to a 1 byte boundary and be one byte in
size.
-mno-strict-align
-mstrict-align
-mrelocatable
-mno-relocatable
-mrelocatable-lib
-mno-relocatable-lib
-mno-toc
-mtoc
-mlittle
-mlittle-endian
-mbig
-mbig-endian
-mcall-sysv
-mcall-sysv-eabi
-mcall-sysv-noeabi
-mcall-aix
-mcall-solaris
-mcall-linux
-mprototype
-mno-prototype
-msim
-mmvme
-mads
-myellowknife
-memb
-meabi
-mno-eabi
-meabi
means that the stack is aligned to an 8 byte boundary, a function
__eabi is called to from main to set up the eabi
environment, and the `-msdata' option can use both r2 and
r13 to point to two separate small data areas. Selecting
-mno-eabi means that the stack is aligned to a 16 byte boundary,
do not call an initialization function from main, and the
`-msdata' option will only use r13 to point to a single
small data area. The `-meabi' option is on by default if you
configured GCC using one of the `powerpc*-*-eabi*' options.
-msdata=eabi
const global and static data in the `.sdata2' section, which
is pointed to by register r2. Put small initialized
non-const global and static data in the `.sdata' section,
which is pointed to by register r13. Put small uninitialized
global and static data in the `.sbss' section, which is adjacent to
the `.sdata' section. The `-msdata=eabi' option is
incompatible with the `-mrelocatable' option. The
`-msdata=eabi' option also sets the `-memb' option.
-msdata=sysv
r13. Put small uninitialized global and static data in the
`.sbss' section, which is adjacent to the `.sdata' section.
The `-msdata=sysv' option is incompatible with the
`-mrelocatable' option.
-msdata=default
-msdata
-msdata-data
r13
to address small data however. This is the default behavior unless
other `-msdata' options are used.
-msdata=none
-mno-sdata
-G num
-mregnames
-mno-regnames
These `-m' options are defined for the IBM RT PC:
-min-line-mul
-mcall-lib-mul
lmul$$ for integer multiples.
-mfull-fp-blocks
-mminimum-fp-blocks
-mfp-arg-in-fpregs
varargs.h and stdargs.h will not work with
floating point operands if this option is specified.
-mfp-arg-in-gregs
-mhc-struct-return
-mnohc-struct-return
These `-m' options are defined for the MIPS family of computers:
-mcpu=cpu type
-mips1
-mips2
-mips3
-mips4
-mfp32
-mfp64
-mgp32
-mgp64
-mint64
-mlong64
-mlong32
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
-mmips-as
-mgas
-msplit-addresses
-mno-split-addresses
gcc to optimize away redundant loads of the high order
bits of addresses. This optimization requires GNU as and GNU ld.
This optimization is enabled by default for some embedded targets where
GNU as and GNU ld are standard.
-mrnames
-mno-rnames
-mgpopt
-mno-gpopt
-mstats
-mno-stats
-mmemcpy
-mno-memcpy
-mmips-tfile
-mno-mips-tfile
-msoft-float
-mhard-float
-mabicalls
-mno-abicalls
-mlong-calls
-mno-long-calls
-mhalf-pic
-mno-half-pic
-membedded-pic
-mno-embedded-pic
-membedded-data
-mno-embedded-data
-msingle-float
-mdouble-float
-mmad
-mno-mad
-m4650
-mips16
-mno-mips16
-mentry
-EL
-EB
-G num
-nocpp
These `-m' options are defined for the i386 family of computers:
-mcpu=cpu type
| `i386' | `i486' | `i586' | `i686' |
| `pentium' | `pentiumpro' | `k6' |
-march=cpu type
-m386
-m486
-mpentium
-mpentiumpro
-mieee-fp
-mno-ieee-fp
-msoft-float
-mno-fp-ret-in-387
float and double in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate
an FPU.
The option `-mno-fp-ret-in-387' causes such values to be returned
in ordinary CPU registers instead.
-mno-fancy-math-387
sin, cos and
sqrt instructions for the 387. Specify this option to avoid
generating those instructions. This option is the default on FreeBSD.
As of revision 2.6.1, these instructions are not generated unless you
also use the `-ffast-math' switch.
-malign-double
-mno-align-double
double, long double, and
long long variables on a two word boundary or a one word
boundary. Aligning double variables on a two word boundary will
produce code that runs somewhat faster on a `Pentium' at the
expense of more memory.
Warning: if you use the `-malign-double' switch,
structures containing the above types will be aligned differently than
the published application binary interface specifications for the 386.
-msvr3-shlib
-mno-svr3-shlib
bss or
data. `-msvr3-shlib' places these locals into bss.
These options are meaningful only on System V Release 3.
-mno-wide-multiply
-mwide-multiply
mul and imul that produce
64 bit results in eax:edx from 32 bit operands to do long
long multiplies and 32-bit division by constants.
-mrtd
ret num
instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments
there.
You can specify that an individual function is called with this calling
sequence with the function attribute `stdcall'. You can also
override the `-mrtd' option by using the function attribute
`cdecl'. See section Declaring Attributes of Functions.
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
-mreg-alloc=regs
a allocate EAX; b allocate EBX;
c allocate ECX; d allocate EDX; S allocate ESI;
D allocate EDI; B allocate EBP.
-mregparm=num
-malign-loops=num
-malign-jumps=num
-malign-functions=num
-mpreferred-stack-boundary=num
double and long double values should be
aligned to an 8 byte boundary (see `-malign-double') or suffer
significant run time performance penalties. On Pentium III, the
Streaming SIMD Extention (SSE) data type __m128 suffers similar
penalties if it is not 16 byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary
must be as aligned as that required by any value stored on the stack.
Further, every function must be generated such that it keeps the stack
aligned. Thus calling a function compiled with a higher preferred
stack boundary from a function compiled with a lower preferred stack
boundary will most likely misalign the stack. It is recommended that
libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space. Code that is sensitive
to stack space usage, such as embedded systems and operating system kernels,
may want to reduce the preferred alignment to
`-mpreferred-stack-boundary=2'.
These `-m' options are defined for the HPPA family of computers:
-march=architecture type
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
-mbig-switch
-mjump-in-delay
-mdisable-fpregs
-mdisable-indexing
-mno-space-regs
-mfast-indirect-calls
-mspace
-mlong-load-store
-mportable-runtime
-mgas
-mschedule=cpu type
-mlinker-opt
-msoft-float
These `-m' options are defined for the Intel 960 implementations:
-mcpu type
-mnumerics
-msoft-float
-mleaf-procedures
-mno-leaf-procedures
bal instruction as well as call. This will result in more
efficient code for explicit calls when the bal instruction can be
substituted by the assembler or linker, but less efficient code in other
cases, such as calls via function pointers, or using a linker that doesn't
support this optimization.
-mtail-call
-mno-tail-call
-mcomplex-addr
-mno-complex-addr
-mcode-align
-mno-code-align
-mic-compat
-mic2.0-compat
-mic3.0-compat
-masm-compat
-mintel-asm
-mstrict-align
-mno-strict-align
-mold-align
-mlong-double-64
These `-m' options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
-msoft-float is specified,
functions in `libgcc1.c' will be used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines will issue floating-point
operations. If you are compiling for an Alpha without floating-point
operations, you must ensure that the library is built so as not to call
them.
Note that Alpha implementations without floating-point operations are
required to have floating-point registers.
-mfp-reg
-mno-fp-regs
-mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating point operands are passed in integer
registers as if they were integers and floating-point results are passed
in $0 instead of $f0. This is a non-standard calling sequence, so any
function with a floating-point argument or return value called by code
compiled with -mno-fp-regs must also be compiled with that
option.
A typical use of this option is building a kernel that does not use,
and hence need not save and restore, any floating-point registers.
-mieee
_IEEE_FP is defined
during compilation. The option is a shorthand for: `-D_IEEE_FP
-mfp-trap-mode=su -mtrap-precision=i -mieee-conformant'. The resulting
code is less efficient but is able to correctly support denormalized
numbers and exceptional IEEE values such as not-a-number and plus/minus
infinity. Other Alpha compilers call this option
-ieee_with_no_inexact.
-mieee-with-inexact
-mfp-trap-mode=trap mode
-mfp-rounding-mode=rounding mode
-mtrap-precision=trap precision
-mieee-conformant
-mbuild-constants
-malpha-as
-mgas
-mbwx
-mno-bwx
-mcix
-mno-cix
-mmax
-mno-max
-mcpu=cpu_type
-mmemory-latency=time
These `-m' options are defined for the Clipper implementations:
-mc300
-mc400
These `-m' options are defined for the H8/300 implementations:
-mrelax
ld and the H8/300' in Using ld, for a fuller description.
-mh
-ms
-mint32
int data 32 bits by default.
-malign-300
These `-m' options are defined for the SH implementations:
-m1
-m2
-m3
-m3e
-mb
-ml
-mdalign
-mrelax
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G
-Qy
.ident assembler directive in the output.
-Qn
.ident directives to the output file (this is
the default).
-YP,dirs
-Ym,dir
These `-m' options are defined for TMS320C3x/C4x implementations:
-mcpu=cpu_type
-mbig-memory
-mbig
-msmall-memory
-msmall
-mbk
-mno-bk
-mdb
-mno-db
-mdp-isr-reload
-mparanoid
-mmpyi
-mno-mpyi
-mfast-fix
-mno-fast-fix
-mrptb
-mno-rptb
-mrpts=count
-mno-rpts
-mloop-unsigned
-mno-loop-unsigned
-mti
-mregparm
-mmemparm
-mparallel-insns
-mno-parallel-insns
-mparallel-mpy
-mno-parallel-mpy
These `-m' options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
-mno-ep
-mep
ep register, and
use the shorter sld and sst instructions. The `-mep'
option is on by default if you optimize.
-mno-prolog-function
-mprolog-function
-mspace
-mtda=n
ep points to. The tiny data
area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
gp points to. The small data
area can hold up to 64 kilobytes.
-mzda=n
-mv850
-mbig-switch
These options are defined for ARC implementations:
-EL
-EB
-mmangle-cpu
-mcpu=cpu
-mtext=text section
-mdata=data section
-mrodata=readonly data section
section attribute.
See section Specifying Attributes of Variables.
These are the `-m' options defined for the 32000 series. The default values for these options depends on which style of 32000 was selected when the compiler was configured; the defaults for the most common choices are given below.
-m32032
-m32032
-m32332
-m32332
-m32532
-m32532
-m32081
-m32381
-mmulti-add
polyF
and dotF. This option is only available if the `-m32381'
option is in effect. Using these instructions requires changes to to
register allocation which generally has a negative impact on
performance. This option should only be enabled when compiling code
particularly likely to make heavy use of multiply-add instructions.
-mnomulti-add
polyF and dotF. This is the default on all platforms.
-msoft-float
-mnobitfield
-mbitfield
-mrtd
ret instruction.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
This option takes its name from the 680x0 rtd instruction.
-mregparam
-mnoregparam
-msb
-mnosb
-mhimem
-mnohimem
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing `no-' or adding it.
-fexceptions
-fpcc-struct-return
struct and union values in memory like
longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between
GCC-compiled files and files compiled with other compilers.
The precise convention for returning structures in memory depends
on the target configuration macros.
Short structures and unions are those whose size and alignment match
that of some integer type.
-freg-struct-return
struct and union values are
returned in registers when possible. This is more efficient for small
structures than `-fpcc-struct-return'.
If you specify neither `-fpcc-struct-return' nor its contrary
`-freg-struct-return', GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC
defaults to `-fpcc-struct-return', except on targets where GCC
is the principal compiler. In those cases, we can choose the standard,
and we chose the more efficient register return alternative.
-fshort-enums
enum type only as many bytes as it needs for the
declared range of possible values. Specifically, the enum type
will be equivalent to the smallest integer type which has enough room.
-fshort-double
double as for float.
-fshared-data
const variables of this
compilation be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private data
exists in one copy per process.
-fno-common
extern) in
two different compilations, you will get an error when you link them.
The only reason this might be useful is if you wish to verify that the
program will work on other systems which always work this way.
-fno-ident
-fno-gnu-linker
collect2 program to make sure the system linker includes
constructors and destructors. (collect2 is included in the GCC
distribution.) For systems which must use collect2, the
compiler driver gcc is configured to do this automatically.
-finhibit-size-directive
.size assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option is
used when compiling `crtstuff.c'; you should not need to use it
for anything else.
-fverbose-asm
-fvolatile
-fvolatile-global
-fvolatile-static
-fpic
-fPIC
-ffixed-reg
REGISTER_NAMES
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
-fcall-saved-reg
-fpack-struct
-fcheck-memory-usage
read), you might not be able to recompile the library and
specify this option. In that case, you can enable the
`-fprefix-function-name' option, which requests GCC to encapsulate
your code and make other functions look as if they were compiled with
`-fcheck-memory-usage'. This is done by calling "stubs",
which are provided by the detector. If you cannot find or build
stubs for every function you call, you might have to specify
`-fcheck-memory-usage' without `-fprefix-function-name'.
If you specify this option, you can not use the asm or
__asm__ keywords in functions with memory checking enabled. The
compiler cannot understand what the asm statement will do, and
therefore cannot generate the appropriate code, so it is rejected.
However, the function attribute no_check_memory_usage will
disable memory checking within a function, and asm statements can
be put inside such functions. Inline expansion of a non-checked
function within a checked function is permitted; the inline function's
memory accesses won't be checked, but the rest will.
If you move your asm statements to non-checked inline functions,
but they do access memory, you can add calls to the support code in your
inline function, to indicate any reads, writes, or copies being done.
These calls would be similar to those done in the stubs described above.
-fprefix-function-name
extern void bar (int);
void
foo (int a)
{
return bar (a + 5);
}
GCC will compile the code as if it was written:
extern void prefix_bar (int);
void
prefix_foo (int a)
{
return prefix_bar (a + 5);
}
This option is designed to be used with `-fcheck-memory-usage'.
-finstrument-functions
__builtin_return_address does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn, void *call_site);The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use `extern inline' in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) A function may be given the attribute
no_instrument_function, in
which case this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
-fstack-check
-fargument-alias
-fargument-noalias
-fargument-noalias-global
-fleading-underscore
This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see section Options for Directory Search). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
LC_CTYPE and LC_MESSAGES if it has been configured to do
so. These locale categories can be set to any value supported by your
installation. A typical value is `en_UK' for English in the United
Kingdom.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries in
a string; this is needed for some multibyte encodings that contain quote
and escape characters that would otherwise be interpreted as a string
end or escape.
The LC_MESSAGES environment variable specifies the language to
use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE
and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
TMPDIR
TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for example,
the output of the preprocessor, which is the input to the compiler
proper.
GCC_EXEC_PREFIX
GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is added
when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
`prefix/lib/gcc-lib/' where prefix is the value
of prefix when you ran the `configure' script.
Other prefixes specified with `-B' take precedence over this prefix.
This prefix is also used for finding files such as `crt0.o' that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with `/usr/local/lib/gcc-lib'
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with `-Bfoo/', GCC will search
`foo/bar' where it would normally search `/usr/local/lib/bar'.
These alternate directories are searched first; the standard directories
come next.
COMPILER_PATH
COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a native compiler,
GCC tries the directories thus specified when searching for special
linker files, if it can't find them using GCC_EXEC_PREFIX. Linking
using GCC also uses these directories when searching for ordinary
libraries for the `-l' option (but directories specified with
`-L' come first).
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
PATH. When GCC searches for header files, it tries the
directories listed in the variable for the language you are using, after
the directories specified with `-I' but before the standard header
file directories.
DEPENDENCIES_OUTPUT
DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
`file target', in which case the rules are written to
file file using target as the target name.
LANG
LANG are recognized:
C-JIS
C-SJIS
C-EUCJP
LANG is not defined, or if it has some other value, then the
compiler will use mblen and mbtowc as defined by the default locale to
recognize and translate multibyte characters.
The program protoize is an optional part of GNU C. You can use
it to add prototypes to a program, thus converting the program to ANSI
C in one respect. The companion program unprotoize does the
reverse: it removes argument types from any prototypes that are found.
When you run these programs, you must specify a set of source files as command line arguments. The conversion programs start out by compiling these files to see what functions they define. The information gathered about a file foo is saved in a file named `foo.X'.
After scanning comes actual conversion. The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.
But not all the eligible files are converted. By default,
protoize and unprotoize convert only source and header
files in the current directory. You can specify additional directories
whose files should be converted with the `-d directory'
option. You can also specify particular files to exclude with the
`-x file' option. A file is converted if it is eligible, its
directory name matches one of the specified directory names, and its
name within the directory has not been excluded.
Basic conversion with protoize consists of rewriting most
function definitions and function declarations to specify the types of
the arguments. The only ones not rewritten are those for varargs
functions.
protoize optionally inserts prototype declarations at the
beginning of the source file, to make them available for any calls that
precede the function's definition. Or it can insert prototype
declarations with block scope in the blocks where undeclared functions
are called.
Basic conversion with unprotoize consists of rewriting most
function declarations to remove any argument types, and rewriting
function definitions to the old-style pre-ANSI form.
Both conversion programs print a warning for any function declaration or definition that they can't convert. You can suppress these warnings with `-q'.
The output from protoize or unprotoize replaces the
original source file. The original file is renamed to a name ending
with `.save'. If the `.save' file already exists, then
the source file is simply discarded.
protoize and unprotoize both depend on GCC itself to
scan the program and collect information about the functions it uses.
So neither of these programs will work until GCC is installed.
Here is a table of the options you can use with protoize and
unprotoize. Each option works with both programs unless
otherwise stated.
-B directory
protoize.
-c compilation-options
gcc to
produce the `.X' files. The special option `-aux-info' is
always passed in addition, to tell gcc to write a `.X' file.
Note that the compilation options must be given as a single argument to
protoize or unprotoize. If you want to specify several
gcc options, you must quote the entire set of compilation options
to make them a single word in the shell.
There are certain gcc arguments that you cannot use, because they
would produce the wrong kind of output. These include `-g',
`-O', `-c', `-S', and `-o' If you include these in
the compilation-options, they are ignored.
-C
protoize.
-g
protoize.
-i string
protoize.
unprotoize converts prototyped function definitions to old-style
function definitions, where the arguments are declared between the
argument list and the initial `{'. By default, unprotoize
uses five spaces as the indentation. If you want to indent with just
one space instead, use `-i " "'.
-k
-l
protoize with `-l' inserts
a prototype declaration for each function in each block which calls the
function without any declaration. This option applies only to
protoize.
-n
-N
-p program
-q
-v
gcc.
If you need special compiler options to compile one of your program's
source files, then you should generate that file's `.X' file
specially, by running gcc on that source file with the
appropriate options and the option `-aux-info'. Then run
protoize on the entire set of files. protoize will use
the existing `.X' file because it is newer than the source file.
For example:
gcc -Dfoo=bar file1.c -aux-info protoize *.c
You need to include the special files along with the rest in the
protoize command, even though their `.X' files already
exist, because otherwise they won't get converted.
See section Caveats of using protoize, for more information on how to use
protoize successfully.
Note most of this information is out of date and superceded by the EGCS install procedures. It is provided for historical reference only.
Here is the procedure for installing GNU CC on a GNU or Unix system. See section Installing GNU CC on VMS, for VMS systems. In this section we assume you compile in the same directory that contains the source files; see section Compilation in a Separate Directory, to find out how to compile in a separate directory on Unix systems.
You cannot install GNU C by itself on MSDOS; it will not compile under any MSDOS compiler except itself. You need to get the complete compilation package DJGPP, which includes binaries as well as sources, and includes all the necessary compilation tools and libraries.
PATH. The cc command in
`/usr/ucb' uses libraries which have bugs.
PATH environment variable such that the necessary GNU tools come
before the standard system tools.
./configure --host=sparc-sun-sunos4.1A configuration name may be canonical or it may be more or less abbreviated. A canonical configuration name has three parts, separated by dashes. It looks like this: `cpu-company-system'. (The three parts may themselves contain dashes; `configure' can figure out which dashes serve which purpose.) For example, `m68k-sun-sunos4.1' specifies a Sun 3. You can also replace parts of the configuration by nicknames or aliases. For example, `sun3' stands for `m68k-sun', so `sun3-sunos4.1' is another way to specify a Sun 3. You can also use simply `sun3-sunos', since the version of SunOS is assumed by default to be version 4. You can specify a version number after any of the system types, and some of the CPU types. In most cases, the version is irrelevant, and will be ignored. So you might as well specify the version if you know it. See section Configurations Supported by GNU CC, for a list of supported configuration names and notes on many of the configurations. You should check the notes in that section before proceeding any further with the installation of GNU CC.
configure, you may also need to specify certain
additional options that describe variant hardware and software
configurations. These are `--with-gnu-as', `--with-gnu-ld',
`--with-stabs' and `--nfp'.
as in various directories; if the program it finds is GAS, then
it runs GAS. If you are not sure where GNU CC finds the assembler it is
using, try specifying `-v' when you run it.
The systems where it makes a difference whether you use GAS areconfigure
will print out whether the Haifa scheduler is enabled when it is run.
configure. Normally
you need not be concerned with these files.
gettext libraries, and falls back on GCC's copy of the GNU
gettext library only if the host libraries do not suffice. The
`--with-included-gettext' option causes the build procedure to
prefer its copy of GNU gettext.
gettext but has the
inferior catgets interface, the GCC build procedure normally
ignores catgets and instead uses GCC's copy of the GNU
gettext library. The `--with-catgets' option causes the
build procedure to use the host's catgets in this situation.
configure.
--with-local-prefix option below. The directory
you specify need not exist, but its parent directory must exist.
fixincludes script.
Indications are that people who use this option use it based on
mistaken ideas of what it is for. People use it as if it specified
where to install part of GNU CC. Perhaps they make this assumption
because installing GNU CC creates the directory.
make stage1The files are moved into a subdirectory named `stage1'. Once installation is complete, you may wish to delete these files with
rm -r stage1.
PATH environment variable such that the necessary GNU tools come
before the standard system tools.
make CC="stage1/xgcc -Bstage1/" CFLAGS="-g -O2"This is called making the stage 2 compiler. The command shown above builds compilers for all the supported languages. If you don't want them all, you can specify the languages to build by typing the argument `LANGUAGES="list"'. list should contain one or more words from the list `c', `c++', `objective-c', and `proto'. Separate the words with spaces. `proto' stands for the programs
protoize and
unprotoize; they are not a separate language, but you use
LANGUAGES to enable or disable their installation.
If you are going to build the stage 3 compiler, then you might want to
build only the C language in stage 2.
Once you have built the stage 2 compiler, if you are short of disk
space, you can delete the subdirectory `stage1'.
On a 68000 or 68020 system lacking floating point hardware,
unless you have selected a `tm.h' file that expects by default
that there is no such hardware, do this instead:
make CC="stage1/xgcc -Bstage1/" CFLAGS="-g -O2 -msoft-float"
make stage2 make CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O2"This is called making the stage 3 compiler. Aside from the `-B' option, the compiler options should be the same as when you made the stage 2 compiler. But the
LANGUAGES option need not be the
same. The command shown above builds compilers for all the supported
languages; if you don't want them all, you can specify the languages to
build by typing the argument `LANGUAGES="list"', as described
above.
If you do not have to install any additional GNU tools, you may use the
command
make bootstrap LANGUAGES=language-list BOOT_CFLAGS=option-listinstead of making `stage1', `stage2', and performing the two compiler builds.
make compareThis will mention any object files that differ between stage 2 and stage 3. Any difference, no matter how innocuous, indicates that the stage 2 compiler has compiled GNU CC incorrectly, and is therefore a potentially serious bug which you should investigate and report (see section Reporting Bugs). If your system does not put time stamps in the object files, then this is a faster way to compare them (using the Bourne shell):
for file in *.o; do cmp $file stage2/$file doneIf you have built the compiler with the `-mno-mips-tfile' option on MIPS machines, you will not be able to compare the files.
CC,
CFLAGS and LANGUAGES that you used when compiling the
files that are being installed. One reason this is necessary is that
some versions of Make have bugs and recompile files gratuitously when
you do this step. If you use the same variable values, those files will
be recompiled properly.
For example, if you have built the stage 2 compiler, you can use the
following command:
make install CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O" LANGUAGES="list"This copies the files `cc1', `cpp' and `libgcc.a' to files `cc1', `cpp' and `libgcc.a' in the directory `/usr/local/lib/gcc-lib/target/version', which is where the compiler driver program looks for them. Here target is the canonicalized form of target machine type specified when you ran `configure', and version is the version number of GNU CC. This naming scheme permits various versions and/or cross-compilers to coexist. It also copies the executables for compilers for other languages (e.g., `cc1plus' for C++) to the same directory. This also copies the driver program `xgcc' into `/usr/local/bin/gcc', so that it appears in typical execution search paths. It also copies `gcc.1' into `/usr/local/man/man1' and info pages into `/usr/local/info'. On some systems, this command causes recompilation of some files. This is usually due to bugs in
make. You should either ignore this
problem, or use GNU Make.
Warning: there is a bug in alloca in the Sun library. To
avoid this bug, be sure to install the executables of GNU CC that were
compiled by GNU CC. (That is, the executables from stage 2 or 3, not
stage 1.) They use alloca as a built-in function and never the
one in the library.
(It is usually better to install GNU CC executables from stage 2 or 3,
since they usually run faster than the ones compiled with some other
compiler.)
$ CXX=gcc ./configure configure-options $ make $ make install
make CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O2" OBJC_THREAD_FILE=thr-singleBelow is a list of the currently available back-ends.
configure
Here we spell out what files will be set up by configure. Normally
you need not be concerned with these files.
Here are the possible CPU types:
1750a, a29k, alpha, arm, cn, clipper, dsp16xx, elxsi, h8300, hppa1.0, hppa1.1, i370, i386, i486, i586, i860, i960, m32r, m68000, m68k, m88k, mips, mipsel, mips64, mips64el, ns32k, powerpc, powerpcle, pyramid, romp, rs6000, sh, sparc, sparclite, sparc64, vax, we32k.
Here are the recognized company names. As you can see, customary abbreviations are used rather than the longer official names.
acorn, alliant, altos, apollo, apple, att, bull, cbm, convergent, convex, crds, dec, dg, dolphin, elxsi, encore, harris, hitachi, hp, ibm, intergraph, isi, mips, motorola, ncr, next, ns, omron, plexus, sequent, sgi, sony, sun, tti, unicom, wrs.
The company name is meaningful only to disambiguate when the rest of the information supplied is insufficient. You can omit it, writing just `cpu-system', if it is not needed. For example, `vax-ultrix4.2' is equivalent to `vax-dec-ultrix4.2'.
Here is a list of system types:
386bsd, aix, acis, amigaos, aos, aout, aux, bosx, bsd, clix, coff, ctix, cxux, dgux, dynix, ebmon, ecoff, elf, esix, freebsd, hms, genix, gnu, linux-gnu, hiux, hpux, iris, irix, isc, luna, lynxos, mach, minix, msdos, mvs, netbsd, newsos, nindy, ns, osf, osfrose, ptx, riscix, riscos, rtu, sco, sim, solaris, sunos, sym, sysv, udi, ultrix, unicos, uniplus, unos, vms, vsta, vxworks, winnt, xenix.
You can omit the system type; then `configure' guesses the operating system from the CPU and company.
You can add a version number to the system type; this may or may not make a difference. For example, you can write `bsd4.3' or `bsd4.4' to distinguish versions of BSD. In practice, the version number is most needed for `sysv3' and `sysv4', which are often treated differently.
If you specify an impossible combination such as `i860-dg-vms', then you may get an error message from `configure', or it may ignore part of the information and do the best it can with the rest. `configure' always prints the canonical name for the alternative that it used. GNU CC does not support all possible alternatives.
Often a particular model of machine has a name. Many machine names are recognized as aliases for CPU/company combinations. Thus, the machine name `sun3', mentioned above, is an alias for `m68k-sun'. Sometimes we accept a company name as a machine name, when the name is popularly used for a particular machine. Here is a table of the known machine names:
3300, 3b1, 3bn, 7300, altos3068, altos, apollo68, att-7300, balance, convex-cn, crds, decstation-3100, decstation, delta, encore, fx2800, gmicro, hp7nn, hp8nn, hp9k2nn, hp9k3nn, hp9k7nn, hp9k8nn, iris4d, iris, isi68, m3230, magnum, merlin, miniframe, mmax, news-3600, news800, news, next, pbd, pc532, pmax, powerpc, powerpcle, ps2, risc-news, rtpc, sun2, sun386i, sun386, sun3, sun4, symmetry, tower-32, tower.
Remember that a machine name specifies both the cpu type and the company name. If you want to install your own homemade configuration files, you can use `local' as the company name to access them. If you use configuration `cpu-local', the configuration name without the cpu prefix is used to form the configuration file names.
Thus, if you specify `m68k-local', configuration uses files `m68k.md', `local.h', `m68k.c', `xm-local.h', `t-local', and `x-local', all in the directory `config/m68k'.
Here is a list of configurations that have special treatment or special things you must know:
as1750, an assembler/linker available under the GNU Public
License for the 1750A. as1750 can be obtained at
ftp://ftp.fta-berlin.de/pub/crossgcc/1750gals/.
A similarly licensed simulator for
the 1750A is available from same address.
You should ignore a fatal error during the building of libgcc (libgcc is
not yet implemented for the 1750A.)
The as1750 assembler requires the file `ms1750.inc', which is
found in the directory `config/1750a'.
GNU CC produced the same sections as the Fairchild F9450 C Compiler,
namely:
Normal
Static
Konst
Init
make compare may fail on old versions of DEC Unix unless you add
`-save-temps' to CFLAGS. On these systems, the name of the
assembler input file is stored in the object file, and that makes
comparison fail if it differs between the stage1 and
stage2 compilations. The option `-save-temps' forces a
fixed name to be used for the assembler input file, instead of a
randomly chosen name in `/tmp'. Do not add `-save-temps'
unless the comparisons fail without that option. If you add
`-save-temps', you will have to manually delete the `.i' and
`.s' files after each series of compilations.
GNU CC now supports both the native (ECOFF) debugging format used by DBX
and GDB and an encapsulated STABS format for use only with GDB. See the
discussion of the `--with-stabs' option of `configure' above
for more information on these formats and how to select them.
There is a bug in DEC's assembler that produces incorrect line numbers
for ECOFF format when the `.align' directive is used. To work
around this problem, GNU CC will not emit such alignment directives
while writing ECOFF format debugging information even if optimization is
being performed. Unfortunately, this has the very undesirable
side-effect that code addresses when `-O' is specified are
different depending on whether or not `-g' is also specified.
To avoid this behavior, specify `-gstabs+' and use GDB instead of
DBX. DEC is now aware of this problem with the assembler and hopes to
provide a fix shortly.
CC make variable and use the MIPS
compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'.
mrs@cygnus.com for more details.
/bin,
/usr/bin, and /usr/ccs/bin in your search path. You
should install GAS before you build GNU CC.
To enable debugging, you must configure GNU CC with the `--with-gnu-as'
option before building.
configure winnt from an MSDOS console window or from the
program manager dialog box. `configure.bat' assumes you have
already installed and have in your path a Unix-like `sed' program
which is used to create a working `Makefile' from `Makefile.in'.
`Makefile' uses the Microsoft Nmake program maintenance utility and
the Visual C/C++ V8.00 compiler to build GNU CC. You need only have the
utilities `sed' and `touch' to use this installation method,
which only automatically builds the compiler itself. You must then
examine what `fixinc.winnt' does, edit the header files by hand and
build `libgcc.a' manually.
law@cygnus.com
to get binaries of GNU CC for bootstrapping.
configure.
Note the C compiler that comes
with this system cannot compile GNU CC. You can find binaries of GNU CC
for bootstrapping on jagubox.gsfc.nasa.gov.
You will also a patched version of `/bin/ld' there that
raises some of the arbitrary limits found in the original.
obstack_free in the file
`tree.c' with _obstack_free.
make to get the first-stage GNU CC.
F.Pierresteguy@frcl.bull.fr.
casm instead of as. For some
strange reason linking `/bin/as' to `/bin/casm' changes the
behavior, and does not work. So, when installing GNU CC, you should
install the following script as `as' in the subdirectory where
the passes of GCC are installed:
#!/bin/sh casm $*The default Unos library is named `libunos.a' instead of `libc.a'. To allow GNU CC to function, either change all references to `-lc' in `gcc.c' to `-lunos' or link `/lib/libc.a' to `/lib/libunos.a'. When compiling GNU CC with the standard compiler, to overcome bugs in the support of
alloca, do not use `-O' when making stage 2.
Then use the stage 2 compiler with `-O' to make the stage 3
compiler. This compiler will have the same characteristics as the usual
stage 2 compiler on other systems. Use it to make a stage 4 compiler
and compare that with stage 3 to verify proper compilation.
(Perhaps simply defining ALLOCA in `x-crds' as described in
the comments there will make the above paragraph superfluous. Please
inform us of whether this works.)
Unos uses memory segmentation instead of demand paging, so you will need
a lot of memory. 5 Mb is barely enough if no other tasks are running.
If linking `cc1' fails, try putting the object files into a library
and linking from that library.
memcpy, memcmp, and memset. If your system lacks
these, you must remove or undo the definition of
TARGET_MEM_FUNCTIONS in `mips-bsd.h'.
The MIPS C compiler needs to be told to increase its table size
for switch statements with the `-Wf,-XNg1500' option in
order to compile `cp/parse.c'. If you use the `-O2'
optimization option, you also need to use `-Olimit 3000'.
Both of these options are automatically generated in the
`Makefile' that the shell script `configure' builds.
If you override the CC make variable and use the MIPS
compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'.
CC make variable and use the MIPS
compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'.
MIPS computers running RISC-OS can support four different
personalities: default, BSD 4.3, System V.3, and System V.4
(older versions of RISC-OS don't support V.4). To configure GCC
for these platforms use the following configurations:
rev'
rev.
revbsd'
rev.
revsysv4'
rev.
revsysv'
rev.
rev mentioned above is the revision of
RISC-OS to use. You must reconfigure GCC when going from a
RISC-OS revision 4 to RISC-OS revision 5. This has the effect of
avoiding a linker
bug (see section Installation Problems, for more details).
make compare may fail on version 5 of IRIX unless you add
`-save-temps' to CFLAGS. On these systems, the name of the
assembler input file is stored in the object file, and that makes
comparison fail if it differs between the stage1 and
stage2 compilations. The option `-save-temps' forces a
fixed name to be used for the assembler input file, instead of a
randomly chosen name in `/tmp'. Do not add `-save-temps'
unless the comparisons fail without that option. If you do you
`-save-temps', you will have to manually delete the `.i' and
`.s' files after each series of compilations.
The MIPS C compiler needs to be told to increase its table size
for switch statements with the `-Wf,-XNg1500' option in
order to compile `cp/parse.c'. If you use the `-O2'
optimization option, you also need to use `-Olimit 3000'.
Both of these options are automatically generated in the
`Makefile' that the shell script `configure' builds.
If you override the CC make variable and use the MIPS
compilers, you may need to add `-Wf,-XNg1500 -Olimit 3000'.
On Irix version 4.0.5F, and perhaps on some other versions as well,
there is an assembler bug that reorders instructions incorrectly. To
work around it, specify the target configuration
`mips-sgi-irix4loser'. This configuration inhibits assembler
optimization.
In a compiler configured with target `mips-sgi-irix4', you can turn
off assembler optimization by using the `-noasmopt' option. This
compiler option passes the option `-O0' to the assembler, to
inhibit reordering.
The `-noasmopt' option can be useful for testing whether a problem
is due to erroneous assembler reordering. Even if a problem does not go
away with `-noasmopt', it may still be due to assembler
reordering--perhaps GNU CC itself was miscompiled as a result.
To enable debugging under Irix 5, you must use GNU as 2.5 or later,
and use the `--with-gnu-as' configure option when configuring gcc.
GNU as is distributed as part of the binutils package.
alloca
and malloc; you must get the compiled versions of these from GNU
Emacs.
hc, the Metaware compiler, it will work, but you will get
mismatches between the stage 2 and stage 3 compilers in various files.
These errors are minor differences in some floating-point constants and
can be safely ignored; the stage 3 compiler is correct.
vcc). It produces incorrect code
in some cases (for example, when alloca is used).
Meanwhile, compiling `cp/parse.c' with pcc does not work because of
an internal table size limitation in that compiler. To avoid this
problem, compile just the GNU C compiler first, and use it to recompile
building all the languages that you want to run.
mv /lib/cpp /lib/cpp.att
cp cpp /lib/cpp.gnu
echo '/lib/cpp.gnu -traditional ${1+"$@"}' > /lib/cpp
chmod +x /lib/cpp
The system's compiler produces bad code for some of the GNU CC
optimization files. So you must build the stage 2 compiler without
optimization. Then build a stage 3 compiler with optimization.
That executable should work. Here are the necessary commands:
make LANGUAGES=c CC=stage1/xgcc CFLAGS="-Bstage1/ -g" make stage2 make CC=stage2/xgcc CFLAGS="-Bstage2/ -g -O"You may need to raise the ULIMIT setting to build a C++ compiler, as the file `cc1plus' is larger than one megabyte.
If you wish to build the object files and executables in a directory other than the one containing the source files, here is what you must do differently:
VPATH
feature. (GNU Make supports it, as do Make versions on most BSD
systems.)
make distclean
mkdir gcc-sun3 cd gcc-sun3On systems that do not support symbolic links, this directory must be on the same file system as the source code directory.
../gcc/configure ...This also tells
configure where to find the compiler sources;
configure takes the directory from the file name that was used to
invoke it. But if you want to be sure, you can specify the source
directory with the `--srcdir' option, like this:
../gcc/configure --srcdir=../gcc other optionsThe directory you specify with `--srcdir' need not be the same as the one that
configure is found in.
Now, you can run make in that directory. You need not repeat the
configuration steps shown above, when ordinary source files change. You
must, however, run configure again when the configuration files
change, if your system does not support symbolic links.
GNU CC can function as a cross-compiler for many machines, but not all.
Since GNU CC generates assembler code, you probably need a cross-assembler that GNU CC can run, in order to produce object files. If you want to link on other than the target machine, you need a cross-linker as well. You also need header files and libraries suitable for the target machine that you can install on the host machine.
To compile and run a program using a cross-compiler involves several steps:
It is most convenient to do all of these steps on the same host machine, since then you can do it all with a single invocation of GNU CC. This requires a suitable cross-assembler and cross-linker. For some targets, the GNU assembler and linker are available.
To build GNU CC as a cross-compiler, you start out by running `configure'. Use the `--target=target' to specify the target type. If `configure' was unable to correctly identify the system you are running on, also specify the `--build=build' option. For example, here is how to configure for a cross-compiler that produces code for an HP 68030 system running BSD on a system that `configure' can correctly identify:
./configure --target=m68k-hp-bsd4.3
If you have a cross-assembler and cross-linker available, you should install them now. Put them in the directory `/usr/local/target/bin'. Here is a table of the tools you should put in this directory:
The installation of GNU CC will find these programs in that directory, and copy or link them to the proper place to for the cross-compiler to find them when run later.
The easiest way to provide these files is to build the Binutils package and GAS. Configure them with the same `--host' and `--target' options that you use for configuring GNU CC, then build and install them. They install their executables automatically into the proper directory. Alas, they do not support all the targets that GNU CC supports.
If you want to install libraries to use with the cross-compiler, such as a standard C library, put them in the directory `/usr/local/target/lib'; installation of GNU CC copies all the files in that subdirectory into the proper place for GNU CC to find them and link with them. Here's an example of copying some libraries from a target machine:
ftp target-machine lcd /usr/local/target/lib cd /lib get libc.a cd /usr/lib get libg.a get libm.a quit
The precise set of libraries you'll need, and their locations on the target machine, vary depending on its operating system.
Many targets require "start files" such as `crt0.o' and
`crtn.o' which are linked into each executable; these too should be
placed in `/usr/local/target/lib'. There may be several
alternatives for `crt0.o', for use with profiling or other
compilation options. Check your target's definition of
STARTFILE_SPEC to find out what start files it uses.
Here's an example of copying these files from a target machine:
ftp target-machine lcd /usr/local/target/lib prompt cd /lib mget *crt*.o cd /usr/lib mget *crt*.o quit
Code compiled by GNU CC uses certain runtime support functions implicitly. Some of these functions can be compiled successfully with GNU CC itself, but a few cannot be. These problem functions are in the source file `libgcc1.c'; the library made from them is called `libgcc1.a'.
When you build a native compiler, these functions are compiled with some other compiler--the one that you use for bootstrapping GNU CC. Presumably it knows how to open code these operations, or else knows how to call the run-time emulation facilities that the machine comes with. But this approach doesn't work for building a cross-compiler. The compiler that you use for building knows about the host system, not the target system.
So, when you build a cross-compiler you have to supply a suitable library `libgcc1.a' that does the job it is expected to do.
To compile `libgcc1.c' with the cross-compiler itself does not work. The functions in this file are supposed to implement arithmetic operations that GNU CC does not know how to open code for your target machine. If these functions are compiled with GNU CC itself, they will compile into infinite recursion.
On any given target, most of these functions are not needed. If GNU CC can open code an arithmetic operation, it will not call these functions to perform the operation. It is possible that on your target machine, none of these functions is needed. If so, you can supply an empty library as `libgcc1.a'.
Many targets need library support only for multiplication and division.
If you are linking with a library that contains functions for
multiplication and division, you can tell GNU CC to call them directly
by defining the macros MULSI3_LIBCALL, and the like. These
macros need to be defined in the target description macro file. For
some targets, they are defined already. This may be sufficient to
avoid the need for libgcc1.a; if so, you can supply an empty library.
Some targets do not have floating point instructions; they need other functions in `libgcc1.a', which do floating arithmetic. Recent versions of GNU CC have a file which emulates floating point. With a certain amount of work, you should be able to construct a floating point emulator that can be used as `libgcc1.a'. Perhaps future versions will contain code to do this automatically and conveniently. That depends on whether someone wants to implement it.
Some embedded targets come with all the necessary `libgcc1.a' routines written in C or assembler. These targets build `libgcc1.a' automatically and you do not need to do anything special for them. Other embedded targets do not need any `libgcc1.a' routines since all the necessary operations are supported by the hardware.
If your target system has another C compiler, you can configure GNU CC as a native compiler on that machine, build just `libgcc1.a' with `make libgcc1.a' on that machine, and use the resulting file with the cross-compiler. To do this, execute the following on the target machine:
cd target-build-dir ./configure --host=sparc --target=sun3 make libgcc1.a
And then this on the host machine:
ftp target-machine binary cd target-build-dir get libgcc1.a quit
Another way to provide the functions you need in `libgcc1.a' is to
define the appropriate perform_... macros for those
functions. If these definitions do not use the C arithmetic operators
that they are meant to implement, you should be able to compile them
with the cross-compiler you are building. (If these definitions already
exist for your target file, then you are all set.)
To build `libgcc1.a' using the perform macros, use
`LIBGCC1=libgcc1.a OLDCC=./xgcc' when building the compiler.
Otherwise, you should place your replacement library under the name
`libgcc1.a' in the directory in which you will build the
cross-compiler, before you run make.
If you are cross-compiling a standalone program or a program for an embedded system, then you may not need any header files except the few that are part of GNU CC (and those of your program). However, if you intend to link your program with a standard C library such as `libc.a', then you probably need to compile with the header files that go with the library you use.
The GNU C compiler does not come with these files, because (1) they are system-specific, and (2) they belong in a C library, not in a compiler.
If the GNU C library supports your target machine, then you can get the header files from there (assuming you actually use the GNU library when you link your program).
If your target machine comes with a C compiler, it probably comes with suitable header files also. If you make these files accessible from the host machine, the cross-compiler can use them also.
Otherwise, you're on your own in finding header files to use when cross-compiling.
When you have found suitable header files, put them in the directory `/usr/local/target/include', before building the cross compiler. Then installation will run fixincludes properly and install the corrected versions of the header files where the compiler will use them.
Provide the header files before you build the cross-compiler, because the build stage actually runs the cross-compiler to produce parts of `libgcc.a'. (These are the parts that can be compiled with GNU CC.) Some of them need suitable header files.
Here's an example showing how to copy the header files from a target machine. On the target machine, do this:
(cd /usr/include; tar cf - .) > tarfile
Then, on the host machine, do this:
ftp target-machine lcd /usr/local/target/include get tarfile quit tar xf tarfile
Now you can proceed just as for compiling a single-machine compiler through the step of building stage 1. If you have not provided some sort of `libgcc1.a', then compilation will give up at the point where it needs that file, printing a suitable error message. If you do provide `libgcc1.a', then building the compiler will automatically compile and link a test program called `libgcc1-test'; if you get errors in the linking, it means that not all of the necessary routines in `libgcc1.a' are available.
You must provide the header file `float.h'. One way to do this is to compile `enquire' and run it on your target machine. The job of `enquire' is to run on the target machine and figure out by experiment the nature of its floating point representation. `enquire' records its findings in the header file `float.h'. If you can't produce this file by running `enquire' on the target machine, then you will need to come up with a suitable `float.h' in some other way (or else, avoid using it in your programs).
Do not try to build stage 2 for a cross-compiler. It doesn't work to rebuild GNU CC as a cross-compiler using the cross-compiler, because that would produce a program that runs on the target machine, not on the host. For example, if you compile a 386-to-68030 cross-compiler with itself, the result will not be right either for the 386 (because it was compiled into 68030 code) or for the 68030 (because it was configured for a 386 as the host). If you want to compile GNU CC into 68030 code, whether you compile it on a 68030 or with a cross-compiler on a 386, you must specify a 68030 as the host when you configure it.
To install the cross-compiler, use `make install', as usual.
On Solaris, do not use the linker or other tools in
`/usr/ucb' to build GNU CC. Use /usr/ccs/bin.
If the assembler reports `Error: misaligned data' when bootstrapping,
you are probably using an obsolete version of the GNU assembler. Upgrade
to the latest version of GNU binutils, or use the Solaris assembler.
Make sure the environment variable FLOAT_OPTION is not set when
you compile `libgcc.a'. If this option were set to f68881
when `libgcc.a' is compiled, the resulting code would demand to be
linked with a special startup file and would not link properly without
special pains.
There is a bug in alloca in certain versions of the Sun library.
To avoid this bug, install the binaries of GNU CC that were compiled by
GNU CC. They use alloca as a built-in function and never the one
in the library.
Some versions of the Sun compiler crash when compiling GNU CC. The problem is a segmentation fault in cpp. This problem seems to be due to the bulk of data in the environment variables. You may be able to avoid it by using the following command to compile GNU CC with Sun CC:
make CC="TERMCAP=x OBJS=x LIBFUNCS=x STAGESTUFF=x cc"
SunOS 4.1.3 and 4.1.3_U1 have bugs that can cause intermittent core dumps when compiling GNU CC. A common symptom is an internal compiler error which does not recur if you run it again. To fix the problem, install Sun recommended patch 100726 (for SunOS 4.1.3) or 101508 (for SunOS 4.1.3_U1), or upgrade to a later SunOS release.
The VMS version of GNU CC is distributed in a backup saveset containing both source code and precompiled binaries.
To install the `gcc' command so you can use the compiler easily, in the same manner as you use the VMS C compiler, you must install the VMS CLD file for GNU CC as follows:
$ assign /system /translation=concealed - disk:[gcc.] gnu_cc $ assign /system /translation=concealed - disk:[gcc.include.] gnu_cc_includewith the appropriate disk and directory names. These commands can be placed in your system startup file so they will be executed whenever the machine is rebooted. You may, if you choose, do this via the `GCC_INSTALL.COM' script in the `[GCC]' directory.
$ set command /table=sys$common:[syslib]dcltables - /output=sys$common:[syslib]dcltables gnu_cc:[000000]gcc $ install replace sys$common:[syslib]dcltables
$ library/help sys$library:helplib.hlb gcc.hlpNow you can invoke the compiler with a command like `gcc /verbose file.c', which is equivalent to the command `gcc -v -c file.c' in Unix.
If you wish to use GNU C++ you must first install GNU CC, and then perform the following steps:
$ assign /system /translation=concealed - disk:[gcc.gxx_include.] gnu_gxx_includewith the appropriate disk and directory name. If you are going to be using a C++ runtime library, this is where its install procedure will install its header files.
We try to put corresponding binaries and sources on the VMS distribution tape. But sometimes the binaries will be from an older version than the sources, because we don't always have time to update them. (Use the `/version' option to determine the version number of the binaries and compare it with the source file `version.c' to tell whether this is so.) In this case, you should use the binaries you get to recompile the sources. If you must recompile, here is how:
$ @vmsconfig.com
$ assign /system /translation=concealed - disk:[bison.] gnu_bisonYou may, if you choose, use the `INSTALL_BISON.COM' script in the `[BISON]' directory.
$ set command /table=sys$common:[syslib]dcltables - /output=sys$common:[syslib]dcltables - gnu_bison:[000000]bison $ install replace sys$common:[syslib]dcltables
$ library gnu_cc:[000000]gcclib/delete=(new,eprintf) $ library gnu_cc:[000000]gcclib/delete=L_* $ library libgcc2/extract=*/output=libgcc2.obj $ library gnu_cc:[000000]gcclib libgcc2.objThe first command simply removes old modules that will be replaced with modules from `libgcc2' under different module names. The modules
new and eprintf may not actually be present in your
`gcclib.olb'---if the VMS librarian complains about those modules
not being present, simply ignore the message and continue on with the
next command. The second command removes the modules that came from the
previous version of the library `libgcc2.c'.
Whenever you update the compiler on your system, you should also update the
library with the above procedure.
$ assign dua0:[gcc.build_dir.]/translation=concealed, -
dua1:[gcc.source_dir.]/translation=concealed gcc_build
$ set default gcc_build:[000000]
where the directory `dua1:[gcc.source_dir]' contains the source
code, and the directory `dua0:[gcc.build_dir]' is meant to contain
all of the generated object files and executables. Once you have done
this, you can proceed building GCC as described above. (Keep in mind
that `gcc_build' is a rooted logical name, and thus the device
names in each element of the search list must be an actual physical
device name rather than another rooted logical name).
extern const variables will not have
the read-only bit set, and the linker will generate warning messages
about mismatched psect attributes for these variables. These warning
messages are merely a nuisance, and can safely be ignored.
If you are compiling with a version of GNU CC older than 1.33, specify
`/DEFINE=("inline=")' as an option in all the compilations. This
requires editing all the gcc commands in `make-cc1.com'.
(The older versions had problems supporting inline.) Once you
have a working 1.33 or newer GNU CC, you can change this file back.
CC, CFLAGS, and
LIBS. See comments in those files. However, you must
also have a working version of the GNU assembler (GNU as, aka GAS) as
it is used as the back-end for GNU CC to produce binary object modules
and is not included in the GNU CC sources. GAS is also needed to
compile `libgcc2' in order to build `gcclib' (see above);
`make-l2.com' expects to be able to find it operational in
`gnu_cc:[000000]gnu-as.exe'.
To use GNU CC on VMS, you need the VMS driver programs
`gcc.exe', `gcc.com', and `gcc.cld'. They are
distributed with the VMS binaries (`gcc-vms') rather than the
GNU CC sources. GAS is also included in `gcc-vms', as is Bison.
Once you have successfully built GNU CC with VAX C, you should use the
resulting compiler to rebuild itself. Before doing this, be sure to
restore the CC, CFLAGS, and LIBS definitions in
`make-cccp.com' and `make-cc1.com'. The second generation
compiler will be able to take advantage of many optimizations that must
be suppressed when building with other compilers.
Under previous versions of GNU CC, the generated code would occasionally give strange results when linked with the sharable `VAXCRTL' library. Now this should work.
Even with this version, however, GNU CC itself should not be linked with
the sharable `VAXCRTL'. The version of qsort in
`VAXCRTL' has a bug (known to be present in VMS versions V4.6
through V5.5) which causes the compiler to fail.
The executables are generated by `make-cc1.com' and
`make-cccp.com' use the object library version of `VAXCRTL' in
order to make use of the qsort routine in `gcclib.olb'. If
you wish to link the compiler executables with the shareable image
version of `VAXCRTL', you should edit the file `tm.h' (created
by `vmsconfig.com') to define the macro QSORT_WORKAROUND.
QSORT_WORKAROUND is always defined when GNU CC is compiled with
VAX C, to avoid a problem in case `gcclib.olb' is not yet
available.
collect2
GNU CC uses a utility called collect2 on nearly all systems to arrange
to call various initialization functions at start time.
The program collect2 works by linking the program once and
looking through the linker output file for symbols with particular names
indicating they are constructor functions. If it finds any, it
creates a new temporary `.c' file containing a table of them,
compiles it, and links the program a second time including that file.
The actual calls to the constructors are carried out by a subroutine
called __main, which is called (automatically) at the beginning
of the body of main (provided main was compiled with GNU
CC). Calling __main is necessary, even when compiling C code, to
allow linking C and C++ object code together. (If you use
`-nostdlib', you get an unresolved reference to __main,
since it's defined in the standard GCC library. Include `-lgcc' at
the end of your compiler command line to resolve this reference.)
The program collect2 is installed as ld in the directory
where the passes of the compiler are installed. When collect2
needs to find the real ld, it tries the following file
names:
PATH.
REAL_LD_FILE_NAME configuration macro,
if specified.
collect2 will not execute itself recursively.
PATH.
"The compiler's search directories" means all the directories where
gcc searches for passes of the compiler. This includes
directories that you specify with `-B'.
Cross-compilers search a little differently:
PATH.
REAL_LD_FILE_NAME configuration macro,
if specified.
PATH.
collect2 explicitly avoids running ld using the file name
under which collect2 itself was invoked. In fact, it remembers
up a list of such names--in case one copy of collect2 finds
another copy (or version) of collect2 installed as ld in a
second place in the search path.
collect2 searches for the utilities nm and strip
using the same algorithm as above for ld.
GCC_INCLUDE_DIR means the same thing for native and cross. It is
where GNU CC stores its private include files, and also where GNU CC
stores the fixed include files. A cross compiled GNU CC runs
fixincludes on the header files in `$(tooldir)/include'.
(If the cross compilation header files need to be fixed, they must be
installed before GNU CC is built. If the cross compilation header files
are already suitable for ANSI C and GNU CC, nothing special need be
done).
GPLUSPLUS_INCLUDE_DIR means the same thing for native and cross. It
is where g++ looks first for header files. The C++ library
installs only target independent header files in that directory.
LOCAL_INCLUDE_DIR is used only for a native compiler. It is
normally `/usr/local/include'. GNU CC searches this directory so
that users can install header files in `/usr/local/include'.
CROSS_INCLUDE_DIR is used only for a cross compiler. GNU CC
doesn't install anything there.
TOOL_INCLUDE_DIR is used for both native and cross compilers. It
is the place for other packages to install header files that GNU CC will
use. For a cross-compiler, this is the equivalent of
`/usr/include'. When you build a cross-compiler,
fixincludes processes any header files in this directory.
GNU C provides several language features not found in ANSI standard C.
(The `-pedantic' option directs GNU CC to print a warning message if
any of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
__GNUC__, which is always defined under GNU CC.
These extensions are available in C and Objective C. Most of them are also available in C++. See section Extensions to the C++ Language, for extensions that apply only to C++.
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression
for the absolute value of foo ().
The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct. (If you use some other kind of statement
last within the braces, the construct has type void, and thus
effectively no value.)
This feature is especially useful in making macro definitions "safe" (so that they evaluate each operand exactly once). For example, the "maximum" function is commonly defined as a macro in standard C as follows:
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either a or b twice, with bad
results if the operand has side effects. In GNU C, if you know the
type of the operands (here let's assume int), you can define
the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit field, or the initial value of a static variable.
If you don't know the type of the operand, you can still do this, but you
must use typeof (see section Referring to a Type with typeof) or type naming (see section Naming an Expression's Type).
Each statement expression is a scope in which local labels can be
declared. A local label is simply an identifier; you can jump to it
with an ordinary goto statement, but only from within the
statement expression it belongs to.
A local label declaration looks like this:
__label__ label;
or
__label__ label1, label2, ...;
Local label declarations must come at the beginning of the statement expression, right after the `({', before any ordinary declarations.
The label declaration defines the label name, but does not define
the label itself. You must do this in the usual way, with
label:, within the statements of the statement expression.
The local label feature is useful because statement expressions are
often used in macros. If the macro contains nested loops, a goto
can be useful for breaking out of them. However, an ordinary label
whose scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function. A local label avoids this problem. For
example:
#define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})
You can get the address of a label defined in the current function
(or a containing function) with the unary operator `&&'. The
value has type void *. This value is a constant and can be used
wherever a constant of that type is valid. For example:
void *ptr; ... ptr = &&foo;
To use these values, you need to be able to jump to one. This is done
with the computed goto statement(1), goto *exp;. For example,
goto *ptr;
Any expression of type void * is allowed.
One way of using these constants is in initializing a static array that will serve as a jump table:
static void *array[] = { &&foo, &&bar, &&hack };
Then you can select a label with indexing, like this:
goto *array[i];
Note that this does not check whether the subscript is in bounds--array indexing in C never does that.
Such an array of label values serves a purpose much like that of the
switch statement. The switch statement is cleaner, so
use that rather than an array unless the problem does not fit a
switch statement very well.
Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.
You can use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.
A nested function is a function defined inside another function.
(Nested functions are not supported for GNU C++.) The nested function's
name is local to the block where it is defined. For example, here we
define a nested function named square, and call it twice:
foo (double a, double b)
{
double square (double z) { return z * z; }
return square (a) + square (b);
}
The nested function can access all the variables of the containing
function that are visible at the point of its definition. This is
called lexical scoping. For example, here we show a nested
function which uses an inherited variable named offset:
bar (int *array, int offset, int size)
{
int access (int *array, int index)
{ return array[index + offset]; }
int i;
...
for (i = 0; i < size; i++)
... access (array, i) ...
}
Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.
It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:
hack (int *array, int size)
{
void store (int index, int value)
{ array[index] = value; }
intermediate (store, size);
}
Here, the function intermediate receives the address of
store as an argument. If intermediate calls store,
the arguments given to store are used to store into array.
But this technique works only so long as the containing function
(hack, in this example) does not exit.
If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.
GNU CC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as `http://master.debian.org/~karlheg/Usenix88-lexic.pdf'.
A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (see section Locally Declared Labels). Such a jump returns instantly to the
containing function, exiting the nested function which did the
goto and any intermediate functions as well. Here is an example:
bar (int *array, int offset, int size)
{
__label__ failure;
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
int i;
...
for (i = 0; i < size; i++)
... access (array, i) ...
...
return 0;
/* Control comes here from access
if it detects an error. */
failure:
return -1;
}
A nested function always has internal linkage. Declaring one with
extern is erroneous. If you need to declare the nested function
before its definition, use auto (which is otherwise meaningless
for function declarations).
bar (int *array, int offset, int size)
{
__label__ failure;
auto int access (int *, int);
...
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
...
}
Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.
You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).
__builtin_apply_args ()
void * to data
describing how to perform a call with the same arguments as were passed
to the current function.
The function saves the arg pointer register, structure value address,
and all registers that might be used to pass arguments to a function
into a block of memory allocated on the stack. Then it returns the
address of that block.
__builtin_apply (function, arguments, size)
void (*)())
with a copy of the parameters described by arguments (type
void *) and size (type int).
The value of arguments should be the value returned by
__builtin_apply_args. The argument size specifies the size
of the stack argument data, in bytes.
This function returns a pointer of type void * to data describing
how to return whatever value was returned by function. The data
is saved in a block of memory allocated on the stack.
It is not always simple to compute the proper value for size. The
value is used by __builtin_apply to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
area.
__builtin_return (result)
__builtin_apply.
You can give a name to the type of an expression using a typedef
declaration with an initializer. Here is how to define name as a
type name for the type of exp:
typedef name = exp;
This is useful in conjunction with the statements-within-expressions feature. Here is how the two together can be used to define a safe "maximum" macro that operates on any arithmetic type:
#define max(a,b) \
({typedef _ta = (a), _tb = (b); \
_ta _a = (a); _tb _b = (b); \
_a > _b ? _a : _b; })
The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for a and b. Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.
typeof
Another way to refer to the type of an expression is with typeof.
The syntax of using of this keyword looks like sizeof, but the
construct acts semantically like a type name defined with typedef.
There are two ways of writing the argument to typeof: with an
expression or with a type. Here is an example with an expression:
typeof (x[0](1))
This assumes that x is an array of functions; the type described
is that of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *)
Here the type described is that of pointers to int.
If you are writing a header file that must work when included in ANSI C
programs, write __typeof__ instead of typeof.
See section Alternate Keywords.
A typeof-construct can be used anywhere a typedef name could be
used. For example, you can use it in a declaration, in a cast, or inside
of sizeof or typeof.
y with the type of what x points to.
typeof (*x) y;
y as an array of such values.
typeof (*x) y[4];
y as an array of pointers to characters:
typeof (typeof (char *)[4]) y;It is equivalent to the following traditional C declaration:
char *y[4];To see the meaning of the declaration using
typeof, and why it
might be a useful way to write, let's rewrite it with these macros:
#define pointer(T) typeof(T *) #define array(T, N) typeof(T [N])Now the declaration can be rewritten this way:
array (pointer (char), 4) y;Thus,
array (pointer (char), 4) is the type of arrays of 4
pointers to char.
Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them.
Standard C++ allows compound expressions and conditional expressions as lvalues, and permits casts to reference type, so use of this extension is deprecated for C++ code.
For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent:
(a, b) += 5 a, (b += 5)
Similarly, the address of the compound expression can be taken. These two expressions are equivalent:
&(a, b) a, &b
A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent:
(a ? b : c) = 5 (a ? b = 5 : (c = 5))
A cast is a valid lvalue if its operand is an lvalue. A simple
assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the
inner left-hand side expression. After this is stored, the value is
converted back to the specified type to become the value of the
assignment. Thus, if a has type char *, the following two
expressions are equivalent:
(int)a = 5 (int)(a = (char *)(int)5)
An assignment-with-arithmetic operation such as `+=' applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent:
(int)a += 5 (int)(a = (char *)(int) ((int)a + 5))
You cannot take the address of an lvalue cast, because the use of its
address would not work out coherently. Suppose that &(int)f were
permitted, where f has type float. Then the following
statement would try to store an integer bit-pattern where a floating
point number belongs:
*&(int)f = 1;
This is quite different from what (int)f = 1 would do--that
would convert 1 to floating point and store it. Rather than cause this
inconsistency, we think it is better to prohibit use of `&' on a cast.
If you really do want an int * pointer with the address of
f, you can simply write (int *)&f.
The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.
Therefore, the expression
x ? : y
has the value of x if that is nonzero; otherwise, the value of
y.
This example is perfectly equivalent to
x ? x : y
In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.
GNU C supports data types for integers that are twice as long as
int. Simply write long long int for a signed integer, or
unsigned long long int for an unsigned integer. To make an
integer constant of type long long int, add the suffix LL
to the integer. To make an integer constant of type unsigned long
long int, add the suffix ULL to the integer.
You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GNU CC.
There may be pitfalls when you use long long types for function
arguments, unless you declare function prototypes. If a function
expects type int for its argument, and you pass a value of type
long long int, confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects long long int and you pass
int. The best way to avoid such problems is to use prototypes.
GNU C supports complex data types. You can declare both complex integer
types and complex floating types, using the keyword __complex__.
For example, `__complex__ double x;' declares x as a
variable whose real part and imaginary part are both of type
double. `__complex__ short int y;' declares y to
have real and imaginary parts of type short int; this is not
likely to be useful, but it shows that the set of complex types is
complete.
To write a constant with a complex data type, use the suffix `i' or
`j' (either one; they are equivalent). For example, 2.5fi
has type __complex__ float and 3i has type
__complex__ int. Such a constant always has a pure imaginary
value, but you can form any complex value you like by adding one to a
real constant.
To extract the real part of a complex-valued expression exp, write
__real__ exp. Likewise, use __imag__ to
extract the imaginary part.
The operator `~' performs complex conjugation when used on a value with a complex type.
GNU CC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa). None of the
supported debugging info formats has a way to represent noncontiguous
allocation like this, so GNU CC describes a noncontiguous complex
variable as if it were two separate variables of noncomplex type.
If the variable's actual name is foo, the two fictitious
variables are named foo$real and foo$imag. You can
examine and set these two fictitious variables with your debugger.
A future version of GDB will know how to recognize such pairs and treat them as a single variable with a complex type.
GNU CC recognizes floating-point numbers written not only in the usual
decimal notation, such as 1.55e1, but also numbers such as
0x1.fp3 written in hexadecimal format. In that format the
0x hex introducer and the p or P exponent field are
mandatory. The exponent is a decimal number that indicates the power of
2 by which the significand part will be multiplied. Thus 0x1.f is
1 15/16, p3 multiplies it by 8, and the value of 0x1.fp3
is the same as 1.55e1.
Unlike for floating-point numbers in the decimal notation the exponent
is always required in the hexadecimal notation. Otherwise the compiler
would not be able to resolve the ambiguity of, e.g., 0x1.f. This
could mean 1.0f or 1.9375 since f is also the
extension for floating-point constants of type float.
Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:
struct line {
int length;
char contents[0];
};
{
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
}
In standard C, you would have to give contents a length of 1, which
means either you waste space or complicate the argument to malloc.
Variable-length automatic arrays are allowed in GNU C. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:
FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
return fopen (str, mode);
}
Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.
You can use the function alloca to get an effect much like
variable-length arrays. The function alloca is available in
many other C implementations (but not in all). On the other hand,
variable-length arrays are more elegant.
There are other differences between these two methods. Space allocated
with alloca exists until the containing function returns.
The space for a variable-length array is deallocated as soon as the array
name's scope ends. (If you use both variable-length arrays and
alloca in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with alloca.)
You can also use variable-length arrays as arguments to functions:
struct entry
tester (int len, char data[len][len])
{
...
}
The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
sizeof.
If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list--another GNU extension.
struct entry
tester (int len; char data[len][len], int len)
{
...
}
The `int len' before the semicolon is a parameter forward
declaration, and it serves the purpose of making the name len
known when the declaration of data is parsed.
You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the "real" parameter declarations. Each forward declaration must match a "real" declaration in parameter name and data type.
In GNU C, a macro can accept a variable number of arguments, much as a function can. The syntax for defining the macro looks much like that used for a function. Here is an example:
#define eprintf(format, args...) \ fprintf (stderr, format , ## args)
Here args is a rest argument: it takes in zero or more
arguments, as many as the call contains. All of them plus the commas
between them form the value of args, which is substituted into
the macro body where args is used. Thus, we have this expansion:
eprintf ("%s:%d: ", input_file_name, line_number)
==>
fprintf (stderr, "%s:%d: " , input_file_name, line_number)
Note that the comma after the string constant comes from the definition
of eprintf, whereas the last comma comes from the value of
args.
The reason for using `##' is to handle the case when args
matches no arguments at all. In this case, args has an empty
value. In this case, the second comma in the definition becomes an
embarrassment: if it got through to the expansion of the macro, we would
get something like this:
fprintf (stderr, "success!\n" , )
which is invalid C syntax. `##' gets rid of the comma, so we get the following instead:
fprintf (stderr, "success!\n")
This is a special feature of the GNU C preprocessor: `##' before a rest argument that is empty discards the preceding sequence of non-whitespace characters from the macro definition. (If another macro argument precedes, none of it is discarded.)
It might be better to discard the last preprocessor token instead of the last preceding sequence of non-whitespace characters; in fact, we may someday change this feature to do so. We advise you to write the macro definition so that the preceding sequence of non-whitespace characters is just a single token, so that the meaning will not change if we change the definition of this feature.
Subscripting is allowed on arrays that are not lvalues, even though the unary `&' operator is not. For example, this is valid in GNU C though not valid in other C dialects:
struct foo {int a[4];};
struct foo f();
bar (int index)
{
return f().a[index];
}
void- and Function-Pointers
In GNU C, addition and subtraction operations are supported on pointers to
void and on pointers to functions. This is done by treating the
size of a void or of a function as 1.
A consequence of this is that sizeof is also allowed on void
and on function types, and returns 1.
The option `-Wpointer-arith' requests a warning if these extensions are used.
As in standard C++, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:
foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
...
}
GNU C supports constructor expressions. A constructor looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer.
Usually, the specified type is a structure. Assume that
struct foo and structure are declared as shown:
struct foo {int a; char b[2];} structure;
Here is an example of constructing a struct foo with a constructor:
structure = ((struct foo) {x + y, 'a', 0});
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
You can also construct an array. If all the elements of the constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here:
char **foo = (char *[]) { "x", "y", "z" };
Array constructors whose elements are not simple constants are
not very useful, because the constructor is not an lvalue. There
are only two valid ways to use it: to subscript it, or initialize
an array variable with it. The former is probably slower than a
switch statement, while the latter does the same thing an
ordinary C initializer would do. Here is an example of
subscripting an array constructor:
output = ((int[]) { 2, x, 28 }) [input];
Constructor expressions for scalar types and union types are is also allowed, but then the constructor expression is equivalent to a cast.
Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.
In GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to. This extension is not implemented in GNU C++.
To specify an array index, write `[index]' or `[index] =' before the element value. For example,
int a[6] = { [4] 29, [2] = 15 };
is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
The index values must be constant expressions, even if the array being initialized is automatic.
To initialize a range of elements to the same value, write `[first ... last] = value'. For example,
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
Note that the length of the array is the highest value specified plus one.
In a structure initializer, specify the name of a field to initialize with `fieldname:' before the element value. For example, given the following structure,
struct point { int x, y; };
the following initialization
struct point p = { y: yvalue, x: xvalue };
is equivalent to
struct point p = { xvalue, yvalue };
Another syntax which has the same meaning is `.fieldname ='., as shown here:
struct point p = { .y = yvalue, .x = xvalue };
You can also use an element label (with either the colon syntax or the period-equal syntax) when initializing a union, to specify which element of the union should be used. For example,
union foo { int i; double d; };
union foo f = { d: 4 };
will convert 4 to a double to store it in the union using
the second element. By contrast, casting 4 to type union foo
would store it into the union as the integer i, since it is
an integer. (See section Cast to a Union Type.)
You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a label applies to the next consecutive element of the array or structure. For example,
int a[6] = { [1] = v1, v2, [4] = v4 };
is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an enum type.
For example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
You can specify a range of consecutive values in a single case label,
like this:
case low ... high:
This has the same effect as the proper number of individual case
labels, one for each integer value from low to high, inclusive.
This feature is especially useful for ranges of ASCII character codes:
case 'A' ... 'Z':
Be careful: Write spaces around the ..., for otherwise
it may be parsed wrong when you use it with integer values. For example,
write this:
case 1 ... 5:
rather than this:
case 1...5:
A cast to union type is similar to other casts, except that the type
specified is a union type. You can specify the type either with
union tag or with a typedef name. A cast to union is actually
a constructor though, not a cast, and hence does not yield an lvalue like
normal casts. (See section Constructor Expressions.)
The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:
union foo { int i; double d; };
int x;
double y;
both x and y can be cast to type union foo.
Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:
union foo u; ... u = (union foo) x == u.i = x u = (union foo) y == u.d = y
You can also use the union cast as a function argument:
void hack (union foo); ... hack ((union foo) x);
In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.
The keyword __attribute__ allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. Nine attributes,
noreturn, const, format,
no_instrument_function, section,
constructor, destructor, unused and weak are
currently defined for functions. Other attributes, including
section are supported for variables declarations (see section Specifying Attributes of Variables) and for types (see section Specifying Attributes of Types).
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __noreturn__ instead of noreturn.
noreturn
abort and exit,
cannot return. GNU CC knows this automatically. Some programs define
their own functions that never return. You can declare them
noreturn to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (...)
{
... /* Print error message. */ ...
exit (1);
}
The noreturn keyword tells the compiler to assume that
fatal cannot return. It can then optimize without regard to what
would happen if fatal ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
Do not assume that registers saved by the calling function are
restored before calling the noreturn function.
It does not make sense for a noreturn function to have a return
type other than void.
The attribute noreturn is not implemented in GNU C versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:
typedef void voidfn (); volatile voidfn fatal;
const
const. For example,
int square (int) __attribute__ ((const));says that the hypothetical function
square is safe to call
fewer times than the program says.
The attribute const is not implemented in GNU C versions earlier
than 2.5. An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:
typedef int intfn (); extern const intfn square;This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value. Note that a function that has pointer arguments and examines the data pointed to must not be declared
const. Likewise, a
function that calls a non-const function usually must not be
const. It does not make sense for a const function to
return void.
format (archetype, string-index, first-to-check)
format attribute specifies that a function takes printf,
scanf, or strftime style arguments which should be type-checked
against a format string. For example, the declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to my_printf
for consistency with the printf style format string argument
my_format.
The parameter archetype determines how the format string is
interpreted, and should be either printf, scanf, or
strftime. The
parameter string-index specifies which argument is the format
string argument (starting from 1), while first-to-check is the
number of the first argument to check against the format string. For
functions where the arguments are not available to be checked (such as
vprintf), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency.
In the example above, the format string (my_format) is the second
argument of the function my_print, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
The format attribute allows you to identify your own functions
which take format strings as arguments, so that GNU CC can check the
calls to these functions for errors. The compiler always checks formats
for the ANSI library functions printf, fprintf,
sprintf, scanf, fscanf, sscanf, strftime,
vprintf, vfprintf and vsprintf whenever such
warnings are requested (using `-Wformat'), so there is no need to
modify the header file `stdio.h'.
format_arg (string-index)
format_arg attribute specifies that a function takes
printf or scanf style arguments, modifies it (for example,
to translate it into another language), and passes it to a printf
or scanf style function. For example, the declaration:
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
causes the compiler to check the arguments in calls to
my_dgettext whose result is passed to a printf,
scanf, or strftime type function for consistency with the
printf style format string argument my_format.
The parameter string-index specifies which argument is the format
string argument (starting from 1).
The format-arg attribute allows you to identify your own
functions which modify format strings, so that GNU CC can check the
calls to printf, scanf, or strftime function whose
operands are a call to one of your own function. The compiler always
treats gettext, dgettext, and dcgettext in this
manner.
no_instrument_function
section ("section-name")
text section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The section
attribute specifies that a function lives in a particular section.
For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the function foobar in the bar section.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
constructor
destructor
constructor attribute causes the function to be called
automatically before execution enters main (). Similarly, the
destructor attribute causes the function to be called
automatically after main () has completed or exit () has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
These attributes are not currently implemented for Objective C.
unused
weak
weak attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.
alias ("target")
alias attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));
declares `f' to be a weak alias for `__f'. In C++, the
mangled name for the target must be used.
Not all target machines support this attribute.
no_check_memory_usage
asm statements
are not allowed. Declaring a function with this attribute disables the
memory checking code for that function, permitting the use of asm
statements without requiring separate compilation with different
options, and allowing you to write support routines of your own if you
wish, without getting infinite recursion if they get compiled with this
option.
regparm (number)
regparm attribute causes the compiler to
pass up to number integer arguments in registers EAX,
EDX, and ECX instead of on the stack. Functions that take a
variable number of arguments will continue to be passed all of their
arguments on the stack.
stdcall
stdcall attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.
The PowerPC compiler for Windows NT currently ignores the stdcall
attribute.
cdecl
cdecl attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments. This is
useful to override the effects of the `-mrtd' switch.
The PowerPC compiler for Windows NT currently ignores the cdecl
attribute.
longcall
longcall attribute causes the
compiler to always call the function via a pointer, so that functions
which reside further than 64 megabytes (67,108,864 bytes) from the
current location can be called.
dllimport
dllimport attribute causes
the compiler to call the function via a global pointer to the function
pointer that is set up by the Windows NT dll library. The pointer name
is formed by combining __imp_ and the function name.
dllexport
dllexport attribute causes
the compiler to provide a global pointer to the function pointer, so
that it can be called with the dllimport attribute. The pointer
name is formed by combining __imp_ and the function name.
exception (except-func [, except-arg])
exception attribute causes
the compiler to modify the structured exception table entry it emits for
the declared function. The string or identifier except-func is
placed in the third entry of the structured exception table. It
represents a function, which is called by the exception handling
mechanism if an exception occurs. If it was specified, the string or
identifier except-arg is placed in the fourth entry of the
structured exception table.
function_vector
interrupt_handler
eightbit_data
tiny_data
interrupt
model (model-name)
small, medium,
or large, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction), and are
callable with the bl instruction.
Medium model objects may live anywhere in the 32 bit address space (the
compiler will generate seth/add3 instructions to load their addresses),
and are callable with the bl instruction.
Large model objects may live anywhere in the 32 bit address space (the
compiler will generate seth/add3 instructions to load their addresses),
and may not be reachable with the bl instruction (the compiler will
generate the much slower seth/add3/jl instruction sequence).
You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.
Some people object to the __attribute__ feature, suggesting that ANSI C's
#pragma should be used instead. There are two reasons for not
doing this.
#pragma commands from a macro.
#pragma might mean in another
compiler.
These two reasons apply to almost any application that might be proposed
for #pragma. It is basically a mistake to use #pragma for
anything.
GNU C extends ANSI C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:
/* Use prototypes unless the compiler is old-fashioned. */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif
/* Prototype function declaration. */
int isroot P((uid_t));
/* Old-style function definition. */
int
isroot (x) /* ??? lossage here ??? */
uid_t x;
{
return x == 0;
}
Suppose the type uid_t happens to be short. ANSI C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted. Therefore in this example the
function definition's argument is really an int, which does not
match the prototype argument type of short.
This restriction of ANSI C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the uid_t type is short, int, or
long. Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition. More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion. Thus in GNU C the above example is
equivalent to the following:
int isroot (uid_t);
int
isroot (uid_t x)
{
return x == 0;
}
GNU C++ does not support old-style function definitions, so this extension is irrelevant.
In GNU C, you may use C++ style comments, which start with `//' and
continue until the end of the line. Many other C implementations allow
such comments, and they are likely to be in a future C standard.
However, C++ style comments are not recognized if you specify
`-ansi' or `-traditional', since they are incompatible
with traditional constructs like dividend//*comment*/divisor.
In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.
You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC.
The keyword __alignof__ allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type. Its
syntax is just like sizeof.
For example, if the target machine requires a double value to be
aligned on an 8-byte boundary, then __alignof__ (double) is 8.
This is true on many RISC machines. On more traditional machine
designs, __alignof__ (double) is 4 or even 2.
Some machines never actually require alignment; they allow reference to any
data type even at an odd addresses. For these machines, __alignof__
reports the recommended alignment of a type.
When the operand of __alignof__ is an lvalue rather than a type, the
value is the largest alignment that the lvalue is known to have. It may
have this alignment as a result of its data type, or because it is part of
a structure and inherits alignment from that structure. For example, after
this declaration:
struct foo { int x; char y; } foo1;
the value of __alignof__ (foo1.y) is probably 2 or 4, the same as
__alignof__ (int), even though the data type of foo1.y
does not itself demand any alignment.
A related feature which lets you specify the alignment of an object is
__attribute__ ((aligned (alignment))); see the following
section.
The keyword __attribute__ allows you to specify special
attributes of variables or structure fields. This keyword is followed
by an attribute specification inside double parentheses. Eight
attributes are currently defined for variables: aligned,
mode, nocommon, packed, section,
transparent_union, unused, and weak. Other
attributes are available for functions (see section Declaring Attributes of Functions) and
for types (see section Specifying Attributes of Types).
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __aligned__ instead of aligned.
aligned (alignment)
int x __attribute__ ((aligned (16))) = 0;causes the compiler to allocate the global variable
x on a
16-byte boundary. On a 68040, this could be used in conjunction with
an asm expression to access the move16 instruction which
requires 16-byte aligned operands.
You can also specify the alignment of structure fields. For example, to
create a double-word aligned int pair, you could write:
struct foo { int x[2] __attribute__ ((aligned (8))); };
This is an alternative to creating a union with a double member
that forces the union to be double-word aligned.
It is not possible to specify the alignment of functions; the alignment
of functions is determined by the machine's requirements and cannot be
changed. You cannot specify alignment for a typedef name because such a
name is just an alias, not a distinct type.
As in the preceding examples, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given variable or
structure field. Alternatively, you can leave out the alignment factor
and just ask the compiler to align a variable or field to the maximum
useful alignment for the target machine you are compiling for. For
example, you could write:
short array[3] __attribute__ ((aligned));Whenever you leave out the alignment factor in an
aligned attribute
specification, the compiler automatically sets the alignment for the declared
variable or field to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often make
copy operations more efficient, because the compiler can use whatever
instructions copy the biggest chunks of memory when performing copies to
or from the variables or fields that you have aligned this way.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
mode (mode)
nocommon
nocommon attribute for a variable provides an
initialization of zeros. A variable may only be initialized in one
source file.
packed
packed attribute specifies that a variable or structure field
should have the smallest possible alignment--one byte for a variable,
and one bit for a field, unless you specify a larger value with the
aligned attribute.
Here is a structure in which the field x is packed, so that it
immediately follows a:
struct foo
{
char a;
int x[2] __attribute__ ((packed));
};
section ("section-name")
data and bss. Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections,
for example to map to special hardware. The section
attribute specifies that a variable (or function) lives in a particular
section. For example, this small program uses several specific section names:
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA"))) = 0;
main()
{
/* Initialize stack pointer */
init_sp (stack + sizeof (stack));
/* Initialize initialized data */
memcpy (&init_data, &data, &edata - &data);
/* Turn on the serial ports */
init_duart (&a);
init_duart (&b);
}
Use the section attribute with an initialized definition
of a global variable, as shown in the example. GNU CC issues
a warning and otherwise ignores the section attribute in
uninitialized variable declarations.
You may only use the section attribute with a fully initialized
global definition because of the way linkers work. The linker requires
each object be defined once, with the exception that uninitialized
variables tentatively go in the common (or bss) section
and can be multiply "defined". You can force a variable to be
initialized with the `-fno-common' flag or the nocommon
attribute.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
transparent_union
typedef for a union data type; then it
applies to all function parameters with that type.
unused
weak
weak attribute is described in See section Declaring Attributes of Functions.
model (model-name)
small, medium,
or large, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction).
Medium and large model objects may live anywhere in the 32 bit address space
(the compiler will generate seth/add3 instructions to load their
addresses).
To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.
The keyword __attribute__ allows you to specify special
attributes of struct and union types when you define such
types. This keyword is followed by an attribute specification inside
double parentheses. Three attributes are currently defined for types:
aligned, packed, and transparent_union. Other
attributes are defined for functions (see section Declaring Attributes of Functions) and
for variables (see section Specifying Attributes of Variables).
You may also specify any one of these attributes with `__'
preceding and following its keyword. This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name. For example, you may use __aligned__
instead of aligned.
You may specify the aligned and transparent_union
attributes either in a typedef declaration or just past the
closing curly brace of a complete enum, struct or union type
definition and the packed attribute only past the closing
brace of a definition.
You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.
aligned (alignment)
struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
force the compiler to insure (as far as it can) that each variable whose
type is struct S or more_aligned_int will be allocated and
aligned at least on a 8-byte boundary. On a Sparc, having all
variables of type struct S aligned to 8-byte boundaries allows
the compiler to use the ldd and std (doubleword load and
store) instructions when copying one variable of type struct S to
another, thus improving run-time efficiency.
Note that the alignment of any given struct or union type
is required by the ANSI C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the struct or union in question. This means that you can
effectively adjust the alignment of a struct or union
type by attaching an aligned attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire struct or union type.
As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given struct
or union type. Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for. For
example, you could write:
struct S { short f[3]; } __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables which have types that you have aligned
this way.
In the example above, if the size of each short is 2 bytes, then
the size of the entire struct S type is 6 bytes. The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire struct S type to 8
bytes.
Note that although you can ask the compiler to select a time-efficient
alignment for a given type and then declare only individual stand-alone
objects of that type, the compiler's ability to select a time-efficient
alignment is primarily useful only when you plan to create arrays of
variables having the relevant (efficiently aligned) type. If you
declare or use arrays of variables of an efficiently-aligned type, then
it is likely that your program will also be doing pointer arithmetic (or
subscripting, which amounts to the same thing) on pointers to the
relevant type, and the code that the compiler generates for these
pointer arithmetic operations will often be more efficient for
efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
packed
enum, struct, or
union type definition, specified that the minimum required memory
be used to represent the type.
Specifying this attribute for struct and union types is
equivalent to specifying the packed attribute on each of the
structure or union members. Specifying the `-fshort-enums'
flag on the line is equivalent to specifying the packed
attribute on all enum definitions.
You may only specify this attribute after a closing curly brace on an
enum definition, not in a typedef declaration, unless that
declaration also contains the definition of the enum.
transparent_union
union type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.
First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required. Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like const on
the referenced type must be respected, just as with normal pointer
conversions.
Second, the argument is passed to the function using the calling
conventions of first member of the transparent union, not the calling
conventions of the union itself. All members of the union must have the
same machine representation; this is necessary for this argument passing
to work properly.
Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons. For example, suppose the
wait function must accept either a value of type int * to
comply with Posix, or a value of type union wait * to comply with
the 4.1BSD interface. If wait's parameter were void *,
wait would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful. Instead, <sys/wait.h> might define the interface
as follows:
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
pid_t wait (wait_status_ptr_t);
This interface allows either int * or union wait *
arguments to be passed, using the int * calling convention.
The program can call wait with arguments of either type:
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
With this interface, wait's implementation might look like this:
pid_t wait (wait_status_ptr_t p)
{
return waitpid (-1, p.__ip, 0);
}
unused
union or a struct),
this attribute means that variables of that type are meant to appear
possibly unused. GNU CC will not produce a warning for any variables of
that type, even if the variable appears to do nothing. This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.
To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.
By declaring a function inline, you can direct GNU CC to
integrate that function's code into the code for its callers. This
makes execution faster by eliminating the function-call overhead; in
addition, if any of the actual argument values are constant, their known
values may permit simplifications at compile time so that not all of the
inline function's code needs to be included. The effect on code size is
less predictable; object code may be larger or smaller with function
inlining, depending on the particular case. Inlining of functions is an
optimization and it really "works" only in optimizing compilation. If
you don't use `-O', no function is really inline.
To declare a function inline, use the inline keyword in its
declaration, like this:
inline int
inc (int *a)
{
(*a)++;
}
(If you are writing a header file to be included in ANSI C programs, write
__inline__ instead of inline. See section Alternate Keywords.)
You can also make all "simple enough" functions inline with the option
`-finline-functions'.
Note that certain usages in a function definition can make it unsuitable
for inline substitution. Among these usages are: use of varargs, use of
alloca, use of variable sized data types (see section Arrays of Variable Length),
use of computed goto (see section Labels as Values), use of nonlocal goto,
and nested functions (see section Nested Functions). Using `-Winline'
will warn when a function marked inline could not be substituted,
and will give the reason for the failure.
Note that in C and Objective C, unlike C++, the inline keyword
does not affect the linkage of the function.
GNU CC automatically inlines member functions defined within the class
body of C++ programs even if they are not explicitly declared
inline. (You can override this with `-fno-default-inline';
see section Options Controlling C++ Dialect.)
When a function is both inline and static, if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GNU CC does not actually output assembler code for the
function, unless you specify the option `-fkeep-inline-functions'.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition). If there is a
nonintegrated call, then the function is compiled to assembler code as
usual. The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.
When an inline function is not static, then the compiler must assume
that there may be calls from other source files; since a global symbol can
be defined only once in any program, the function must not be defined in
the other source files, so the calls therein cannot be integrated.
Therefore, a non-static inline function is always compiled on its
own in the usual fashion.
If you specify both inline and extern in the function
definition, then the definition is used only for inlining. In no case
is the function compiled on its own, not even if you refer to its
address explicitly. Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.
This combination of inline and extern has almost the
effect of a macro. The way to use it is to put a function definition in
a header file with these keywords, and put another copy of the
definition (lacking inline and extern) in a library file.
The definition in the header file will cause most calls to the function
to be inlined. If any uses of the function remain, they will refer to
the single copy in the library.
GNU C does not inline any functions when not optimizing. It is not clear whether it is better to inline or not, in this case, but we found that a correct implementation when not optimizing was difficult. So we did the easy thing, and turned it off.
In an assembler instruction using asm, you can specify the
operands of the instruction using C expressions. This means you need not
guess which registers or memory locations will contain the data you want
to use.
You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.
For example, here is how to use the 68881's fsinx instruction:
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
Here angle is the C expression for the input operand while
result is that of the output operand. Each has `"f"' as its
operand constraint, saying that a floating point register is required.
The `=' in `=f' indicates that the operand is an output; all
output operands' constraints must use `='. The constraints use the
same language used in the machine description (see section Constraints for asm Operands).
Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is limited to ten or to the maximum number of operands in any instruction pattern in the machine description, whichever is greater.
If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.
Output operand expressions must be lvalues; the compiler can check this.
The input operands need not be lvalues. The compiler cannot check
whether the operands have data types that are reasonable for the
instruction being executed. It does not parse the assembler instruction
template and does not know what it means or even whether it is valid
assembler input. The extended asm feature is most often used for
machine instructions the compiler itself does not know exist. If
the output expression cannot be directly addressed (for example, it is a
bit field), your constraint must allow a register. In that case, GNU CC
will use the register as the output of the asm, and then store
that register into the output.
The ordinary output operands must be write-only; GNU CC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character `+' to indicate such an operand and list it with the output operands.
When the constraints for the read-write operand (or the operand in which
only some of the bits are to be changed) allows a register, you may, as
an alternative, logically split its function into two separate operands,
one input operand and one write-only output operand. The connection
between them is expressed by constraints which say they need to be in
the same location when the instruction executes. You can use the same C
expression for both operands, or different expressions. For example,
here we write the (fictitious) `combine' instruction with
bar as its read-only source operand and foo as its
read-write destination:
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A digit in constraint is allowed only in an input operand and it must refer to an output operand.
Only a digit in the constraint can guarantee that one operand will be in
the same place as another. The mere fact that foo is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code. The following would not
work reliably:
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
Various optimizations or reloading could cause operands 0 and 1 to be in
different registers; GNU CC knows no reason not to do so. For example, the
compiler might find a copy of the value of foo in one register and
use it for operand 1, but generate the output operand 0 in a different
register (copying it afterward to foo's own address). Of course,
since the register for operand 1 is not even mentioned in the assembler
code, the result will not work, but GNU CC can't tell that.
Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX:
asm volatile ("movc3 %0,%1,%2"
: /* no outputs */
: "g" (from), "g" (to), "g" (count)
: "r0", "r1", "r2", "r3", "r4", "r5");
It is an error for a clobber description to overlap an input or output
operand (for example, an operand describing a register class with one
member, mentioned in the clobber list). Most notably, it is invalid to
describe that an input operand is modified, but unused as output. It has
to be specified as an input and output operand anyway. Note that if there
are only unused output operands, you will then also need to specify
volatile for the asm construct, as described below.
If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some assemblers, the register names begin with `%'; to produce one `%' in the assembler code, you must write `%%' in the input.
If your assembler instruction can alter the condition code register, add `cc' to the list of clobbered registers. GNU CC on some machines represents the condition codes as a specific hardware register; `cc' serves to name this register. On other machines, the condition code is handled differently, and specifying `cc' has no effect. But it is valid no matter what the machine.
If your assembler instruction modifies memory in an unpredictable fashion, add `memory' to the list of clobbered registers. This will cause GNU CC to not keep memory values cached in registers across the assembler instruction.
You can put multiple assembler instructions together in a single
asm template, separated either with newlines (written as
`\n') or with semicolons if the assembler allows such semicolons.
The GNU assembler allows semicolons and most Unix assemblers seem to do
so. The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like. Here
is an example of multiple instructions in a template; it assumes the
subroutine _foo accepts arguments in registers 9 and 10:
asm ("movl %0,r9;movl %1,r10;call _foo"
: /* no outputs */
: "g" (from), "g" (to)
: "r9", "r10");
Unless an output operand has the `&' constraint modifier, GNU CC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. See section Constraint Modifier Characters.
If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the asm
construct, as follows:
asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:"
: "g" (result)
: "g" (input));
This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.
Speaking of labels, jumps from one asm to another are not
supported. The compiler's optimizers do not know about these jumps, and
therefore they cannot take account of them when deciding how to
optimize.
Usually the most convenient way to use these asm instructions is to
encapsulate them in macros that look like functions. For example,
#define sin(x) \
({ double __value, __arg = (x); \
asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
__value; })
Here the variable __arg is used to make sure that the instruction
operates on a proper double value, and to accept only those
arguments x which can convert automatically to a double.
Another way to make sure the instruction operates on the correct data
type is to use a cast in the asm. This is different from using a
variable __arg in that it converts more different types. For
example, if the desired type were int, casting the argument to
int would accept a pointer with no complaint, while assigning the
argument to an int variable named __arg would warn about
using a pointer unless the caller explicitly casts it.
If an asm has output operands, GNU CC assumes for optimization
purposes the instruction has no side effects except to change the output
operands. This does not mean instructions with a side effect cannot be
used, but you must be careful, because the compiler may eliminate them
if the output operands aren't used, or move them out of loops, or
replace two with one if they constitute a common subexpression. Also,
if your instruction does have a side effect on a variable that otherwise
appears not to change, the old value of the variable may be reused later
if it happens to be found in a register.
You can prevent an asm instruction from being deleted, moved
significantly, or combined, by writing the keyword volatile after
the asm. For example:
#define get_and_set_priority(new) \
({ int __old; \
asm volatile ("get_and_set_priority %0, %1": "=g" (__old) : "g" (new)); \
__old; })
If you write an asm instruction with no outputs, GNU CC will know
the instruction has side-effects and will not delete the instruction or
move it outside of loops. If the side-effects of your instruction are
not purely external, but will affect variables in your program in ways
other than reading the inputs and clobbering the specified registers or
memory, you should write the volatile keyword to prevent future
versions of GNU CC from moving the instruction around within a core
region.
An asm instruction without any operands or clobbers (and "old
style" asm) will not be deleted or moved significantly,
regardless, unless it is unreachable, the same wasy as if you had
written a volatile keyword.
Note that even a volatile asm instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions. You can't expect a sequence of volatile asm
instructions to remain perfectly consecutive. If you want consecutive
output, use a single asm.
It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following "store" instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary "test" and "compare" instructions because they don't have any output operands.
If you are writing a header file that should be includable in ANSI C
programs, write __asm__ instead of asm. See section Alternate Keywords.
There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs:
asm ("foo" : "=t" (a) : "f" (b));
This asm says that input B is not popped by the asm, and that
the asm pushes a result onto the reg-stack, ie, the stack is one
deeper after the asm than it was before. But, it is possible that
reload will think that it can use the same reg for both the input and
the output, if input B dies in this insn.
If any input operand uses the f constraint, all output reg
constraints must use the & earlyclobber.
The asm above would be written as
asm ("foo" : "=&t" (a) : "f" (b));
=f is not allowed: the operand
constraints must select a class with a single reg.
Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs.
asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
This asm takes two inputs, which are popped by the fyl2xp1 opcode,
and replaces them with one output. The user must code the st(1)
clobber for reg-stack.c to know that fyl2xp1 pops both inputs.
asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
asm Operands
Here are specific details on what constraint letters you can use with
asm operands.
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have. Constraints can also require two operands to match.
The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:
const_double) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).
const_double) is
allowed.
asm distinguishes. For example, an add instruction uses
two input operands and an output operand, but on most CISC
machines an add instruction really has only two operands, one of them an
input-output operand:
addl #35,r12Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.
address_operand
as the predicate in the match_operand. This predicate interprets
the mode specified in the match_operand as the mode of the memory
reference for which the address would be valid.
Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.
These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative.
If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the `?' and `!' characters:
?
!
Here are constraint modifier characters.
Whenever possible, you should use the general-purpose constraint letters
in asm arguments, since they will convey meaning more readily to
people reading your code. Failing that, use the constraint letters
that usually have very similar meanings across architectures. The most
commonly used constraints are `m' and `r' (for memory and
general-purpose registers respectively; see section Simple Constraints), and
`I', usually the letter indicating the most common
immediate-constant format.
For each machine architecture, the `config/machine.h' file
defines additional constraints. These constraints are used by the
compiler itself for instruction generation, as well as for asm
statements; therefore, some of the constraints are not particularly
interesting for asm. The constraints are defined through these
macros:
REG_CLASS_FROM_LETTER
CONST_OK_FOR_LETTER_P
CONST_DOUBLE_OK_FOR_LETTER_P
EXTRA_CONSTRAINT
Inspecting these macro definitions in the compiler source for your machine is the best way to be certain you have the right constraints. However, here is a summary of the machine-dependent constraints available on some particular machines.
f
F
G
I
J
K
L
M
Q
asm statements)
R
S
l
b
q
h
A
a
f
I
J
K
L
M
N
O
P
G
H
asm statements, use the machine
independent `E' or `F' instead)
b
f
h
q
c
l
x
y
z
I
J
K
L
M
N
O
P
G
Q
asm statements)
R
S
U
q
b, c, or d register
A
d register (for 64-bit ints)
f
t
u
a
b
c
d
D
S
I
J
K
L
M
lea instruction)
N
out instruction)
G
f
fp0 to fp3)
l
r0 to r15)
b
g0 to g15)
d
I
J
K
G
H
d
f
h
l
x
y
z
I
J
K
L
lui)
M
N
O
P
G
Q
asm statements)
R
asm statements)
S
asm statements)
a
d
f
x
y
I
J
K
L
M
G
H
f
e
I
J
K
sethi instruction)
G
H
Q
asm statements)
S
T
U
You can specify the name to be used in the assembler code for a C
function or variable by writing the asm (or __asm__)
keyword after the declarator as follows:
int foo asm ("myfoo") = 2;
This specifies that the name to be used for the variable foo in
the assembler code should be `myfoo' rather than the usual
`_foo'.
On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.
You cannot use asm in this way in a function definition; but
you can get the same effect by writing a declaration for the function
before its definition and putting asm there, like this:
extern func () asm ("FUNC");
func (x, y)
int x, y;
...
It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GNU CC does not as yet have the ability to store static variables in registers. Perhaps that will be added.
GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.
asm feature (see section Assembler Instructions with C Expression Operands), if you want to write one
output of the assembler instruction directly into a particular register.
(This will work provided the register you specify fits the constraints
specified for that operand in the asm.)
You can define a global register variable in GNU C like this:
register int *foo asm ("a5");
Here a5 is the name of the register which should be used. Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.
Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type. The register
a5 would be a good choice on a 68000 for a variable of pointer
type. On machines with register windows, be sure to choose a "global"
register that is not affected magically by the function call mechanism.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.
Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.
It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).
It is not safe for one function that uses a global register variable to
call another such function foo by way of a third function
lose that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared). This is
because lose might save the register and put some other value there.
For example, you can't expect a global register variable to be available in
the comparison-function that you pass to qsort, since qsort
might have put something else in that register. (If you are prepared to
recompile qsort with the same global register variable, you can
solve this problem.)
If you want to recompile qsort or other source files which do not
actually use your global register variable, so that they will not use that
register for any other purpose, then it suffices to specify the compiler
option `-ffixed-reg'. You need not actually add a global
register declaration to their source code.
A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.
On most machines, longjmp will restore to each global register
variable the value it had at the time of the setjmp. On some
machines, however, longjmp will not change the value of global
register variables. To be portable, the function that called setjmp
should make other arrangements to save the values of the global register
variables, and to restore them in a longjmp. This way, the same
thing will happen regardless of what longjmp does.
All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.
Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.
On the Sparc, there are reports that g3 ... g7 are suitable
registers, but certain library functions, such as getwd, as well
as the subroutines for division and remainder, modify g3 and g4. g1 and
g2 are local temporaries.
On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those.
You can define a local register variable with a specified register like this:
register int *foo asm ("a5");
Here a5 is the name of the register which should be used. Note
that this is the same syntax used for defining global register
variables, but for a local variable it would appear within a function.
Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see section Assembler Instructions with C Expression Operands). Both of these things generally require that you conditionalize your program according to cpu type.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass; excessive use of this feature leaves the compiler too few available registers to compile certain functions.
This option does not guarantee that GNU CC will generate code that has
this variable in the register you specify at all times. You may not
code an explicit reference to this register in an asm statement
and assume it will always refer to this variable.
Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.
The option `-traditional' disables certain keywords; `-ansi'
disables certain others. This causes trouble when you want to use GNU C
extensions, or ANSI C features, in a general-purpose header file that
should be usable by all programs, including ANSI C programs and traditional
ones. The keywords asm, typeof and inline cannot be
used since they won't work in a program compiled with `-ansi', while
the keywords const, volatile, signed, typeof
and inline won't work in a program compiled with
`-traditional'.
The way to solve these problems is to put `__' at the beginning and
end of each problematical keyword. For example, use __asm__
instead of asm, __const__ instead of const, and
__inline__ instead of inline.
Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:
#ifndef __GNUC__ #define __asm__ asm #endif
`-pedantic' causes warnings for many GNU C extensions. You can
prevent such warnings within one expression by writing
__extension__ before the expression. __extension__ has no
effect aside from this.
enum Types
You can define an enum tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
struct foo without describing the elements. A later declaration
which does specify the possible values completes the type.
You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.
This extension may not be very useful, but it makes the handling of
enum more consistent with the way struct and union
are handled.
This extension is not supported by GNU C++.
GNU CC predefines two string variables to be the name of the current function.
The variable __FUNCTION__ is the name of the function as it appears
in the source. The variable __PRETTY_FUNCTION__ is the name of
the function pretty printed in a language specific fashion.
These names are always the same in a C function, but in a C++ function they may be different. For example, this program:
extern "C" {
extern int printf (char *, ...);
}
class a {
public:
sub (int i)
{
printf ("__FUNCTION__ = %s\n", __FUNCTION__);
printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
}
};
int
main (void)
{
a ax;
ax.sub (0);
return 0;
}
gives this output:
__FUNCTION__ = sub __PRETTY_FUNCTION__ = int a::sub (int)
These names are not macros: they are predefined string variables.
For example, `#ifdef __FUNCTION__' does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier __FUNCTION__.
These functions may be used to get information about the callers of a function.
__builtin_return_address (level)
0 yields the return address
of the current function, a value of 1 yields the return address
of the caller of the current function, and so forth.
The level argument must be a constant integer.
On some machines it may be impossible to determine the return address of
any function other than the current one; in such cases, or when the top
of the stack has been reached, this function will return 0.
This function should only be used with a non-zero argument for debugging
purposes.
__builtin_frame_address (level)
__builtin_return_address, but it
returns the address of the function frame rather than the return address
of the function. Calling __builtin_frame_address with a value of
0 yields the frame address of the current function, a value of
1 yields the frame address of the caller of the current function,
and so forth.
The frame is the area on the stack which holds local variables and saved
registers. The frame address is normally the address of the first word
pushed on to the stack by the function. However, the exact definition
depends upon the processor and the calling convention. If the processor
has a dedicated frame pointer register, and the function has a frame,
then __builtin_frame_address will return the value of the frame
pointer register.
The caveats that apply to __builtin_return_address apply to this
function as well.
GNU CC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.
The remaining functions are provided for optimization purposes.
GNU CC includes builtin versions of many of the functions in the
standard C library. These will always be treated as having the same
meaning as the C library function even if you specify the
`-fno-builtin' (see section Options Controlling C Dialect) option. These functions
correspond to the C library functions alloca, ffs,
abs, fabsf, fabs, fabsl, labs,
memcpy, memcmp, strcmp, strcpy,
strlen, sqrtf, sqrt, sqrtl, sinf,
sin, sinl, cosf, cos, and cosl.
You can use the builtin function __builtin_constant_p to
determine if a value is known to be constant at compile-time and hence
that GNU CC can perform constant-folding on expressions involving that
value. The argument of the function is the value to test. The function
returns the integer 1 if the argument is known to be a compile-time
constant and 0 if it is not known to be a compile-time constant. A
return of 0 does not indicate that the value is not a constant,
but merely that GNU CC cannot prove it is a constant with the specified
value of the `-O' option.
You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:
#define Scale_Value(X) \ (__builtin_constant_p (X) ? ((X) * SCALE + OFFSET) : Scale (X))
You may use this builtin function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the builtin, GNU CC will never return 1 when you call the inline function with a string constant or constructor expression (see section Constructor Expressions) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the `-O' option.
In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superceded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.
While the list below is not exhaustive, it documents some of the options that are now deprecated:
-fthis-is-variable
-fexternal-templates
-falt-external-templates
The GNU compiler provides these extensions to the C++ language (and you
can also use most of the C language extensions in your C++ programs). If you
want to write code that checks whether these features are available, you can
test for the GNU compiler the same way as for C programs: check for a
predefined macro __GNUC__. You can also use __GNUG__ to
test specifically for GNU C++ (see section `Standard Predefined Macros' in The C Preprocessor).
GNU C++ extends the function-definition syntax to allow you to specify a name for the result of a function outside the body of the definition, in C++ programs:
type
functionname (args) return resultname;
{
...
body
...
}
You can use this feature to avoid an extra constructor call when
a function result has a class type. For example, consider a function
m, declared as `X v = m ();', whose result is of class
X:
X
m ()
{
X b;
b.a = 23;
return b;
}
Although m appears to have no arguments, in fact it has one implicit
argument: the address of the return value. At invocation, the address
of enough space to hold v is sent in as the implicit argument.
Then b is constructed and its a field is set to the value
23. Finally, a copy constructor (a constructor of the form `X(X&)')
is applied to b, with the (implicit) return value location as the
target, so that v is now bound to the return value.
But this is wasteful. The local b is declared just to hold
something that will be copied right out. While a compiler that
combined an "elision" algorithm with interprocedural data flow
analysis could conceivably eliminate all of this, it is much more
practical to allow you to assist the compiler in generating
efficient code by manipulating the return value explicitly,
thus avoiding the local variable and copy constructor altogether.
Using the extended GNU C++ function-definition syntax, you can avoid the
temporary allocation and copying by naming r as your return value
at the outset, and assigning to its a field directly:
X
m () return r;
{
r.a = 23;
}
The declaration of r is a standard, proper declaration, whose effects
are executed before any of the body of m.
Functions of this type impose no additional restrictions; in particular,
you can execute return statements, or return implicitly by
reaching the end of the function body ("falling off the edge").
Cases like
X
m () return r (23);
{
return;
}
(or even `X m () return r (23); { }') are unambiguous, since
the return value r has been initialized in either case. The
following code may be hard to read, but also works predictably:
X
m () return r;
{
X b;
return b;
}
The return value slot denoted by r is initialized at the outset,
but the statement `return b;' overrides this value. The compiler
deals with this by destroying r (calling the destructor if there
is one, or doing nothing if there is not), and then reinitializing
r with b.
This extension is provided primarily to help people who use overloaded operators, where there is a great need to control not just the arguments, but the return values of functions. For classes where the copy constructor incurs a heavy performance penalty (especially in the common case where there is a quick default constructor), this is a major savings. The disadvantage of this extension is that you do not control when the default constructor for the return value is called: it is always called at the beginning.
It is very convenient to have operators which return the "minimum" or the "maximum" of two arguments. In GNU C++ (but not in GNU C),
a <? b
a >? b
These operations are not primitive in ordinary C++, since you can use a macro to return the minimum of two things in C++, as in the following example.
#define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
You might then use `int min = MIN (i, j);' to set min to the minimum value of variables i and j.
However, side effects in X or Y may cause unintended
behavior. For example, MIN (i++, j++) will fail, incrementing
the smaller counter twice. A GNU C extension allows you to write safe
macros that avoid this kind of problem (see section Naming an Expression's Type). However, writing MIN and MAX as
macros also forces you to use function-call notation for a
fundamental arithmetic operation. Using GNU C++ extensions, you can
write `int min = i <? j;' instead.
Since <? and >? are built into the compiler, they properly
handle expressions with side-effects; `int min = i++ <? j++;'
works correctly.
goto and Destructors in GNU C++
In C++ programs, you can safely use the goto statement. When you
use it to exit a block which contains aggregates requiring destructors,
the destructors will run before the goto transfers control.
The compiler still forbids using goto to enter a scope
that requires constructors.
C++ object definitions can be quite complex. In principle, your source code will need two kinds of things for each object that you use across more than one source file. First, you need an interface specification, describing its structure with type declarations and function prototypes. Second, you need the implementation itself. It can be tedious to maintain a separate interface description in a header file, in parallel to the actual implementation. It is also dangerous, since separate interface and implementation definitions may not remain parallel.
With GNU C++, you can use a single header file for both purposes.
Warning: The mechanism to specify this is in transition. For the nonce, you must use one of two
#pragmacommands; in a future release of GNU C++, an alternative mechanism will make these#pragmacommands unnecessary.
The header file contains the full definitions, but is marked with
`#pragma interface' in the source code. This allows the compiler
to use the header file only as an interface specification when ordinary
source files incorporate it with #include. In the single source
file where the full implementation belongs, you can use either a naming
convention or `#pragma implementation' to indicate this alternate
use of the header file.
#pragma interface
#pragma interface "subdir/objects.h"
#pragma implementation
#pragma implementation "objects.h"
`#pragma implementation' and `#pragma interface' also have an effect on function inlining.
If you define a class in a header file marked with `#pragma
interface', the effect on a function defined in that class is similar to
an explicit extern declaration--the compiler emits no code at
all to define an independent version of the function. Its definition
is used only for inlining with its callers.
Conversely, when you include the same header file in a main source file that declares it as `#pragma implementation', the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with `-fno-implement-inlines'. If any calls were not inlined, you will get linker errors.
C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which I will refer to as the Borland model and the Cfront model.
When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the Borland model. On other systems, g++ implements neither automatic model.
A future version of g++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations. The linker will then combine duplicate instantiations.
In the mean time, you have the following options for dealing with template instantiations:
#include <tmethods.cc> to the end of each template header.
For library code, if you want the library to provide all of the template
instantiations it needs, just try to link all of its object files
together; the link will fail, but cause the instantiations to be
generated as a side effect. Be warned, however, that this may cause
conflicts if multiple libraries try to provide the same instantiations.
For greater control, use explicit instantiation as described in the next
option.
#include "Foo.h"
#include "Foo.cc"
template class Foo<int>;
template ostream& operator <<
(ostream&, const Foo<int>&);
for each of the instances you need, and create a template instantiation
library from those.
If you are using Cfront-model code, you can probably get away with not
using `-fno-implicit-templates' when compiling files that don't
`#include' the member template definitions.
If you use one big file to do the instantiations, you may want to
compile it without `-fno-implicit-templates' so you get all of the
instances required by your explicit instantiations (but not by any
other files) without having to specify them as well.
g++ has extended the template instantiation syntax outlined in the
Working Paper to allow forward declaration of explicit instantiations
and instantiation of the compiler support data for a template class
(i.e. the vtable) without instantiating any of its members:
extern template int max (int, int); inline template class Foo<int>;
template class A<int>; template ostream& operator << (ostream&, const A<int>&);This strategy will work with code written for either model. If you are using code written for the Cfront model, the file containing a class template and the file containing its member templates should be implemented in the same translation unit. A slight variation on this approach is to instead use the flag `-falt-external-templates'; this flag causes template instances to be emitted in the translation unit that implements the header where they are first instantiated, rather than the one which implements the file where the templates are defined. This header must be the same in all translation units, or things are likely to break. See section Declarations and Definitions in One Header, for more discussion of these pragmas.
In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the `this' pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time.
Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls.
The syntax for this extension is
extern A a; extern int (A::*fp)(); typedef int (*fptr)(A *); fptr p = (fptr)(a.*fp);
You must specify `-Wno-pmf-conversions' to use this extension.
In GNU C++, you can use the keyword signature to define a
completely abstract class interface as a datatype. You can connect this
abstraction with actual classes using signature pointers. If you want
to use signatures, run the GNU compiler with the
`-fhandle-signatures' command-line option. (With this option, the
compiler reserves a second keyword sigof as well, for a future
extension.)
Roughly, signatures are type abstractions or interfaces of classes.
Some other languages have similar facilities. C++ signatures are
related to ML's signatures, Haskell's type classes, definition modules
in Modula-2, interface modules in Modula-3, abstract types in Emerald,
type modules in Trellis/Owl, categories in Scratchpad II, and types in
POOL-I. For a more detailed discussion of signatures, see
Signatures: A Language Extension for Improving Type Abstraction and
Subtype Polymorphism in C++
by Gerald Baumgartner and Vincent F. Russo (Tech report
CSD--TR--95--051, Dept. of Computer Sciences, Purdue University,
August 1995, a slightly improved version appeared in
Software--Practice & Experience, 25(8), pp. 863--889,
August 1995). You can get the tech report by anonymous FTP from
ftp.cs.purdue.edu in `pub/gb/Signature-design.ps.gz'.
Syntactically, a signature declaration is a collection of
member function declarations and nested type declarations.
For example, this signature declaration defines a new abstract type
S with member functions `int foo ()' and `int bar (int)':
signature S
{
int foo ();
int bar (int);
};
Since signature types do not include implementation definitions, you cannot write an instance of a signature directly. Instead, you can define a pointer to any class that contains the required interfaces as a signature pointer. Such a class implements the signature type.
To use a class as an implementation of S, you must ensure that
the class has public member functions `int foo ()' and `int
bar (int)'. The class can have other member functions as well, public
or not; as long as it offers what's declared in the signature, it is
suitable as an implementation of that signature type.
For example, suppose that C is a class that meets the
requirements of signature S (C conforms to
S). Then
C obj; S * p = &obj;
defines a signature pointer p and initializes it to point to an
object of type C.
The member function call `int i = p->foo ();'
executes `obj.foo ()'.
Abstract virtual classes provide somewhat similar facilities in standard C++. There are two main advantages to using signatures instead:
T is a subtype of a signature type S independent of
any inheritance hierarchy as long as all the member functions declared
in S are also found in T. So you can define a subtype
hierarchy that is completely independent from any inheritance
(implementation) hierarchy, instead of being forced to use types that
mirror the class inheritance hierarchy.
There is one more detail about signatures. A signature declaration can
contain member function definitions as well as member function
declarations. A signature member function with a full definition is
called a default implementation; classes need not contain that
particular interface in order to conform. For example, a
class C can conform to the signature
signature T
{
int f (int);
int f0 () { return f (0); };
};
whether or not C implements the member function `int f0 ()'.
If you define C::f0, that definition takes precedence;
otherwise, the default implementation S::f0 applies.
gcov: a Test Coverage Program
gcov is a tool you can use in conjunction with GNU CC to
test code coverage in your programs.
This chapter describes version 1.5 of gcov.
gcov
gcov is a test coverage program. Use it in concert with GNU
CC to analyze your programs to help create more efficient, faster
running code. You can use gcov as a profiling tool to help
discover where your optimization efforts will best affect your code. You
can also use gcov along with the other profiling tool,
gprof, to assess which parts of your code use the greatest amount
of computing time.
Profiling tools help you analyze your code's performance. Using a
profiler such as gcov or gprof, you can find out some
basic performance statistics, such as:
Once you know these things about how your code works when compiled, you
can look at each module to see which modules should be optimized.
gcov helps you determine where to work on optimization.
Software developers also use coverage testing in concert with testsuites, to make sure software is actually good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product.
You should compile your code without optimization if you plan to use
gcov because the optimization, by combining some lines of code
into one function, may not give you as much information as you need to
look for `hot spots' where the code is using a great deal of computer
time. Likewise, because gcov accumulates statistics by line (at
the lowest resolution), it works best with a programming style that
places only one statement on each line. If you use complicated macros
that expand to loops or to other control structures, the statistics are
less helpful--they only report on the line where the macro call
appears. If your complex macros behave like functions, you can replace
them with inline functions to solve this problem.
gcov creates a logfile called `sourcefile.gcov' which
indicates how many times each line of a source file `sourcefile.c'
has executed. You can use these logfiles along with gprof to aid
in fine-tuning the performance of your programs. gprof gives
timing information you can use along with the information you get from
gcov.
gcov works only on code compiled with GNU CC. It is not
compatible with any other profiling or test coverage mechanism.
gcov [-b] [-v] [-n] [-l] [-f] [-o directory] sourcefile
-b
-v
gcov version number (on the standard error stream).
-n
gcov output file.
-l
gcov on the file `a.c' will produce
an output file called `a.c.x.h.gcov' instead of `x.h.gcov'.
This can be useful if `x.h' is included in multiple source files.
-f
-o
.bb,
.bbg, and .da files in this directory.
When using gcov, you must first compile your program with two
special GNU CC options: `-fprofile-arcs -ftest-coverage'.
This tells the compiler to generate additional information needed by
gcov (basically a flow graph of the program) and also includes
additional code in the object files for generating the extra profiling
information needed by gcov. These additional files are placed in the
directory where the source code is located.
Running the program will cause profile output to be generated. For each
source file compiled with -fprofile-arcs, an accompanying .da
file will be placed in the source directory.
Running gcov with your program's source file names as arguments
will now produce a listing of the code along with frequency of execution
for each line. For example, if your program is called `tmp.c', this
is what you see when you use the basic gcov facility:
$ gcc -fprofile-arcs -ftest-coverage tmp.c $ a.out $ gcov tmp.c 87.50% of 8 source lines executed in file tmp.c Creating tmp.c.gcov.
The file `tmp.c.gcov' contains output from gcov.
Here is a sample:
main()
{
1 int i, total;
1 total = 0;
11 for (i = 0; i < 10; i++)
10 total += i;
1 if (total != 45)
###### printf ("Failure\n");
else
1 printf ("Success\n");
1 }
When you use the `-b' option, your output looks like this:
$ gcov -b tmp.c 87.50% of 8 source lines executed in file tmp.c 80.00% of 5 branches executed in file tmp.c 80.00% of 5 branches taken at least once in file tmp.c 50.00% of 2 calls executed in file tmp.c Creating tmp.c.gcov.
Here is a sample of a resulting `tmp.c.gcov' file:
main()
{
1 int i, total;
1 total = 0;
11 for (i = 0; i < 10; i++)
branch 0 taken = 91%
branch 1 taken = 100%
branch 2 taken = 100%
10 total += i;
1 if (total != 45)
branch 0 taken = 100%
###### printf ("Failure\n");
call 0 never executed
branch 1 never executed
else
1 printf ("Success\n");
call 0 returns = 100%
1 }
For each basic block, a line is printed after the last line of the basic block describing the branch or call that ends the basic block. There can be multiple branches and calls listed for a single source line if there are multiple basic blocks that end on that line. In this case, the branches and calls are each given a number. There is no simple way to map these branches and calls back to source constructs. In general, though, the lowest numbered branch or call will correspond to the leftmost construct on the source line.
For a branch, if it was executed at least once, then a percentage indicating the number of times the branch was taken divided by the number of times the branch was executed will be printed. Otherwise, the message "never executed" is printed.
For a call, if it was executed at least once, then a percentage
indicating the number of times the call returned divided by the number
of times the call was executed will be printed. This will usually be
100%, but may be less for functions call exit or longjmp,
and thus may not return everytime they are called.
The execution counts are cumulative. If the example program were
executed again without removing the .da file, the count for the
number of times each line in the source was executed would be added to
the results of the previous run(s). This is potentially useful in
several ways. For example, it could be used to accumulate data over a
number of program runs as part of a test verification suite, or to
provide more accurate long-term information over a large number of
program runs.
The data in the .da files is saved immediately before the program
exits. For each source file compiled with -fprofile-arcs, the profiling
code first attempts to read in an existing .da file; if the file
doesn't match the executable (differing number of basic block counts) it
will ignore the contents of the file. It then adds in the new execution
counts and finally writes the data to the file.
gcov with GCC Optimization
If you plan to use gcov to help optimize your code, you must
first compile your program with two special GNU CC options:
`-fprofile-arcs -ftest-coverage'. Aside from that, you can use any
other GNU CC options; but if you want to prove that every single line
in your program was executed, you should not compile with optimization
at the same time. On some machines the optimizer can eliminate some
simple code lines by combining them with other lines. For example, code
like this:
if (a != b) c = 1; else c = 0;
can be compiled into one instruction on some machines. In this case,
there is no way for gcov to calculate separate execution counts
for each line because there isn't separate code for each line. Hence
the gcov output looks like this if you compiled the program with
optimization:
100 if (a != b)
100 c = 1;
100 else
100 c = 0;
The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1.
gcov data files
gcov uses three files for doing profiling. The names of these
files are derived from the original source file by substituting
the file suffix with either .bb, .bbg, or .da. All
of these files are placed in the same directory as the source file, and
contain data stored in a platform-independent method.
The .bb and .bbg files are generated when the source file
is compiled with the GNU CC `-ftest-coverage' option. The
.bb file contains a list of source files (including headers),
functions within those files, and line numbers corresponding to each
basic block in the source file.
The .bb file format consists of several lists of 4-byte integers
which correspond to the line numbers of each basic block in the
file. Each list is terminated by a line number of 0. A line number of -1
is used to designate that the source file name (padded to a 4-byte
boundary and followed by another -1) follows. In addition, a line number
of -2 is used to designate that the name of a function (also padded to a
4-byte boundary and followed by a -2) follows.
The .bbg file is used to reconstruct the program flow graph for
the source file. It contains a list of the program flow arcs (possible
branches taken from one basic block to another) for each function which,
in combination with the .bb file, enables gcov to reconstruct the
program flow.
In the .bbg file, the format is:
number of basic blocks for function #0 (4-byte number)
total number of arcs for function #0 (4-byte number)
count of arcs in basic block #0 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
destination basic block of arc #1 (4-byte number)
flag bits (4-byte number)
...
destination basic block of arc #N (4-byte number)
flag bits (4-byte number)
count of arcs in basic block #1 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
...
A -1 (stored as a 4-byte number) is used to separate each function's list of basic blocks, and to verify that the file has been read correctly.
The .da file is generated when a program containing object files
built with the GNU CC `-fprofile-arcs' option is executed. A
separate .da file is created for each source file compiled with
this option, and the name of the .da file is stored as an
absolute pathname in the resulting object file. This path name is
derived from the source file name by substituting a .da suffix.
The format of the .da file is fairly simple. The first 8-byte
number is the number of counts in the file, followed by the counts
(stored as 8-byte numbers). Each count corresponds to the number of
times each arc in the program is executed. The counts are cumulative;
each time the program is executed, it attemps to combine the existing
.da files with the new counts for this invocation of the
program. It ignores the contents of any .da files whose number of
arcs doesn't correspond to the current program, and merely overwrites
them instead.
All three of these files use the functions in gcov-io.h to store
integers; the functions in this header provide a machine-independent
mechanism for storing and retrieving data from a stream.
This section describes known problems that affect users of GCC. Most of these are not GCC bugs per se--if they were, we would fix them. But the result for a user may be like the result of a bug.
Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people's opinions differ as to what is best.
fixincludes script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while fixincludes is running. This would seem to be a
bug in the automounter. We don't know any good way to work around it.
fixproto script will sometimes add prototypes for the
sigsetjmp and siglongjmp functions that reference the
jmp_buf type before that type is defined. To work around this,
edit the offending file and place the typedef in front of the
prototypes.
This is a list of problems (and some apparent problems which don't really mean anything is wrong) that show up during installation of GNU CC.
CC can interfere with the functioning of make.
fixincludes if the
System V file system doesn't support symbolic links. These problems
result in a failure to fix the declaration of size_t in
`sys/types.h'. If you find that size_t is a signed type and
that type mismatches occur, this could be the cause.
The solution is not to use such a directory for building GCC.
gcc driver program looked for
as and ld in various places; for example, in files
beginning with `/usr/local/lib/gcc-'. GCC version 2 looks for
them in the directory
`/usr/local/lib/gcc-lib/target/version'.
Thus, to use a version of as or ld that is not the system
default, for example gas or GNU ld, you must put them in
that directory (or make links to them from that directory).
make. These failures, which
are often due to files that were not found, are expected, and can safely
be ignored.
make recompiles parts of the compiler when installing
the compiler. In one case, this was traced down to a bug in
make. Either ignore the problem or switch to GNU Make.
enquire, which is part of building
GCC. The fix is to get rid of the file real-ld which purify
installs--so that GCC won't try to use it.
__GNU_LIBRARY__ conditional
around line 31 to `#if 1'.
enquire hangs due to a hardware problem in the motherboard--it
reports floating point exceptions to the kernel incorrectly. You can
install GCC except for `float.h' by patching out the command to
run enquire. You may also be able to fix the problem for real by
getting a replacement motherboard. This problem was observed in
Revision E of the Micronics motherboard, and is fixed in Revision F.
It has also been observed in the MYLEX MXA-33 motherboard.
If you encounter this problem, you may also want to consider removing
the FPU from the socket during the compilation. Alternatively, if you
are running SCO Unix, you can reboot and force the FPU to be ignored.
To do this, type `hd(40)unix auto ignorefpu'.
ln /etc/emulator.rel1 /etc/emulatorand then reboot the system. (The default emulator file remains present under the name `emulator.dflt'.) Try using `/etc/emulator.att', if you have such a problem on the SCO system. Another system which has this problem is Esix. We don't know whether it has an alternate emulator that works. On NetBSD 0.8, a similar problem manifests itself as these error messages:
enquire.c: In function `fprop': enquire.c:2328: floating overflow
genflags or genoutput while building GCC. This is said to
be due to a bug in sh. You can probably get around it by running
genflags or genoutput manually and then retrying the
make.
pkginfo command. To add an optional package, use the
pkgadd command. For further details, see the Solaris
documentation.
For Solaris 2.0 and 2.1, GCC needs six packages: `SUNWarc',
`SUNWbtool', `SUNWesu', `SUNWhea', `SUNWlibm', and
`SUNWtoo'.
For Solaris 2.2, GCC needs an additional seventh package: `SUNWsprot'.
PATH.
add.d.
fixincludes. This causes
problems in building GCC. Once GCC is installed, the problems go
away.
To work around this problem, when making the stage 1 compiler, specify
this option to Make:
GCC_FOR_TARGET="./xgcc -B./ -I./include"When making stage 2 and stage 3, specify this option:
CFLAGS="-g -I./include"
alloca against shared
libraries on RISC-OS 5.0, and DEC's OSF/1 systems. This is a bug
in the linker, that is supposed to be fixed in future revisions.
To protect against this, GCC passes `-non_shared' to the
linker unless you pass an explicit `-shared' or
`-call_shared' switch.
ld fatal: failed to write symbol name something in strings table for file whateverThis probably indicates that the disk is full or your ULIMIT won't allow the file to be as large as it needs to be. This problem can also result because the kernel parameter
MAXUMEM
is too small. If so, you must regenerate the kernel and make the value
much larger. The default value is reported to be 1024; a value of 32768
is said to work. Smaller values may also work.
/usr/local/lib/bison.simple: In function `yyparse': /usr/local/lib/bison.simple:625: virtual memory exhaustedthat too indicates a problem with disk space, ULIMIT, or
MAXUMEM.
MAXUMEM = 4096
_floatdisf cc1: warning: `-g' option not supported on this version of GCC cc1: warning: `-g1' option not supported on this version of GCC ./xgcc: Internal compiler error: program as got fatal signal 11A patched version of the assembler is available by anonymous ftp from
altdorf.ai.mit.edu as the file
`archive/cph/hpux-8.0-assembler'. If you have HP software support,
the patch can also be obtained directly from HP, as described in the
following note:
This patch is also known as PHCO_4484.This is the patched assembler, to patch SR#1653-010439, where the assembler aborts on floating point constants.
The bug is not really in the assembler, but in the shared library version of the function "cvtnum(3c)". The bug on "cvtnum(3c)" is SR#4701-078451. Anyway, the attached assembler uses the archive library version of "cvtnum(3c)" and thus does not exhibit the bug.
fixproto shell script triggers a bug in the system shell.
If you encounter this problem, upgrade your operating system or
use BASH (the GNU shell) to run fixproto.
muldi3 in file `libgcc2.c'.
You may be able to succeed by getting GCC version 1, installing it,
and using it to compile GCC version 2. The bug in the Pyramid C
compiler does not seem to affect GCC version 1.
va_arg when you build GCC.
If this happens, then you need to link most programs with the library
`iclib.a'. You must also modify `stdio.h' as follows: before
the lines
#if defined(__i860__) && !defined(_VA_LIST) #include <va_list.h>insert the line
#if __PGC__and after the lines
extern int vprintf(const char *, va_list ); extern int vsprintf(char *, const char *, va_list ); #endifinsert the line
#endif /* __PGC__ */These problems don't exist in operating system version 1.1.
./fixproto: sh internal 1K buffer overflowTo fix this, change the first line of the fixproto script to look like:
#!/bin/ksh
You may run into problems with cross compilation on certain machines, for several reasons.
REAL_VALUE_TYPE. But doing so is a substantial amount of
work for each target machine.
See section `Cross Compilation and Floating Point Format' in Using and Porting GCC.
This section lists various difficulties encountered in using GNU C or GNU C++ together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems.
fixincludes. The corrected header files go in a new
directory; GCC searches this directory before `/usr/include'.
If you use `-I/usr/include', this tells GCC to search
`/usr/include' earlier on, before the corrected headers. The
result is that you get the uncorrected header files.
Instead, you should use these options (when compiling C programs):
-I/usr/local/lib/gcc-lib/target/version/include -I/usr/includeFor C++ programs, GCC also uses a special directory that defines C++ interfaces to standard C subroutines. This directory is meant to be searched before other standard include directories, so that it takes precedence. If you are compiling C++ programs and specifying include directories explicitly, use this option first, then the two options above:
-I/usr/local/lib/g++-include
double on an 8-byte
boundary, and it expects every double to be so aligned. The Sun
compiler usually gives double values 8-byte alignment, with one
exception: function arguments of type double may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an
argument of type double and passes this pointer of type
double * to a function compiled with GCC, dereferencing the
pointer may cause a fatal signal.
One way to solve this problem is to compile your entire program with GNU
CC. Another solution is to modify the function that is compiled with
Sun CC to copy the argument into a local variable; local variables
are always properly aligned. A third solution is to modify the function
that uses the pointer to dereference it via the following function
access_double instead of directly with `*':
inline double
access_double (double *unaligned_ptr)
{
union d2i { double d; int i[2]; };
union d2i *p = (union d2i *) unaligned_ptr;
union d2i u;
u.i[0] = p->i[0];
u.i[1] = p->i[1];
return u.d;
}
Storing into the pointer can be done likewise with the same union.
malloc function in the `libmalloc.a' library
may allocate memory that is only 4 byte aligned. Since GCC on the
Sparc assumes that doubles are 8 byte aligned, this may result in a
fatal signal if doubles are stored in memory allocated by the
`libmalloc.a' library.
The solution is to not use the `libmalloc.a' library. Use instead
malloc and related functions from `libc.a'; they do not have
this problem.
_dlclose, _dlsym or _dlopen
when linking, compile and link against the file
`mit/util/misc/dlsym.c' from the MIT version of X windows.
cc does not
compile GCC correctly. We do not yet know why. However, GCC
compiled on earlier HP-UX versions works properly on HP-UX 9.01 and can
compile itself properly on 9.01.
alloca or variable-size arrays. This is because GCC doesn't
generate HP-UX unwind descriptors for such functions. It may even be
impossible to generate them.
(warning) Use of GR3 when frame >= 8192 may cause conflict.These warnings are harmless and can be safely ignored.
as -u < /dev/nullIf the command exits normally, the assembler fix already is installed. If the assembler complains that "-u" is an unknown flag, you need to order the fix.
extern int foo; ... foo ... static int foo;will cause the linker to report an undefined symbol
foo.
Although this behavior differs from most other systems, it is not a
bug because redefining an extern variable as static
is undefined in ANSI C.
size_t. You should change `sys/types.h' by adding these
lines around the definition of size_t:
#ifndef _SIZE_T #define _SIZE_T actual typedef here #endif
-fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5
-L/usr/local/lib/gcc-lib/we32k-att-sysv/2.8.1 -lgcc -lc_sThe first specifies where to find the library `libgcc.a' specified with the `-lgcc' option. GCC does linking by invoking
ld, just as cc does, and
there is no reason why it should matter which compilation program
you use to invoke ld. If someone tracks this problem down,
it can probably be fixed easily.
ecvt, fcvt and gcvt. Given valid
floating point numbers, they sometimes print `NaN'.
Certain programs have problems compiling.
#ifdef __STDC__ #define NeedFunctionPrototypes 0 #endif
-traditional -Dvolatile=__volatile__ -I/usr/include/sun -I/usr/ucbinclude -fpcc-struct-returnmost of which are unnecessary with GCC 2.4.5 and newer versions. You can make a properly working Perl by setting
ccflags to
`-fwritable-strings' (implied by the `-traditional' in the
original options) and cppflags to empty in `config.sh', then
typing `./doSH; make depend; make'.
MALLOC=/usr/local/lib/libgmalloc.aAlternatively, if you have compiled `gmalloc.c' from Emacs 19, copy the object file to `gmalloc.o' and use this option when you relink GCC:
MALLOC=gmalloc.o
There are several noteworthy incompatibilities between GNU C and most existing (non-ANSI) versions of C. The `-traditional' option eliminates many of these incompatibilities, but not all, by telling GNU C to behave like the other C compilers.
mktemp with a string
constant argument. The function mktemp always alters the
string its argument points to.
Another consequence is that sscanf does not work on some systems
when passed a string constant as its format control string or input.
This is because sscanf incorrectly tries to write into the string
constant. Likewise fscanf and scanf.
The best solution to these problems is to change the program to use
char-array variables with initialization strings for these
purposes instead of string constants. But if this is not possible,
you can use the `-fwritable-strings' flag, which directs GCC
to handle string constants the same way most C compilers do.
`-traditional' also has this effect, among others.
-2147483648 is positive.
This is because 2147483648 cannot fit in the type int, so
(following the ANSI C rules) its data type is unsigned long int.
Negating this value yields 2147483648 again.
#define foo(a) "a"will produce output
"a" regardless of what the argument a is.
The `-traditional' option directs GCC to handle such cases
(among others) in the old-fashioned (non-ANSI) fashion.
setjmp and longjmp, the only automatic
variables guaranteed to remain valid are those declared
volatile. This is a consequence of automatic register
allocation. Consider this function:
jmp_buf j;
foo ()
{
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* longjmp (j) may occur in fun3. */
return a + fun3 ();
}
Here a may or may not be restored to its first value when the
longjmp occurs. If a is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.
If you use the `-W' option with the `-O' option, you will
get a warning when GCC thinks such a problem might be possible.
The `-traditional' option directs GNU C to put variables in
the stack by default, rather than in registers, in functions that
call setjmp. This results in the behavior found in
traditional C compilers.
foobar (
#define luser
hack)
ANSI C does not permit such a construct. It would make sense to support
it when `-traditional' is used, but it is too much work to
implement.
extern declaration affects all the
rest of the file even if it happens within a block.
The `-traditional' option directs GNU C to treat all extern
declarations as global, like traditional compilers.
long, etc., with a typedef name,
as shown here:
typedef int foo; typedef long foo bar;In ANSI C, this is not allowed:
long and other type modifiers
require an explicit int. Because this criterion is expressed
by Bison grammar rules rather than C code, the `-traditional'
flag cannot alter it.
#if 0 You can't expect this to work. #endifThe best solution to such a problem is to put the text into an actual C comment delimited by `/*...*/'. However, `-traditional' suppresses these error messages.
time, so it did not matter what type your program declared it to
return. But in systems with ANSI C headers, time is declared to
return time_t, and if that is not the same as long, then
`long time ();' is erroneous.
The solution is to change your program to use time_t as the return
type of time.
float, PCC converts it to
a double. GCC actually returns a float. If you are concerned
with PCC compatibility, you should declare your functions to return
double; you might as well say what you mean.
STRUCT_VALUE and
STRUCT_INCOMING_VALUE tell GCC where to pass this address.
By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted. GCC does not use this method because it is
slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all
structure and union returning. GCC on most of these machines uses a
compatible convention when returning structures and unions in memory,
but still returns small structures and unions in registers.
You can tell GCC to use a compatible convention for all structure and
union returning with the option `-fpcc-struct-return'.
GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GCC unless they are changed. Some have bugs, some are incompatible with ANSI C, and some depend on special features of other compilers.
Installing GCC automatically creates and installs the fixed header
files, by running a program called fixincludes (or for certain
targets an alternative such as fixinc.svr4). Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.
fixincludes script to fail.
This means you will encounter problems due to bugs in the system header
files. It may be no comfort that they aren't GCC's fault, but it
does mean that there's nothing for us to do about them.
GCC by itself attempts to be what the ISO/ANSI C standard calls a conforming freestanding implementation. This means all ANSI C language features are available, as well as the contents of `float.h', `limits.h', `stdarg.h', and `stddef.h'. The rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using `-Wall') that you don't expect.
For example, the sprintf function on SunOS 4.1.3 returns
char * while the C standard says that sprintf returns an
int. The fixincludes program could make the prototype for
this function match the Standard, but that would be wrong, since the
function will still return char *.
If you need a Standard compliant library, then you need to find one, as
GCC does not provide one. The GNU C library (called glibc)
has been ported to a number of operating systems, and provides ANSI/ISO,
POSIX, BSD and SystemV compatibility. You could also ask your operating
system vendor if newer libraries are available.
These problems are perhaps regrettable, but we don't know any practical way around them.
int foo (struct mumble *);
struct mumble { ... };
int foo (struct mumble *x)
{ ... }
This code really is erroneous, because the scope of struct
mumble in the prototype is limited to the argument list containing it.
It does not refer to the struct mumble defined with file scope
immediately below--they are two unrelated types with similar names in
different scopes.
But in the definition of foo, the file-scope type is used
because that is available to be inherited. Thus, the definition and
the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ANSI standard specifies.
It is easy enough for you to make your code work by moving the
definition of struct mumble above the prototype. It's not worth
being incompatible with ANSI C just to avoid an error for the example
shown above.
include; then do
`make install' again.
double in memory.
Compiled code moves values between memory and floating point registers
at its convenience, and moving them into memory truncates them.
You can partially avoid this problem by using the `-ffloat-store'
option (see section Options That Control Optimization).
C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort.
When a class has static data members, it is not enough to declare the static member; you must also define it. For example:
class Foo
{
...
void method();
static int bar;
};
This declaration only establishes that the class Foo has an
int named Foo::bar, and a member function named
Foo::method. But you still need to define both
method and bar elsewhere. According to the draft ANSI
standard, you must supply an initializer in one (and only one) source
file, such as:
int Foo::bar = 0;
Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to g++ from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: g++ reports as undefined
symbols any static data members that lack definitions.
It is dangerous to use pointers or references to portions of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like string classes,
especially ones that define a conversion function to type char *
or const char * -- which is one reason why the standard
string class requires you to call the c_str member
function. However, any class that returns a pointer to some internal
structure is potentially subject to this problem.
For example, a program may use a function strfunc that returns
string objects, and another function charfunc that
operates on pointers to char:
string strfunc ();
void charfunc (const char *);
void
f ()
{
const char *p = strfunc().c_str();
...
charfunc (p);
...
charfunc (p);
}
In this situation, it may seem reasonable to save a pointer to the C
string returned by the c_str member function and use that rather
than call c_str repeatedly. However, the temporary string
created by the call to strfunc is destroyed after p is
initialized, at which point p is left pointing to freed memory.
Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable.
The safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example:
string& tmp = strfunc (); charfunc (tmp.c_str ());
When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example:
struct Base{
char *name;
Base(char *n) : name(strdup(n)){}
Base& operator= (const Base& other){
free (name);
name = strdup (other.name);
}
};
struct A:virtual Base{
int val;
A():Base("A"){}
};
struct B:virtual Base{
int bval;
B():Base("B"){}
};
struct Derived:public A, public B{
Derived():Base("Derived"){}
};
void func(Derived &d1, Derived &d2)
{
d1 = d2;
}
The C++ standard specifies that `Base::Base' is only called once when constructing or copy-constructing a Derived object. It is unspecified whether `Base::operator=' is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside `func' in the example).
g++ implements the "intuitive" algorithm for copy-assignment: assign all
direct bases, then assign all members. In that algorithm, the virtual
base subobject can be encountered many times. In the example, copying
proceeds in the following order: `val', `name' (via
strdup), `bval', and `name' again.
If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned.
protoize
The conversion programs protoize and unprotoize can
sometimes change a source file in a way that won't work unless you
rearrange it.
protoize can insert references to a type name or type tag before
the definition, or in a file where they are not defined.
If this happens, compiler error messages should show you where the new
references are, so fixing the file by hand is straightforward.
protoize cannot figure out.
For example, it can't determine argument types for declaring a
pointer-to-function variable; this you must do by hand. protoize
inserts a comment containing `???' each time it finds such a
variable; so you can find all such variables by searching for this
string. ANSI C does not require declaring the argument types of
pointer-to-function types.
unprotoize can easily introduce bugs. If the program
relied on prototypes to bring about conversion of arguments, these
conversions will not take place in the program without prototypes.
One case in which you can be sure unprotoize is safe is when
you are removing prototypes that were made with protoize; if
the program worked before without any prototypes, it will work again
without them.
You can find all the places where this problem might occur by compiling
the program with the `-Wconversion' option. It prints a warning
whenever an argument is converted.
protoize cannot get the argument types for a function whose
definition was not actually compiled due to preprocessing conditionals.
When this happens, protoize changes nothing in regard to such
a function. protoize tries to detect such instances and warn
about them.
You can generally work around this problem by using protoize step
by step, each time specifying a different set of `-D' options for
compilation, until all of the functions have been converted. There is
no automatic way to verify that you have got them all, however.
unprotoize can become confused when trying to convert a function
definition or declaration which contains a declaration for a
pointer-to-function formal argument which has the same name as the
function being defined or declared. We recommand you avoid such choices
of formal parameter names.
This section lists changes that people frequently request, but which we do not make because we think GCC is better without them.
void whenever the value isn't
useful.
int is signed or not. This in effect creates two
alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the signed
dialect with `-fsigned-bitfields' and the unsigned dialect with
`-funsigned-bitfields'. However, this leaves open the question of
which dialect to use by default.
Currently, the preferred dialect makes plain bitfields signed, because
this is simplest. Since int is the same as signed int in
every other context, it is cleanest for them to be the same in bitfields
as well.
Some computer manufacturers have published Application Binary Interface
standards which specify that plain bitfields should be unsigned. It is
a mistake, however, to say anything about this issue in an ABI. This is
because the handling of plain bitfields distinguishes two dialects of C.
Both dialects are meaningful on every type of machine. Whether a
particular object file was compiled using signed bitfields or unsigned
is of no concern to other object files, even if they access the same
bitfields in the same data structures.
A given program is written in one or the other of these two dialects.
The program stands a chance to work on most any machine if it is
compiled with the proper dialect. It is unlikely to work at all if
compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bitfields differently on
certain machines.
Occasionally users write programs intended only for a particular machine
type. On these occasions, the users would benefit if the GNU C compiler
were to support by default the same dialect as the other compilers on
that machine. But such applications are rare. And users writing a
program to run on more than one type of machine cannot possibly benefit
from this kind of compatibility.
This is why GCC does and will treat plain bitfields in the same
fashion on all types of machines (by default).
There are some arguments for making bitfields unsigned by default on all
machines. If, for example, this becomes a universal de facto standard,
it would make sense for GCC to go along with it. This is something
to be considered in the future.
(Of course, users strongly concerned about portability should indicate
explicitly in each bitfield whether it is signed or not. In this way,
they write programs which have the same meaning in both C dialects.)
__STDC__ when `-ansi' is not used.
Currently, GCC defines __STDC__ as long as you don't use
`-traditional'. This provides good results in practice.
Programmers normally use conditionals on __STDC__ to ask whether
it is safe to use certain features of ANSI C, such as function
prototypes or ANSI token concatenation. Since plain `gcc' supports
all the features of ANSI C, the correct answer to these questions is
"yes".
Some users try to use __STDC__ to check for the availability of
certain library facilities. This is actually incorrect usage in an ANSI
C program, because the ANSI C standard says that a conforming
freestanding implementation should define __STDC__ even though it
does not have the library facilities. `gcc -ansi -pedantic' is a
conforming freestanding implementation, and it is therefore required to
define __STDC__, even though it does not come with an ANSI C
library.
Sometimes people say that defining __STDC__ in a compiler that
does not completely conform to the ANSI C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that claim to support ANSI C, such as `gcc -ansi'---not for other
compilers such as plain `gcc'. Whatever the ANSI C standard says
is relevant to the design of plain `gcc' without `-ansi' only
for pragmatic reasons, not as a requirement.
GCC normally defines __STDC__ to be 1, and in addition
defines __STRICT_ANSI__ if you specify the `-ansi' option.
On some hosts, system include files use a different convention, where
__STDC__ is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GCC follows the host convention when
processing system include files, but when processing user files it follows
the usual GNU C convention.
__STDC__ in C++.
Programs written to compile with C++-to-C translators get the
value of __STDC__ that goes with the C compiler that is
subsequently used. These programs must test __STDC__
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ANSI C fashion
or in the traditional fashion.
These programs work properly with GNU C++ if __STDC__ is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes in ANSI
C but not in traditional C. Many of these header files can work without
change in C++ provided __STDC__ is defined. If __STDC__
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.
void func (int, int); int i = 2; func (i++, i++);There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first.
func might get the
arguments `2, 3', or it might get `3, 2', or even `2, 2'.
The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:
Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the `-W' options (for instance, `-Wall' requests a variety of useful warnings).
GCC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The `-pedantic' option tells GCC to issue warnings in such cases; `-pedantic-errors' says to make them errors instead. This does not mean that all non-ANSI constructs get warnings or errors.
See section Options to Request or Suppress Warnings, for more detail on these and related command-line options.
Your bug reports play an essential role in making GCC reliable.
When you encounter a problem, the first thing to do is to see if it is already known. See section Known Causes of Trouble with GCC. If it isn't known, then you should report the problem.
Reporting a bug may help you by bringing a solution to your problem, or it may not. (If it does not, look in the service directory; see section How To Get Help with GCC.) In any case, the principal function of a bug report is to help the entire community by making the next version of GCC work better. Bug reports are your contribution to the maintenance of GCC.
Since the maintainers are very overloaded, we cannot respond to every bug report. However, if the bug has not been fixed, we are likely to send you a patch and ask you to tell us whether it works.
In order for a bug report to serve its purpose, you must include the information that makes for fixing the bug.
If you are not sure whether you have found a bug, here are some guidelines:
asm statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.
return
is omitted; it is not a bug when GCC produces different results.
Problems often result from expressions with two increment operators,
as in f (*p++, *p++). Your previous compiler might have
interpreted that expression the way you intended; GCC might
interpret it another way. Neither compiler is wrong. The bug is
in your code.
After you have localized the error to a single source line, it should
be easy to check for these things. If your program is correct and
well defined, you have found a compiler bug.
Send bug reports for the GNU Compiler Collection to `gcc-bugs@gcc.gnu.org'. In accordance with the GNU-wide convention, in which bug reports for tool "foo" are sent to `bug-foo@gnu.org', the address `bug-gcc@gnu.org' may also be used; it will forward to the address given above.
Please read `<URL:http://www.gnu.org/software/gcc/bugs.html>' for bug reporting instructions before you post a bug report.
Often people think of posting bug reports to the newsgroup instead of mailing them. This appears to work, but it has one problem which can be crucial: a newsgroup posting does not contain a mail path back to the sender. Thus, if maintainers need more information, they may be unable to reach you. For this reason, you should always send bug reports by mail to the proper mailing list.
As a last resort, send bug reports on paper to:
GNU Compiler Bugs Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307, USA
You may find additional and/or more up-to-date instructions at `<URL:http://www.gnu.org/software/gcc/bugs.html>'.
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and they conclude that some details don't matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it doesn't, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the compiler into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable someone to fix the bug if it is not known. It isn't very important what happens if the bug is already known. Therefore, always write your bug reports on the assumption that the bug is not known.
Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with.
Try to make your bug report self-contained. If we have to ask you for more information, it is best if you include all the previous information in your response, as well as the information that was missing.
Please report each bug in a separate message. This makes it easier for us to track which bugs have been fixed and to forward your bugs reports to the appropriate maintainer.
To enable someone to investigate the bug, you should include all these things:
configure command when you installed
the compiler.
pr to print the RTL expression that it points
to. (If GDB doesn't run on your machine, use your debugger to call
the function debug_rtx with the RTX as an argument.) In
general, whenever a variable is a pointer, its value is no use
without the data it points to.
Here are some things that are not necessary:
If you would like to write bug fixes or improvements for the GNU C
compiler, that is very helpful. Send suggested fixes to the patches
mailing list, gcc-patches@gcc.gnu.org.
Please follow these guidelines so we can study your patches efficiently. If you don't follow these guidelines, your information might still be useful, but using it will take extra work. Maintaining GNU C is a lot of work in the best of circumstances, and we can't keep up unless you do your best to help.
If you need help installing, using or changing GCC, there are two ways to find it:
gcc-bugs@gcc.gnu.org or bug-gcc@gnu.org, and if that
brings no response, try gcc@gcc.gnu.org.
If you would like to help pretest GCC releases to assure they work
well, or if you would like to work on improving GCC, please contact
the maintainers at gcc@gcc.gnu.org. A pretester should
be willing to try to investigate bugs as well as report them.
If you'd like to work on improvements, please ask for suggested projects
or suggest your own ideas. If you have already written an improvement,
please tell us about it. If you have not yet started work, it is useful
to contact gcc@gcc.gnu.org before you start; the
maintainers may be able to suggest ways to make your extension fit in
better with the rest of GCC and with other development plans.
Here is how to use GCC on VMS.
Due to the differences between the filesystems of Unix and VMS, GCC attempts to translate file names in `#include' into names that VMS will understand. The basic strategy is to prepend a prefix to the specification of the include file, convert the whole filename to a VMS filename, and then try to open the file. GCC tries various prefixes one by one until one of them succeeds:
Include directives of the form:
#include foobar
are a common source of incompatibility between VAX-C and GCC. VAX-C
treats this much like a standard #include <foobar.h> directive.
That is incompatible with the ANSI C behavior implemented by GCC: to
expand the name foobar as a macro. Macro expansion should
eventually yield one of the two standard formats for #include:
#include "file" #include <file>
If you have this problem, the best solution is to modify the source to
convert the #include directives to one of the two standard forms.
That will work with either compiler. If you want a quick and dirty fix,
define the file names as macros with the proper expansion, like this:
#define stdio <stdio.h>
This will work, as long as the name doesn't conflict with anything else in the program.
Another source of incompatibility is that VAX-C assumes that:
#include "foobar"
is actually asking for the file `foobar.h'. GCC does not make this assumption, and instead takes what you ask for literally; it tries to read the file `foobar'. The best way to avoid this problem is to always specify the desired file extension in your include directives.
GCC for VMS is distributed with a set of include files that is
sufficient to compile most general purpose programs. Even though the
GCC distribution does not contain header files to define constants
and structures for some VMS system-specific functions, there is no
reason why you cannot use GCC with any of these functions. You first
may have to generate or create header files, either by using the public
domain utility UNSDL (which can be found on a DECUS tape), or by
extracting the relevant modules from one of the system macro libraries,
and using an editor to construct a C header file.
A #include file name cannot contain a DECNET node name. The
preprocessor reports an I/O error if you attempt to use a node name,
whether explicitly, or implicitly via a logical name.
GCC does not provide the globalref, globaldef and
globalvalue keywords of VAX-C. You can get the same effect with
an obscure feature of GAS, the GNU assembler. (This requires GAS
version 1.39 or later.) The following macros allow you to use this
feature in a fairly natural way:
#ifdef __GNUC__
#define GLOBALREF(TYPE,NAME) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME)
#define GLOBALDEF(TYPE,NAME,VALUE) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \
= VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \
= {VALUE}
#else
#define GLOBALREF(TYPE,NAME) \
globalref TYPE NAME
#define GLOBALDEF(TYPE,NAME,VALUE) \
globaldef TYPE NAME = VALUE
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
globalvalue TYPE NAME = VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
globalvalue TYPE NAME
#endif
(The _$$PsectAttributes_GLOBALSYMBOL prefix at the start of the
name is removed by the assembler, after it has modified the attributes
of the symbol). These macros are provided in the VMS binaries
distribution in a header file `GNU_HACKS.H'. An example of the
usage is:
GLOBALREF (int, ijk); GLOBALDEF (int, jkl, 0);
The macros GLOBALREF and GLOBALDEF cannot be used
straightforwardly for arrays, since there is no way to insert the array
dimension into the declaration at the right place. However, you can
declare an array with these macros if you first define a typedef for the
array type, like this:
typedef int intvector[10]; GLOBALREF (intvector, foo);
Array and structure initializers will also break the macros; you can
define the initializer to be a macro of its own, or you can expand the
GLOBALDEF macro by hand. You may find a case where you wish to
use the GLOBALDEF macro with a large array, but you are not
interested in explicitly initializing each element of the array. In
such cases you can use an initializer like: {0,}, which will
initialize the entire array to 0.
A shortcoming of this implementation is that a variable declared with
GLOBALVALUEREF or GLOBALVALUEDEF is always an array. For
example, the declaration:
GLOBALVALUEREF(int, ijk);
declares the variable ijk as an array of type int [1].
This is done because a globalvalue is actually a constant; its "value"
is what the linker would normally consider an address. That is not how
an integer value works in C, but it is how an array works. So treating
the symbol as an array name gives consistent results--with the
exception that the value seems to have the wrong type. Don't
try to access an element of the array. It doesn't have any elements.
The array "address" may not be the address of actual storage.
The fact that the symbol is an array may lead to warnings where the variable is used. Insert type casts to avoid the warnings. Here is an example; it takes advantage of the ANSI C feature allowing macros that expand to use the same name as the macro itself.
GLOBALVALUEREF (int, ss$_normal); GLOBALVALUEDEF (int, xyzzy,123); #ifdef __GNUC__ #define ss$_normal ((int) ss$_normal) #define xyzzy ((int) xyzzy) #endif
Don't use globaldef or globalref with a variable whose
type is an enumeration type; this is not implemented. Instead, make the
variable an integer, and use a globalvaluedef for each of the
enumeration values. An example of this would be:
#ifdef __GNUC__
GLOBALDEF (int, color, 0);
GLOBALVALUEDEF (int, RED, 0);
GLOBALVALUEDEF (int, BLUE, 1);
GLOBALVALUEDEF (int, GREEN, 3);
#else
enum globaldef color {RED, BLUE, GREEN = 3};
#endif
GCC automatically arranges for main to return 1 by default if
you fail to specify an explicit return value. This will be interpreted
by VMS as a status code indicating a normal successful completion.
Version 1 of GCC did not provide this default.
GCC on VMS works only with the GNU assembler, GAS. You need version 1.37 or later of GAS in order to produce value debugging information for the VMS debugger. Use the ordinary VMS linker with the object files produced by GAS.
Under previous versions of GCC, the generated code would occasionally give strange results when linked to the sharable `VAXCRTL' library. Now this should work.
A caveat for use of const global variables: the const
modifier must be specified in every external declaration of the variable
in all of the source files that use that variable. Otherwise the linker
will issue warnings about conflicting attributes for the variable. Your
program will still work despite the warnings, but the variable will be
placed in writable storage.
Although the VMS linker does distinguish between upper and lower case letters in global symbols, most VMS compilers convert all such symbols into upper case and most run-time library routines also have upper case names. To be able to reliably call such routines, GCC (by means of the assembler GAS) converts global symbols into upper case like other VMS compilers. However, since the usual practice in C is to distinguish case, GCC (via GAS) tries to preserve usual C behavior by augmenting each name that is not all lower case. This means truncating the name to at most 23 characters and then adding more characters at the end which encode the case pattern of those 23. Names which contain at least one dollar sign are an exception; they are converted directly into upper case without augmentation.
Name augmentation yields bad results for programs that use precompiled libraries (such as Xlib) which were generated by another compiler. You can use the compiler option `/NOCASE_HACK' to inhibit augmentation; it makes external C functions and variables case-independent as is usual on VMS. Alternatively, you could write all references to the functions and variables in such libraries using lower case; this will work on VMS, but is not portable to other systems. The compiler option `/NAMES' also provides control over global name handling.
Function and variable names are handled somewhat differently with GNU C++. The GNU C++ compiler performs name mangling on function names, which means that it adds information to the function name to describe the data types of the arguments that the function takes. One result of this is that the name of a function can become very long. Since the VMS linker only recognizes the first 31 characters in a name, special action is taken to ensure that each function and variable has a unique name that can be represented in 31 characters.
If the name (plus a name augmentation, if required) is less than 32 characters in length, then no special action is performed. If the name is longer than 31 characters, the assembler (GAS) will generate a hash string based upon the function name, truncate the function name to 23 characters, and append the hash string to the truncated name. If the `/VERBOSE' compiler option is used, the assembler will print both the full and truncated names of each symbol that is truncated.
The `/NOCASE_HACK' compiler option should not be used when you are
compiling programs that use libg++. libg++ has several instances of
objects (i.e. Filebuf and filebuf) which become
indistinguishable in a case-insensitive environment. This leads to
cases where you need to inhibit augmentation selectively (if you were
using libg++ and Xlib in the same program, for example). There is no
special feature for doing this, but you can get the result by defining a
macro for each mixed case symbol for which you wish to inhibit
augmentation. The macro should expand into the lower case equivalent of
itself. For example:
#define StuDlyCapS studlycaps
These macro definitions can be placed in a header file to minimize the number of changes to your source code.
Here is a list of all the passes of the compiler and their source files. Also included is a description of where debugging dumps can be requested with `-d' options.
if-conditions that are
comparisons, boolean operations or conditional expressions. Tail
recursion is detected at this time also. Decisions are made about how
best to arrange loops and how to output switch statements.
The source files for RTL generation include
`stmt.c',
`calls.c',
`expr.c',
`explow.c',
`expmed.c',
`function.c',
`optabs.c'
and `emit-rtl.c'.
Also, the file
`insn-emit.c', generated from the machine description by the
program genemit, is used in this pass. The header file
`expr.h' is used for communication within this pass.
The header files `insn-flags.h' and `insn-codes.h',
generated from the machine description by the programs genflags
and gencodes, tell this pass which standard names are available
for use and which patterns correspond to them.
Aside from debugging information output, none of the following passes
refers to the tree structure representation of the function (only
part of which is saved).
The decision of whether the function can and should be expanded inline
in its subsequent callers is made at the end of rtl generation. The
function must meet certain criteria, currently related to the size of
the function and the types and number of parameters it has. Note that
this function may contain loops, recursive calls to itself
(tail-recursive functions can be inlined!), gotos, in short, all
constructs supported by GCC. The file `integrate.c' contains
the code to save a function's rtl for later inlining and to inline that
rtl when the function is called. The header file `integrate.h'
is also used for this purpose.
The option `-dr' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.rtl' to
the input file name.
Some additional files are used by all or many passes:
gen* also use these files to read and work with the machine
description RTL.
genconfig.
HARD_REG_SET, a bit-vector
with a bit for each hard register, and some macros to manipulate it.
This type is just int if the machine has few enough hard registers;
otherwise it is an array of int and some of the macros expand
into loops.
Jump to: ! - # - $ - % - & - ' - + - - - . - / - 0 - < - = - > - ? - _ - a - b - c - d - e - f - g - h - i - j - k - l - m - n - o - p - q - r - s - t - u - v - w - x - z
#pragma implementation, implied
#pragma, reason for not using
-lgcc, use with -nodefaultlibs
-lgcc, use with -nostdlib
-nodefaultlibs and unresolved references
-nostdlib and unresolved references
?: extensions, ?: extensions
alias attribute
aligned attribute, aligned attribute
alloca and SunOS
alloca vs variable-length arrays
alloca, for SunOS
alloca, for Unos
asm constraints
asm expressions
inline for C++ member fns
inline
catgets
const applied to function
const function attribute
asm
constructor function attribute
goto
destructor function attribute
goto
asm
?:, extensions, ?:
float as function value type
format function attribute
format_arg function attribute
fscanf, and constant strings
printf, scanf or strftime style arguments
genflags, crash on Sun 4
gettext
longjmp
goto in C++
gprof
#pragma implementation
inline automatic for C++ member fns
g++
libstdc++
long long data types
longjmp and automatic variables
longjmp incompatibilities
longjmp warnings
asm
main and the exit status
inline
mktemp, and constant strings
mode attribute
no_check_memory_usage function attribute
no_instrument_function function attribute
nocommon attribute
noreturn function attribute
asm
packed attribute
prof
qsort, and global register variables
longjmp
main
return, in C++ function header
scanf, and constant strings
section function attribute
section variable attribute
setjmp incompatibilities
signature in C++, advantages
sscanf, and constant strings
tcov
-nodefaultlibs
-nostdlib
longjmp
volatile applied to function
weak attribute
The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.
A file's basename was the name stripped of all leading path information and of trailing suffixes, such as `.h' or `.C' or `.cc'.