Chapter 2 Debugging with GDB

This chapter is compiled from GNU's GDB manual: Debugging with GDB. Although GDB is flexible enough to support debugging of a variety of targets including different languages, LynuxWorks supports GDB only for debugging LynxOS target applications and drivers written in C, C++ or assembly languages in a LynxOS developed environment.

The GNU Source Level Debugger

The purpose of a debugger such as GDB is to allow you to see what is going on inside another program while it executes-or what another program was doing at the moment it crashed.

GDB as Free Software

GDB is free software, protected by the GNU General Public License (GPL). The GPL gives you the freedom to copy or adapt a licensed program-but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms. Fundamentally, the General Public License is a license which says that you have these freedoms and that you cannot take these freedoms away from anyone else.

Controlling GDB

You can alter the way GDB interacts with you by using the set command. For commands controlling how GDB displays data, see "Print Settings".

Prompt

GDB indicates its readiness to read a command by printing a string called the prompt. This string is normally (gdb). You can change the prompt string with the set prompt command. For instance, when debugging GDB with GDB, it is useful to change the prompt in one of the GDB sessions so that you can always tell which one you are talking to.


Note: set prompt no longer adds a space for you after the prompt you set. This allows you to set a prompt which ends in a space or a prompt that does not.

Command Editing

GDB reads its input commands via the readline interface. This GNU library provides consistent behavior for programs which provide a command line interface to the user. Advantages are GNU Emacs-style or vi-style inline editing of commands, csh-like history substitution, and a storage and recall of command history across debugging sessions. You may control the behavior of command line editing in GDB with the set command.


set editing
set editing on	Enable command line editing (enabled by default).
set editing off	Disable command line editing.
show editing	Show whether command line editing is enabled.

Command History

GDB can keep track of the commands you type during your debugging sessions, so that you can be certain of precisely what happened. Use the following commands to manage the GDB command history facility.

Set the name of the GDB command history file to fname. This is the file where GDB reads an initial command history list, and where it writes the command history from this session when it exits. You can access this list through history expansion or through the history command editing characters listed in the following. This file defaults to the value of the GDBHISTFILE environment variable, or to ./.gdb_history if this variable is not set.

Set the number of commands which GDB keeps in its history list. This defaults to the value of the environment variable HISTSIZE, or to 256 if this variable is not set. History expansion assigns special meaning to the exclamation point character (!). Because ! is also the logical not operator in C, history expansion is off by default. If you decide to enable history expansion with the set history expansion on command, you may sometimes need to follow ! (when it is used as logical not, in an expression) with a space or a tab to prevent it from being expanded. The readline history facilities do not attempt substitution on the strings
!= and ! , even when history expansion is enabled. The commands to control history expansion are the following.

show history
show history filename
show history save
show history size
show history expansion

Screen Size

Certain commands to GDB may produce large amounts of information output to the screen. To help you read all of it, GDB pauses and asks you for input at the end of each page of output. Use Return when you want to continue the output, or type q to discard the remaining output. Also, the screen width setting determines when to wrap lines of output. Depending on what is being printed, GDB tries to break the line at a readable place, rather than simply letting it overflow onto the following line.

Normally, GDB knows the size of the screen from the termcap data base together with the value of the TERM environment variable and the stty rows and stty cols settings. If this is not correct, you can override it with the set height and set width commands:

These set commands specify a screen height of lpp lines and a screen width of cpl characters. The associated show commands display the current settings. If you specify a height of zero lines, GDB does not pause during output no matter how long the output is. This is useful if output is to a file or to an editor buffer.

Numbers

You can always enter numbers in octal, decimal, or hexadecimal in GDB by the usual conventions. Octal numbers begin with 0, decimal numbers end with a period (.), and hexadecimal numbers begin with 0x.

Numbers that begin with none of these are, by default, entered in base 10; likewise, the default format for displaying numbers is base 10. You can change the default base for both input and output with the set radix command.

Sets the default base for numeric input. Supported choices for base are decimal 8, 10, or 16. base must itself be specified either unambiguously or using the current default radix; for example, any of set radix 012, set radix 10, or set radix 0xa set the base to decimal. On the other hand, set radix 10 leaves the radix unchanged no matter what it was.

Optional Warnings and Messages

By default, GDB is silent about its inner workings. If you are running on a slow machine, you may want to use the set verbose command. This makes GDB tell you when it does a lengthy internal operation, so you will not think it has crashed.

Currently, the messages controlled by set verbose are those which announce that the symbol table for a source file is being read; see symbolfile in "Commands to Specify Files".

Displays whether set verbose is on or off. By default, if GDB encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (see "Errors Reading Symbol Files").

Permits GDB to output limit complaints about each type of unusual symbols before becoming silent about the problem. Set limit to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed.

By default, GDB is cautious, and asks what sometimes seems to be a lot of stupid questions to confirm certain commands. For example, if you try to run a program which is already running and you had entered a run, command you would see the following message on screen:

If you are willing to unflinchingly face the consequences of your own commands, you can disable this "feature" with the following commands.

Getting In and Out of GDB

The following material discusses invoking the debugger, choosing files, choosing modes, stopping the debugger and some essential shell commands.

Invoking GDB

Invoke GDB by using the command, gdb. Once started, GDB reads commands from the terminal until you tell it to quit.

You can also run GDB with a variety of arguments and options, to specify more of your debugging environment at the outset.

The command-line options described in the following discussions are designed to cover a variety of situations; in some environments, effectively, some of these options may be unavailable.

The usual way to start GDB is with one argument, specifying an executable program that you want to debug.

Taking advantage of the second command-line argument requires a fairly complete operating system; when you use GDB as a remote debugger attached to a bare board, there may not be any notion of process, and there is often no way to get a core dump.

You can run GDB without printing the front material, which describes GDB's non-warranty, by specifying -silent:

You can further control how GDB starts up by using command-line options. GDB itself can remind you of the options available.

To display all available options and briefly describe their use, use
gdb help as input (gdb -h is a shorter equivalent).

All options and command line arguments you give are processed in sequential order. The order makes a difference when using the -x option.

Choosing Files

When GDB starts, it reads any arguments other than options as specifying an executable file and core file or (process ID). This is the same as if the arguments were specified by the -se and -c options, respectively. (GDB reads the first argument that does not have an associated option flag as equivalent to the -se option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the -c option followed by that argument.)

Many options have both long and short forms; both are shown in Table: Choosing Files. GDB also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with -- rather than - , though we illustrate the more usual convention.)

Choosing Files
Long Entry Form	Short Entry Form	Command Definition
-symbols file	-s file	Read symbol table from file, file.
-exec file	-e file	Use file, file, as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.
-se file		Read symbol table from file, file, and use it as the executable file.
-core file	-c file	Use file, file, as a core dump to examine.
-c number		Connect to process ID number, as with the attach command (unless there is a file in coredump format named number, in which case -c specifies that file as a core dump to read).
-command file	-x file	Execute GDB commands from file file (see "Command Files").
-directory directory	-d directory	Add directory to the path to search for source files.
-r	-readnow	Read each symbol file's entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster.

Choosing Modes

You can run GDB in various alternative modes-for example, in batch mode or quiet mode. Table: Choosing Modes shows other available options.

Choosing Modes
Long Entry Form	Short Entry Form	Command Definition
-nx	-n	Do not execute commands from any initialization files (normally called .gdbinit). Normally, the commands in these files are executed after all the command options and arguments have been processed (see "Command Files".
-quiet	-q	Quiet. Do not print the introductory and copyright messages. These messages are also suppressed in batch mode.
-batch		Run in batch mode. Exit with status 0 after processing all the command files specified with -x and all commands from initialization files, if not inhibited with -n. Exit with nonzero status if an error occurs in executing the GDB commands in the command files. Batch mode may be useful for running GDB as a filter. For example, to download and run a program on another computer, in order to make this more useful, the following message does not issue when running in batch mode (ordinarily, the message issues whenever a program running under GDB control terminates). Program exited normally.
-cd directory		Run GDB using directory as its working directory, instead of the current directory.
-fullname	-f	GNU Emacs sets this option when it runs GDB as a subprocess. It tells GDB to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time your program stops). This recognizable format looks like two \032 characters, followed by the file name, line number, and character position separated by colons, and a newline. The Emacs-to-GDB interface program uses the two \032 characters as a signal to display the source code for the frame.
-b bps		Set the line speed (baud rate or bits per second) of any serial interface used by GDB for remote debugging.
-tty device		Run using device for your program's standard input and output.

Quitting GDB

To exit GDB, use the quit command (abbreviated q), or use an end-of-file character (usually Ctrl-d). If you do not supply expression, GDB will terminate normally; otherwise it will terminate using the result of expression as the error code.

An interrupt (often, Ctrl-c) does not exit from GDB, but rather terminates the action of any GDB command that is in progress and returns to GDB command level. It is safe to use the interrupt character at any time because GDB does not allow it to take effect until a time when it is safe.

If you have been using GDB to control an attached process or device, you can release it with the detach command ( see "Debugging an Already-Running Process").

Shell Commands

If you need to execute occasional shell commands during your debugging session, there is no need to leave or suspend GDB. Use the shell command to do this.

The utility make is often needed in development environments. You do not have to use the shell command for this purpose in GDB:

GDB Commands

You can abbreviate a GDB command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDB commands by using Return. You can also use the Tab key to get GDB to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).

Command Syntax

A GDB command is a single line of input. There is no limit on how long it can be. It starts with a command name, which is followed by arguments whose meaning depends on the command name. For example, the command, step, accepts an argument which is the number of times to step, as in step 5. You can also use the step command with no arguments. Some command names do not allow any arguments.

Straight brackets ([ ]) enclose optional parameters. Curly brackets ({ }) enclose choices or selections to be made. Neither of these brackets are typed in, but are inferred.

GDB command names may always be truncated if that abbreviation is unambiguous. Other possible command abbreviations are listed in the documentation for individual commands. In some cases, even ambiguous abbreviations are allowed; for example, s is specially defined as equivalent to step even though there are other commands whose names start with s. You can test abbreviations by using them as arguments to the help command.

A blank line as input to GDB (using Return just once) means to repeat the previous command. Certain commands (for example, run) will not repeat this way; such commands have unintentional repetition which might cause trouble and which it is unlikely you want to repeat.

The list and x commands, when you repeat them with Return key actions, construct new arguments rather than repeating exactly as generated. This permits easy scanning of source or memory.

GDB can also use Return in another way: to partition lengthy output, in a way similar to the common utility (see "Screen Size"). Because it is easy to use Return one too many times in this situation, GDB disables command repetition after any command that generates this sort of display.

Any text from a # to the end of the line is a comment; it does nothing. This is useful mainly in command files (see "Command Files").

Command Completion

GDB can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you, at any time, what the valid possibilities are for the next word in a command. This works for GDB commands, GDB subcommands, and the names of symbols in your program.

Use the Tab key whenever you want GDB to fill out the rest of a word. If there is only one possibility, GDB fills in the word, and waits for you to finish the command (or use Return to enter it). For example, if you type (gdb) info bre, and use the Tab key, GDB fills in the rest of the word breakpoints, because that is the only info subcommand beginning with bre.

You can either use Return at this point, to run the info breakpoints command, or use the Backspace key and enter something else, if breakpoints does not look like the command you expected. (If you were sure you wanted info breakpoints in the first place, you might as well just use Return immediately after info bre, to exploit command abbreviations rather than command completion). If there is more than one possibility for the next word when you use the Tab key, GDB sounds a bell. You can either supply more characters and try again, or just use the Tab key a second time; GDB displays all the possible completions for that word. For example, you might want to set a breakpoint on a subroutine whose name begins with make_, but when you type b make_ and use the Tab key, GDB just sounds the bell. Using the Tab key again displays all the function names in your program that begin with those characters. For example, you type (gdb) make_b and then use the Tab key. GDB sounds the bell; you use the Tab key again, to see the following display.


make_a_section_from_file	make_environ
make_abs_section	make_function_type
make_blockvector	make_pointer_type
make_cleanup	make_reference_type
make_command	make_symbol_completion_list
(gdb) b make_

After displaying the available possibilities, GDB copies your partial input (in the example, b make_) so you can finish the command. If you just want to see the list of alternatives in the first place, you can get help by using the command key sequence, M-? rather than using Tab twice.


Note: M-? means using the META key (if there is one, or else, use ESC) and the ? key. This is a command key sequence with which you may or may not be familiar.

Sometimes the string you need, while logically a word, may contain parentheses or other characters that GDB normally excludes from its notion of a word. To permit word completion to work in this situation, you may enclose words in single quote marks in GDB commands.

The most likely situation where you might need this is in typing the name of a C++ function. This is because C++ allows function overloading (multiple definitions of the same function, distinguished by argument type). For example, when you want to set a breakpoint you may need to distinguish whether you mean the version of name that takes an int parameter, name(int), or the version that takes a float parameter, name(float). To use the word-completion facilities in this situation, type a single quote, ', at the beginning of the function name. This alerts GDB that it may need to consider more information than usual when you use the Tab key or M-? to request word completion, as in the following example:

In some cases, GDB can tell that completing a name requires using quotes. When this happens, GDB inserts the quote for you while (completing as much as it can) if you do not type the quote in the first place:

Getting Help

You can always ask GDB itself for information on its commands, using the command help.

Type help followed by a class name for a list of commands in that class. Type help followed by command name for full documentation. Command name abbreviations are allowed if unambiguous.

The complete args command lists all the possible completions for the beginning of a command. Use args to specify the beginning of the command you want completed. For example: complete i results in the following.

In addition to help, you can use the GDB info and show commands to inquire about the state of your program, or the state of GDB itself. Each command supports many topics of inquiry; this manual introduces each of were in the appropriate context. The listings under info and under show in the Index point to all the subcommands.

This command (abbreviated i) is for describing the state of your program. For example, you can list the arguments given to your program with info args, list the registers currently in use with info registers, or list the breakpoints you have set with info breakpoints. You can get a complete list of the info subcommands with help info.

In contrast to info, show is for describing the state of GDB itself. You can change most of the things you can show, by using the related command, set; for example, you can control what number system is used for displays with set radix, or simply inquire which is currently in use with show radix.

To display all the settable parameters and their current values, you can use show with no arguments; you may also use info set. Both commands produce the same display.

The following are three miscellaneous show subcommands which of no corresponding set commands.

Show what version of GDB is running. You should include this information in GDB bug reports. If multiple versions of GDB are in use at your site, you may occasionally want to determine which version of GDB you are running; as GDB evolves, new commands are introduced, and old ones may wither away. The version number is also announced when you start GDB.

Running Programs under GDB

The following material discusses running your programs with GDB.When you run a program under GDB, you must first generate debugging information when you compile it.

You may start GDB with its arguments, if any, in an environment of your choice. You may redirect your program's input and output, debug an already running process, or kill a child process.

Compiling for Debugging

In order to debug a program effectively, you need to generate debugging information when you compile it. This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.

To request debugging information, specify the -g option when you run the compiler. Many C compilers are unable to handle the -g and -O options together. Using those compilers, you cannot generate optimized executables containing debugging information. GCC, the GNU C compiler, supports
-g with or without -O making it possible to debug optimized code.

We recommend that you always use -g whenever you compile a program. You may think your program is correct, but there is no sense in pushing your luck.

When you debug a program compiled with -g -O, remember that the optimizer is rearranging your code; the debugger shows you what is really there.

Do not be too surprised when the execution path does not exactly match your source file! An extreme example: if you define a variable, but never use it, GDB never sees that variable-because the compiler optimizes it out of existence.

Some things do not work as well with -g -O as with just -g, particularly on machines with instruction scheduling. If in doubt, recompile with -g alone, and if this fixes the problem, please report it to us as a bug (including a test case!).


	Caution! The following discussions about your program's arguments environment, working directory and input/output apply only if you start the debugged program locally from your GDB. If you attach GDB to an already running process, the parameters are already determined. If you start the application program remotely from a GDB subserver, the program arguments are given to GDB server's command line, and the other parameters are inherited from the GDB server process

Starting Your Program

Use the run command to start your program locally under GDB. You must first specify the program name with an argument to GDB or by using the file or exec-file command (see "Getting In and Out of GDB" or "Commands to Specify Files").

If you are running your program in an execution environment that supports processes, run creates an inferior process and makes that process run your program. (In environments without processes, run jumps to the start of your program.)

The execution of a program is affected by certain information it receives from its superior. GDB provides ways to specify this information, which you must do before starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into the following four categories.

Specify the arguments to give your program as the arguments of the run command. If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or variable substitution) in describing the arguments. In UNIX systems, you can control which shell is used with the SHELL environment variable.

Your program normally inherits its environment from GDB, but you can use the GDB commands set environment and unset environment to change parts of the environment that affect your program (see "Your Program's Environment").

Your program normally uses the same device for standard input and standard output as GDB is using. You can redirect input and output in the run command line, or you can use the tty command to set a different device for your program (see "Your Program's Input and Output").

Caution! While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program. If you attempt this, GDB is likely to wind up debugging the wrong program.

	Caution! While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program. If you attempt this, GDB is likely to wind up debugging the wrong program.

When you issue the run command, your program begins to execute immediately. See "Stopping and Continuing" for a discussion of how to arrange for your program to stop. Once your program has stopped, you may call functions in your program, using the print or call commands in "Examining Data".

If the modification time of your symbol file has changed since the last time GDB read its symbols, GDB discards its symbol table, and reads it again. When it does this, GDB tries to retain your current breakpoints.

Your Program's Arguments

The arguments to your program can be specified by the arguments of the run command. They are passed to a shell, which expands wildcard characters and performs redirection of I/O, and thence to your program. Your SHELL environment variable (if it exists) specifies what shell GDB uses. If you do not define SHELL, GDB uses /bin/sh.

run with no arguments uses the same arguments used by the previous run, or those set by the set args command.

Specify the arguments to be used the next time your program is run. If set args has no arguments, run executes your program with no arguments. Once you have run your program with arguments, using set args before the next run is the only way to run it again without arguments.

Your Program's Environment

The environment consists of a set of environment variables and their values. Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run.

Usually you set up environment variables with the shell and they are inherited by all the other programs you run.

When debugging, it can be useful to try running your program with a modified environment without having to start GDB over again.

Add directory to the front of the PATH environment variable (the search path for executables), for both GDB and your program. You may specify several directory names, separated by a colon (:) or a whitespace. If directory is already in the path, it is moved to the front, so it is searched sooner.

You can use the $cwd string to refer to whatever is the current working directory at the time GDB searches the path. If you use a period (.) instead, it refers to the directory where you executed the path command. GDB replaces the period (.) in the directory argument (with the current path) before adding directory to the search path.

Print the value of environment variable varname to be given to your program when it starts. If you do not supply varname, print the names and values of all environment variables to be given to your program. You can abbreviate environment as env.

Set environment variable varname to value. The value changes for your program only, not for GDB itself. value may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself. The value parameter is optional; if it is eliminated, the variable is set to a null value. For example, the command, set env USER = foo, tells a UNIX program, when run, that its user is named foo. (The spaces around = are used for clarity here; they are not actually required.)

Remove variable, varname, from the environment to be passed to your program. This is different from set env varname=; unset environment removes the variable from the environment, rather than assigning it an empty value

Note: GDB runs your program using the shell indicated by your SHELL environment variable if it exists (or /bin/sh if not). If your SHELL variable names a shell that runs an initialization file-such as .cshrc for C-shell, or .bashrc for BASH-any variables you set in that file affect your program. You may wish to move setting of environment variables to files that are only run when you sign on, such as .login or .profile.

.

Note: GDB runs your program using the shell indicated by your SHELL environment variable if it exists (or /bin/sh if not). If your SHELL variable names a shell that runs an initialization file-such as .cshrc for C-shell, or .bashrc for BASH-any variables you set in that file affect your program. You may wish to move setting of environment variables to files that are only run when you sign on, such as .login or .profile.

Your Program's Working Directory

Each time you start your program with run, it inherits its working directory from the current working directory of GDB. The GDB working directory is initially whatever it inherited from its parent process (typically the shell), but you can specify a new working directory in GDB with the cd command.

The GDB working directory also serves as a default for the commands that specify files for GDB to operate on, (see "Commands to Specify Files").

Your Program's Input and Output

By default, the program you run under GDB does input and output to the same terminal that GDB uses. GDB switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program.

You can redirect your program's input and/or output using shell redirection with the run command. For example, run > outfile starts your program, diverting its output to the file outfile. Another way to specify where your program should do input and output is with the tty command. This command accepts a file name as argument, and causes this file to be the default for future run commands.

It also resets the controlling terminal for the child process, for future run commands. For example, tty /dev/ttyb directs that processes started with subsequent run commands default to do input and output on the terminal /dev/ttyb and have that as their controlling terminal.

An explicit redirection in run overrides the tty command's effect no the input/output device, but not its effect on the controlling terminal.

When you use the tty command or redirect input in the run command, only the input for your program is affected. The input for GDB still comes from your terminal.

Debugging an Already-Running Process

This command attaches to a running process-one that was started outside GDB. (info files shows your active targets.) The command takes as argument a process ID. The usual way to find out the process-id of a UNIX process is with the ps utility, or with the jobs -l shell command.

To use attach, your program must be running in an environment which supports processes; for example, attach does not work for programs on bareboard targets that lack an operating system. You must also have permission to send the process a signal.

The first thing GDB does after arranging to debug the specified process is to stop it. You can examine and modify an attached process with all the GDB commands that are ordinarily available when you start processes with run. You can insert breakpoints; you can step and continue; you can modify storage. If you would rather the process continue running, you may use the continue command after attaching GDB to the process.

When you have finished debugging the attached process, you can use the detach command to release it from GDB control. Detaching the process continues its execution. After the detach command, that process and GDB become completely independent once more, and you are ready to attach another process or start one with run. detach does not repeat if you use Return again after executing the command.

If you exit GDB or use the run command while you have an attached process, you kill that process. By default, GDB asks for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the set confirm command (see "Optional Warnings and Messages").

Killing the Child Process

The kill command is also useful if you wish to recompile and relink your program, because on many systems it is impossible to modify an executable file while it is running in a process. In this case, when you next use run, GDB notices that the file has changed, and reads the symbol table again (while trying to preserve your current breakpoint settings).

Debugging Programs with Multiple Threads

In some operating systems, a single program may have more than one thread of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes-except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and perhaps private memory. GDB provides these facilities for debugging multithread programs:

The GDB thread debugging facility allows you to observe all threads while your program runs-but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the current thread. Debugging commands show program information from the perspective of the current thread.

Whenever GDB detects a new thread in your program, it displays the target system's identification for the thread with a message in the [New systag]. systag form is a thread identifier whose form varies, depending on the particular system. For example, on LynxOS, you might see [New process 35 thread 27] when GDB notices a new thread. In contrast, on an SGI system, the systag is simply something like process 368, with no further qualifier.

For debugging purposes, GDB associates its own thread number-always a single integer-with each thread in your program.

An asterisk (*) to the left of the GDB thread number indicates the current thread. Use the following example for clarity.

Make thread number threadno the current thread. The command argument threadno is the internal GDB thread number, as shown in the first field of the info threads display. GDB responds by displaying the system identifier of the thread you selected, and its current stack frame summary:

The thread apply command allows you to apply a command to one or more threads. Specify the numbers of the threads that you want affected with the command argument threadno. threadno is the internal GDB thread number, as shown in the first field of the info threads display. To apply a command to all threads, use thread apply all args.

Whenever GDB stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. GDB alerts you to the context switch with a message of the [Switching to systag] form to identify the thread.

Debugging Programs with Multiple Processes

GDB has no special support for debugging programs which create additional processes using the fork function. When a program forks, GDB will continue to debug the parent process and the child process will run unimpeded.

However, if you want to debug the child process there is a workaround which isn't too painful. Put a call to sleep in the code which the child process executes after the fork. It may be useful to sleep only if a certain environment variable is set, or a certain file exists, so that the delay need not occur when you don't want to run GDB on the child. While the child is sleeping, use the ps program to get its process ID. Then tell GDB (a new invocation of GDB if you are also debugging the parent process) to attach to the child process (see "Debugging an Already-Running Process"). From that point on you can debug the child process just like any other process to which you attached.

Stopping and Continuing

The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and determine causes.

Inside GDB, your program may stop for any of several reasons, such as a signal, a breakpoint, or reaching a new line after a GDB command such as step. You may then examine and change variables, set new breakpoints or remove old ones, and then continue execution. Usually, the messages shown by GDB provide ample explanation of the status of your program-but you can also explicitly request this information at any time.

The following documentation provides more specific discussion on breakpoints, watchpoints, exceptions, and other information regarding stopping and continuing GDB.

Breakpoints, Watchpoints, and Exceptions

A breakpoint makes your program stop whenever a certain point in the program is reached. For each breakpoint, you can add conditions to control in finer detail whether your program stops. You can set breakpoints with the break command and its variants (see "Setting Breakpoints") to specify the place where your program should stop by line, number function name or exact address in the program.

In languages with exception handling (such as GNU C++), you can also set Breakpoints where an exception is raised.

A watchpoint is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints, but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.

You can arrange to have values from your program displayed automatically whenever GDB stops at a breakpoint (see "Automatic Display").

GDB assigns a number to each breakpoint or watchpoint when you create it; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints, you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be ; if disabled, it has no effect on your program until you enable it again.

Setting Breakpoints

Breakpoints are set with the break command (abbreviated b). The debugger convenience variable $bpnum records the number of the breakpoints you have set most recently; see "Convenience Variables" for a discussion of what you can do with convenience variables.

Set a breakpoint at entry to function, function. When using source languages that permit overloading of symbols, such as C++, function may refer to more than one possible place to break.

Set a breakpoint at line linenum in the current source file. That file is the last file whose source text saw printed. This breakpoint stops your program just before it executes any of the code on that line.

Set a breakpoint at entry to function, function, found in file, filename. Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions.

When called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame (see "Examining the Stack"). In any selected frame but the innermost, this makes your program stop as soon as control returns to that frame.

This is similar to the effect of a finish command in the frame inside the selected frame-except that finish does not leave an active breakpoint. If you use break without an argument in the innermost frame, GDB stops the next time it reaches the current location; this may be useful inside loops. GDB normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint.

Set a breakpoint with condition, cond; evaluate the expression, cond, each time the breakpoint is reached, and stop only if the value is non-zero-that is, if cond, evaluates as true. `...' stands for one of the possible arguments described previously (or no argument) specifying where to break.

Set a breakpoint enabled only for one stop. args are the same as for the break command, and the breakpoint is set in the same way, but the breakpoint is automatically deleted after the first time your program stops there.

Set a hardware-assisted breakpoint. args are the same as for the break command and the breakpoint is set in the same way, but the breakpoint requires hardware support and some target hardware may not have this support. The main purpose of this is EPROM/ROM code debugging, so you can set a breakpoint at an instruction without changing the instruction. This can be used with the new trap-generation provided by SPARClite DSU. DSU will generate traps when a program accesses some date or instruction address that is assigned to the debug registers. However, the hardware breakpoint registers can only take two data breakpoints, and GDB will reject this command if more than two are used. Delete or disable used hardware breakpoints before setting new ones.

Set a hardware-assisted breakpoint enabled only for one stop. args are the same as for the hbreak command and the breakpoint is set in the same way. However, like the tbreak command, the breakpoint is automatically deleted after the first time your program stops there. Also, like the hbreak command, the breakpoint requires hardware support and some target hardware may not have this support.

Caution! The current release of LynxOS does not support hardware assisted breakpoints, The above are provided only for information only.

	Caution! The current release of LynxOS does not support hardware assisted breakpoints, The above are provided only for information only.

Set breakpoints on all functions matching the regular expression, regex. This command sets an unconditional breakpoint on all matches, printing a list of all breakpoints it set. Once these breakpoints are set, they are treated just like the breakpoints set with the break command. You can delete them, disable them, or make them conditional the same way as any other breakpoint. When debugging C++ programs, rbreak is useful for setting breakpoints on overloaded functions that are not members of any special classes.

Enabled breakpoints are marked with `y'. `n' marks breakpoints that are not enabled.

Where the breakpoint is in the source for your program, as a file and line number.

If a breakpoint is conditional, info break shows the condition on the line following the affected breakpoint; breakpoint commands, if any, follow.

info break with a breakpoint number n as argument lists only that breakpoint. The convenience variable $_ and the default examining-address for the x command are set to the address of the last breakpoint listed (see "Examining Memory").

info break now displays a count of the number of times the breakpoint has been hit. This is especially useful in conjunction with the ignore command. You can ignore a large number of breakpoint hits, look at the breakpoint info to see how many times the breakpoint was hit, and then run again, ignoring one less than that number. This will get you quickly to the last hit of that breakpoint.

GDB allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional this is even useful. GDB itself sometimes sets breakpoints in your program for special purposes, such as proper handling of longjmp (in C programs). These internal breakpoints are assigned negative numbers, starting with -1; info breakpoints does not display them. You can see these breakpoints with the GDB maintenance command maint info breakpoints.

Using the same format as info breakpoints, display both the breakpoints you have set explicitly, and those GDB is using for internal purposes. Internal breakpoints are shown with negative breakpoint numbers. The type column identifies what kind of breakpoint is shown:

Setting Watchpoints

You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen.

Watchpoints currently execute two orders of magnitude more slowly than other breakpoints, but this can be well worth it to catch errors where you have on clue what part of your program is the culprit.

Set a watchpoint for an expression. GDB will break when expr is written into by the program and its value changes. This can be used with the new trap-generation provided by SPARClite. DSU will generate traps when a program accesses some date or instruction address that is assigned to the debug registers. For the data addresses, DSU facilitates the watch command. However the hardware breakpoint registers can only take two data watchpoints, and both watchpoints must be the same kind. For example, you can set two watchpoints with watch commands, two with commands, or two with awatch commands, but you cannot set one watchpoint with one command and the other with a different command. {No value for "GBDN"} will reject the command if you try to mix watchpoints. Delete or disable unused watchpoint commands before setting new ones.

This command prints a list of watchpoints and breakpoints; it is the same as info break.

Caution! In multithread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, GDB can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression.

	Caution! In multithread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, GDB can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression.

Hardware Watchpoints

Watchpoints can be implemented in Software or Hardware. Hardware watchpoints execute quicker than software watchpoints and allows the debugger to report a change in value at the extact instruction where the change occured. Software watchpoints execute slower, and report a change in value in the statement following the change in value.

When setting a watchpoint, GDB attempts to set a hardware watchpoint first. If it is not possible to set a hardware watchpoint, a software watchpoint is set instead.

Hardware watchpoint num: expr

Note: Hardware Watchpoint support is not included in the default LynxOS kernel. To build the kernel for Hardware Watchpoint, Code Test, and Assertation support, use the following rule when running make:

# make all SYS_DEBUG=true

Note: Hardware Watchpoint support is not included in the default LynxOS kernel. To build the kernel for Hardware Watchpoint, Code Test, and Assertation support, use the following rule when running make: # make all SYS_DEBUG=true

Breakpoints and Exceptions

Some languages, such as GNU C++, implement exception handling. You can use GDB to examine what caused your program to raise an exception, and to list the exceptions your program is prepared to handle at a given point in time.

If you call a function interactively, GDB normally returns control to you when the function has finished executing. If the call raises an exception, however, the call may bypass the mechanism that returns control to you and cause your program to simply continue running until it hits a breakpoint, catches a signal that GDB is listening for, or exits.

Sometimes catch is not the best way to debug exception handling: if you need to know exactly where an exception is raised, it is better to stop before the exception handler is called, because that way you can see the stack before any unwinding takes place. If you set a breakpoint in an exception handler instead, it may not be easy to find out where the exception was raised.

To stop just before an exception handler is called, you need some knowledge of the implementation. In the case of GNU , C++ exceptions are raised by calling a library function named __cp_push_exception which has the following ANSI C interface:

To make the debugger catch all exceptions before any stack unwinding takes place, set a breakpoint on __cp_push_exception (see "Breakpoints, Watchpoints, and Exceptions").

With a conditional breakpoint that depends on the value of id, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to spot your program when any of a number of exceptions are raised.

Deleting Breakpoints

It is often necessary to eliminate a breakpoint or watchpoint once it has done its job and you no longer want your program to stop there. This is called deleting the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.

With the clear command you can delete breakpoints according to where they are in your program. With the delete command you can delete individual breakpoints or watchpoints by specifying their breakpoint numbers.

It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.

Delete any breakpoints at the next instruction to be executed in the selected stacks frame (see "Selecting a Frame"). When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped.

Delete the breakpoints or watchpoints of the numbers specified as arguments. If no argument is specified, delete all breakpoints GDB ( asks confirmation, unless you have set confirm off). You can abbreviate this command as d.

Disabling Breakpoints

Rather than deleting a breakpoint or watchpoint you might prefer to disable it. This makes the breakpoint inoperative as if it had been deleted, but remembers the information no the breakpoint so that you can enable it again later. You disable and enable breakpoints and watchpoints with the enable and disable commands, optionally specifying one or more breakpoint numbers as arguments. Use info break or info watch to print a list of breakpoints or watchpoints if you do not know which numbers to use. A breakpoint or watchpoint can have any of four different states of enablement:

The breakpoint stops your program. A breakpoint set with the break command starts out in this state.

The breakpoint stops your program, but then becomes disabled. A breakpoint set with the tbreak command starts out in this state.

The breakpoint stops your program, but immediately after it does so it is deleted permanently.

You can use the following commands to enable or disable breakpoints and watchpoints.

Disable the specified breakpoints-or all breakpoints, if none are listed. A disabled breakpoint has no effect but is not forgotten. All options such as ignore-counts, conditions and commands are remembered in case the breakpoint is enabled again later. You may abbreviate disable as dis.

Except for a breakpoint set with tbreak (see "Setting Breakpoints"), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the previously discussed commands. (The command until can set and delete a breakpoint of its own, but it does not change the state of your other breakpoints (see "Continuing and Stepping").

Break Conditions

The simplest sort of breakpoint breaks every time your program reaches a specified place. You can also specify a condition for a breakpoint. A condition is just a Boolean expression in your programming language (see "Expressions"). A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.

This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated-that is, when the condition is false. In C, if you want to test an assertion expressed by the condition, assert, you should set the condition ! assert on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not need them, because a watchpoint is inspecting the value of an expression anyhow-but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.

Break conditions can have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, GDB might see the other breakpoint first and stop your program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible for the purpose of performing side effects when a breakpoint is reached (see "Breakpoint Command Lists").

Break conditions can be specified when a breakpoint is set, by using if in the arguments to the break command (see "Setting Breakpoints"). They can also be changed at any time with the condition command. The watch command does not recognize the if keyword; condition is the only way to impose a further condition on a watchpoint.

Specify expression as the break condition for breakpoint or watchpoint number, bnum. After you set a condition, breakpoint bnum stops your program only if the value of expression is true (non-zero, in C). When you use condition, GDB checks expression immediately for syntactic correctness, and to determine whether symbols in it have referents in the context of your breakpoint. GDB does not actually evaluate expression at the time the condition command is given, however (see "Expressions").

A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times. This is so useful that there is a special way to do it, using the ignore count of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if your program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues. As a result, if the ignore count value is n, the breakpoint does not stop the next n times your program reaches it.

Set the ignore count of breakpoint number bnum to count. The next count times the breakpoint is reached, your program's execution does not stop; other than to decrement the ignore count, GDB takes no action.

To make the breakpoint stop the next time it is reached, specify a count of zero.

When you use continue to resume execution of your program from a breakpoint, you can specify an ignore count directly as an argument to continue, rather than using ignore (see "Continuing and Stepping").

If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, GDB resumes checking the condition.

You could achieve the effect of the ignore count with a condition such as $foo-- <= 0 using a debugger convenience variable that is decremented each time (see "Convenience Variables").

Breakpoint Command Lists

You can give any breakpoint or ( watchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints.

With no bnum argument, commands refers to the last breakpoint or watchpoint set (not to the breakpoint most recently encountered).

Using Return as a means of repeating the last GDB command is disabled within a command-list.

Simply use the continue command, or step, or any other command that resumes execution.

Any other commands in the command list are ignored, after a command that resumes execution. This is because any time you resume execution (even with a simple next or step), you may encounter another breakpoint- which could have its own command list, leading to ambiguities about which list to execute.

If the first command you specify in a command list is silent, the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue.

If none of the remaining commands print anything, you see no sign that the breakpoint was reached.

The commands echo, output, and printf allow you to print precisely controlled output, and are often useful in silent breakpoints (see "Commands for Controlled Output".)

For example, the following shows how to use breakpoint commands to print the value of x at entry to foo whenever x is positive.

One application for breakpoint commands is to compensate for one bug so you can test for another. Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them. End with the continue command so that your program does not stop, and start with the silent command so that no output is produced. The following is an example.

Breakpoint Menus

Some programming languages notably ( C++) permit a single function name to be defined several times for application in different contexts. This is called overloading. When a function name is overloaded, "break function" is not enough to tell GDB where you want a breakpoint. If you realize this is a problem, you can use something like "break function(types)" to specify which particular version of the function you want. Otherwise, GDB offers you a menu of numbered choices for different possible breakpoints, and waits for your selection with the > prompt . The first two options are always
[0] cancel and [1] all. Typing 1 sets a breakpoint at each definition of function, and typing 0 aborts the break command without setting any new breakpoints.

For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol String::after. The following shows three particular definitions of that function name:

(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] ;cc. String:file line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
>2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line .578
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the `delete' command to delete unwanted breakpoints.
(gdb)

Continuing and Stepping

Continuing means resuming program execution until your program completes normally. In contrast, stepping means executing just one more "step" of your program, where "step" may mean either one line of source code, or one machine instruction (depending on what particular command you use). Either when continuing or when stepping, your program may stop even sooner, due to a breakpoint or a signal. (If due to a signal, you may want to use handle, or use signal 0 to resume execution; see "Signals".)

Resume program execution, at the address where your program last stopped; any breakpoints set at that address are bypassed. The optional argument, ignore-count, allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of ignore.

The argument, ignore-count, is meaningful only when your program stopped due to a breakpoint. At other times, the argument to continue is ignored.

The synonyms, c and fg are provided purely for convenience, and have exactly the same behavior as continue.

A typical technique for using stepping is to set a breakpoint at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.

Continue running your program until control reaches a different source line, then stop it and return control to GDB. This command is abbreviated s.


	Caution! If you use the step command while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information. Likewise, it will not step into a function which is compiled without debugging information. To step through functions without debugging information, use the stepi command, described in the following.

The step command only stops at the first instruction of a source line. This prevents multiple stops that used to occur in switch statements, for loops, etc. step continues to stop if a function that has debugging information is called within the line.

Also, the step command now only enters a subroutine if there is line number information for the subroutine. Otherwise it acts like the next command. This avoids problems when using cc -gl on MIPS machines. Previously, step entered subroutines if there saw any debugging information about the routine.

Continue to the next source line in the current (innermost) stack frame. This is similar to step, but function calls that appear within the line of code are executed without stopping. Execution stops when control reaches a different line of code at the original stacks level that was executing when you gave the next command. This command is abbreviated n.

The next command now only stops at the first instruction of a source line. This prevents the multiple stops that used to occur in switch statements, for loops, etc.

Continue running until just after function in the selected stack frame returns. Print the returned value (if any). Contrast this with the return command (see "Returning from a Function").

Continue running until a source line past the current line in the current stack frame is reached. This command is used to avoid single stepping through a loop more than once. It is like the next command, except that when until encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump.

This means that when you reach the end of a loop after single stepping through it, until makes your program continue execution until it exits the loop. In contrast, a next command at the end of a loop simply steps back to the beginning of the loop, which forces you to step through the next iteration.

until may produce somewhat counter-intuitive results if the order of machine code does not match the order of the source lines. For instance, in the following example from a debugging session, the f (frame) command shows that execution is stopped at line 206. When we use until, we get to line 195:


(gdb) f
#0 main (argc=4,	argv=0xf7fffae8) at m4.c:206
206	expand_input();
(gdb) until
195	for ( ; argc > 0; NEXTARG) {

This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop-even though the test in a C for-loop is written before the body of the loop.

The until command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement-not in terms of the actual machine code.

until with no argument works by means of single instruction stepping and, hence, is slower than until with an argument.

Continue running your program until either the specified location is reached, or the current stack frame returns. location is any of the forms of argument acceptable to break (see "Setting Breakpoints").

It is often useful to use display/i $pc when stepping by machine instructions. This makes GDB automatically display the next instruction to be executed, each time your program stops. See "Automatic Display".

Signals

The operating system defines the possible kinds of signals, and gives each kind a name and a number. For example, in UNIX, SIGINT is the signal a program gets when you use an interrupt (often Ctrl-c); SIGSEGV is the signal a program gets from referencing a place in memory away from all the areas in use; SIGALRM occurs when the alarm clock timer goes off (which happens only if your program has requested an alarm).

Some signals, including SIGALRM, are a normal part of the functioning of your program. Others, such as SIGSEGV, indicate errors; these signals are fatal (kill your program immediately) if the program has not specified in advance some other way to handle the signal. SIGINT does not indicate an error in your program, but it is normally fatal so it can carry out the purpose of the interrupt: to kill the program.

GDB has the ability to detect any occurrence of a signal in your program. You can tell GDB in advance what to do for each kind of signal.

Normally, GDB is set up to ignore non-erroneous signals like SIGALRM so as not to interfere with their role in the functioning of your program, but to stop your program immediately whenever an error signal happens. You can change these settings with the handle command.

The keywords allowed by the handle command can be abbreviated. Their full names are:

When a signal stops your program, the signal is not visible until you continue. Your program sees the signal then, if pass is in effect for the signal in question at that time. In other words, after GDB reports a signal, you can use the handle command with pass or nopass to control whether your program sees that signal when you continue.

You can also use the signal command to prevent your program from seeing a signal, or cause it to see a signal it normally would not see, or to give it any signal at any time. For example, if your program stopped due to some sort of memory reference error, you might store correct values into the erroneous variables and continue, hoping to see more execution; but your program would probably terminate immediately as a result of the fatal signal once it was the signal. To prevent this, you can continue with signal 0.

Stopping and Starting Multithread Programs

When your program has multiple threads (see "Debugging Programs with Multiple Threads"), you can choose whether to set breakpoints on all threads, or on a particular thread.

Use the qualifier thread threadno with a breakpoint command to specify that you only want GDB to stop the program when a particular thread reaches this breakpoint. threadno is one of the numeric thread identifiers assigned by GDB, shown in the first column of the info threads display.

If you do not specify thread threadno when you set a breakpoint, the breakpoint applies to all threads of your program.

You can use the thread qualifier on conditional breakpoints as well; in this case, place thread threadno before the breakpoint condition, as the following example shows.

Whenever your program stops under GDB for any reason, all threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot.

Conversely, whenever you restart the program, all threads start executing. This is true even when single-stepping with commands such as step or next.

In particular, GDB cannot single-step all threads in lockstep. Because thread scheduling is up to your debugging target's operating system (not controlled by GDB), other threads may execute more than one statement while the current thread completes a single step. Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops.

You might even find your program stopped in another thread after continuing or even single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested.

Examining the Stack

The following documentation discusses GDB, stack frames and other related topics.

When your program has stopped, the first thing you need to know is where it stopped and how it got there.

Each time your program performs a function call, information about the call is generated. That information includes the location of the call in your program, the arguments of the call, and the local variables of the function being called. The information is saved in a block of data called a stack frame. The stack frames are allocated in a region of memory called the call stack. When your program stops, the GDB commands for examining the stack allow you to see all of this information.

One of the stack frames is selected by GDB and many GDB commands refer implicitly to the selected frame. In particular, whenever you ask GDB for the value of a variable in your program, the value is found in the selected frame. There are special GDB commands to select whichever frame you are interested in (see "Selecting a Frame").

When your program stops, GDB automatically selects the currently executing frame and describes it briefly, similar to the frame command (see "Information about a Frame").

Stack Frames

The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing.

When your program is started, the stack has only one frame, that of the function main. This is called the initial frame or the outermost frame. Each time a function is called, a new frame is made. Each time a function returns, the frame for that function invocation is eliminated. If a function is recursive, there can be many frames for the same function. The frame for the function in which execution is actually occurring is called the innermost frame. This is the most recently created of all the stack frames that still exist.

Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this address is kept in a register called the frame pointer register while execution is going on in that frame.

GDB assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward.

These numbers do not really exist in your program; they are assigned by GDB to give you a way of designating stack frames in GDB commands.

Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the -fomit-frame-pointer gcc option generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. GDB has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, GDB nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, GDB has no provision for frameless functions elsewhere in the stack.

The frame command allows you to move from one stack frame to another, and to print the stack frame you select. args may be either the address of the frame of the stack frame number. Without an argument, frame prints the current stack frame.

Backtraces

A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.

Print a backtrace of the entire stack: one line per frame for all frames in the stack. You can stop the backtrace at any time by using the system interrupt character, normally Ctrl-c.

The names where and info stack (abbreviated info s) are additional aliases for backtrace.

Each line in the backtrace shows the frame number and the function name. The program counter value is also shown-unless you use set print address off. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number. Here is an example of a backtrace. It was made with the command bt 3, so it shows the innermost three frames.


#0	m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
	at builtin.c:993
#1	0x6e38 in expand_macro (sym=0x2b600) at macro.c:242
#2	0x6840 in expand_token (obs=0x0, t=177664, td=fffb08)
	at macro.c:71
(More stack frames to follow...)

The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line 993 of builtin.c.

Selecting a Frame

Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.

Select frame number n. Recall that frame zero is the innermost (currently executing) frame, frame one is the frame that called the innermost one, and so on. The highest-numbered frame is the one for main.

Select the frame at address, addr. This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDB to assign numbers properly to all frames. In addition, this can be useful when your program has multiple stacks and switches between them.

On the SPARC architecture, frame needs two addresses to select an arbitrary frame: a frame pointer and a stack pointer.

On the MIPS and Alpha architecture, it needs two addresses: a stack pointer and a program counter.

On the 29k architecture, it needs three addresses: a register stack pointer, a program counter, and a memory stack pointer.

Move n frames down the stack. For positive numbers n, this advances toward the innermost frame, to lower frame numbers, to frames that were created more recently. n defaults to one. You may abbreviate down as do.

All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line. For instance, use the following as an example.


(gdb)up
#1	0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc) at env.c:10
10	read_input_file (argv[i]);

After such a printout, the list command with no arguments prints ten lines centered on the point of execution in the frame (see "Printing Source Lines").

These two commands are variants of up and down, respectively; they differ in that they do their work silently, without causing display of the new frame. They are intended primarily for use in GDB command scripts, where the output might be unnecessary and distracting.

Information about a Frame

There are several other commands to print information about the selected stack frame.

When used without any argument, this command does not change which frame is selected, but prints a brief description of the currently selected stack frame. It can be abbreviated f. With an argument, this command is used to select a stack frame (see "Selecting a Frame").

Print a verbose description of the frame at address addr, without selecting that frame. The selected frame remains unchanged by this command. This requires the same kind of address (more than one for some architectures) that you specify in the frame command (see "Selecting a Frame").

Print the local variables of the selected frame, each on a separate line. These are all variables (declared either static or automatic) accessible at the point of execution of the selected frame.

Print a list of all the exception handlers that are active in the current stack frame at the current point of execution. To see other exception handlers, visit the associated frame (using the up, down, or frame commands); then type: info catch. See "Breakpoints, Watchpoints, and Exceptions"

MIPS Machines and the Function Stack

MIPS based computers use an unusual stack frame, which sometimes requires GDB to search backward in the object code to find the beginning of a function.

To improve response time (especially for embedded applications, where GDB may be restricted to a slow serial line for this search) you may want to limit the size of this search, using one of these commands:

A value of 0 (the default) means there is no limit. However, except for 0, the larger the limit the more bytes heuristic-fence-post must search and therefore the longer it takes to run.

These commands are available only when GDB is configured for debugging programs on MIPS processors.

Examining Source Files

GDB can print parts of your program's source, because the debugging information recorded in the program tells GDB what source files were used to build it. When your program stops, GDB spontaneously prints the line where it stopped. Likewise, when you select a stack frame (see "Selecting a Frame"). GDB prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command.

See the following documentation for more specific discussion on source files and GDB.

If you use GDB through its GNU Emacs interface, you may prefer to use Emacs facilities to view source (see "Using GDB under GNU Emacs").

Printing Source Lines

To print lines from a source file, use the list command (abbreviated l). By default, 10 lines are printed. There are several ways to specify what part of the file you want to print. The following are the forms of the list command most commonly used:

Print more lines. If the last lines printed were printed with a list command, this prints lines following the last lines printed; however, if the last line printed was a solitary line printed as part of displaying a stack frame (see "Examining the Stack"), this prints lines centered around that line.

Repeating a list command using Return discards the argument, so it is equivalent to typing list. This is more useful than listing the same lines again. An exception is made for an argument of -; that argument is preserved in repetition so that each repetition moves up in the source file.

In general, the list command expects you to supply zero, one or two linespecs which specify source lines. There are several ways of writing linespecs, but the effect is always to specify some source line. Here is a complete description of the possible arguments for list:

The following are the ways of specifying a single source line-all the kinds of linespec.

Specifies the line offset lines after the last line printed. When used as the second linespec in a list command that has two, this specifies the line offset lines down from the first linespec.

Specifies the line of the open-brace that begins the body of the function function in the file, filename. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files.

Searching Source Files

There are two commands for searching through the current source file for a regular expression.

The forward-search regexp command checks each line, starting with the one following the last line listed, for a match for regexp. It lists the line that is found. You can use the synonym, search regexp, or abbreviate the command name as fo.

The reverse-search regexp command checks each line, starting with the one before the last line listed and going backward, for a match for regexp. It lists the line that is found. You can abbreviate this command as rev.

Specifying Source Directories

Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. GDB has a list of directories to search for source files; this is called the source path. Each time GDB wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name.


Note: The executable search path is not used for this purpose. Neither is the current working directory, unless it happens to be in the source path.

If GDB cannot find a source file in the source path, and the object program records a directory, GDB tries that directory too. If the source path is empty, and there is no record of the compilation directory, GDB looks in the current directory as a last resort.

Whenever you reset or rearrange the source path, GDB clears out any information it has cached about where source files are found and where each line is in the file. When you start GDB, its source path is empty. To add other directories, use the directory command.

Add directory, dirname, to the front of the source path. Several directory names may be given to this command, separated by a colon (:) or whitespace. You may specify a directory that is already in the source path; this moves it forward, so GDB searches it sooner.

You can use the $cdir string to refer to the compilation directory (if one is recorded), and $cwd to refer to the current working directory. $cwd is not the same as a period (.) -the former tracks the current working directory as it changes during your GDB session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path.

If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation by the following methods.

Source and Machine Code

You can use the info line command to map source lines to program addresses (and vice versa), and the disassemble command to display a range of addresses as machine instructions.

When run under GNU Emacs mode, the info line command now causes the arrow to point to the line specified. Also, info line prints addresses in symbolic form as well as hex.

Print the starting and ending addresses of the compiled code for source line linespec. Specify source lines in any of the ways understood by the list command (see "Printing Source Lines").

For instance, we can use info line to discover the location of the object code for the first line of function, m4_changequote, as in the following example.

We can also inquire (using *addras, the form for linespec) what source line covers a particular address, as in the following example.

After info line, the default address for the x command is changed to the starting address of the line, so that x/i is sufficient to begin examining the machine code (see "Examining Memory"). Also, this address is saved as the value of the convenience variable, $_ (see "Convenience Variables").

This specialized command dumps a range of memory as machine instructions. The default memory range is the function surrounding the program counter of the selected frame. A single argument to this command is a program counter value; GDB dumps the function surrounding this value. Two arguments specify a range of addresses (first inclusive, second exclusive) to dump.

We can use disassemble to inspect the object code range shown in the last info line example (the example shows SPARC machine instructions):

(gdb) disas 0x63e4 0x6404
Dump of assembler code from 0x63e4 to 0x6404:
0x63e4 <builtin_init+5340>: ble 0x63f8<builtin_init+5360>
0x63e8 <builtin_init+5344>: sethi %hi(0x4c00), %o0
0x63ec <builtin_init+5348>: ld [%i1+4], %o0
0x63f0 <builtin_init+5352>: 0x63fc <builtin_init+5364>
0x63f4 <builtin_init+5356>: ld [%o0+4], %o0
0x63f8 <builtin_init+5360>: or %o0, 0x1a4, %o0
0x63fc <builtin_init+5364>: call 0x9288 <path_search>
0x6400 <builtin_init+5368>: nop
End of assembler dump.

Examining Data

The usual way to examine data in your program is with the print command (abbreviated p), or its synonym, inspect. It evaluates and prints the value of an expression of the language your program is written in. See "Using GDB with Different Languages"

exp is an expression (in the source language). By default the value of exp is printed in a format appropriate to its data type; you can choose a different format by specifying /f, where f is a letter specifying the format (see "Output Formats").

A more low-level way of examining data is with the x command. It examines data in memory at a specified address and prints it in a specified format. See "Examining Memory".

If you are interested in information about types, or about how the fields of a struct or class are declared, use the ptype exp command rather than print. See "Examining the Symbol Table"

Expressions

print and many other GDB commands accept an expression and compute its value. Any kind of constant, variable or operator defined by the programming language you are using is valid in an expression in GDB. This includes conditional expressions, function calls, casts and string constants. It unfortunately does not include symbols defined by preprocessor #define commands.

GDB now supports array constants in expressions input by the user. The syntax is element, element .... For example, you can now use the command, print {1 2 3} to build up an array in memory that is memory allocated in the target program.

Note: Because C is so widespread, most of the expressions shown in examples in this manual are in C. See "Using GDB with Different Languages"

In this section, we discuss operators that you can use in GDB expressions regardless of your programming language.

Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer in order to examine a structure at that address in memory.

GDB supports these operators, in addition to those common to programming languages:

Refers to an object of type, type, stored at address, addr, in memory. addr may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast). This construct is allowed regardless of what kind of data is normally supposed to reside at addr.

Program Variables

The most common kind of expression to use is the name of a variable in your program. Variables in expressions are understood in the selected stack frame (see "Selecting a Frame"); they must be either global (or static) or visible according to the scope rules of the programming language from the point of execution in that frame. Consider the following function example.

This means that you can examine and use the variable, a, whenever your program is executing within the function, foo, but you can only use or examine the variable, b, while your program is executing inside the block where b is declared. There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file. But it is possible to have more than one such variable or function with the same name (in different source files). If that happens, referring to that name has unpredictable effects. If you wish, you can specify a static variable in a particular function or file, using the colon-colon notation as in the following example.

Here file or function is the name of the context for the static variable. In the case of file names, you can use quotes to make sure GDB parses the file name as a single word-for example, to print a global value of x defined in f2.c, use (gdb) p 'f2.c'::x.

This use of :: is very rarely in conflict with the very similar use of the same notation in C++. GDB also supports use of the C++ scope resolution operator in GDB expressions.

Caution! Occasionally, a local variable may appear to have the wrong value at certain points in a function-just after entry to a new scope, and just before exit. You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.

	Caution! Occasionally, a local variable may appear to have the wrong value at certain points in a function-just after entry to a new scope, and just before exit. You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.

Artificial Arrays

It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program.

You can do this by referring to a contiguous span of memory as an artificial array, using the @ binary operator. The left operand of @ should be the first element of the desired array and be an individual object. The right operand should be the desired length of the array. The result is an array value whose elements are all of the type of the left argument. The first element is actually the left argument; the second element comes from bytes of memory immediately following those holding the first element, and so on.

The left operand of @ must reside in memory. Array values made with @ in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (see "Value History"), after printing one out.

Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory:

As a convenience, if you leave the array length out, as in (type[])value, GDB calculates a size to fill the value, as sizeof(value)/sizeof(type) as the following example shows.

Sometimes, the artificial array mechanism is not quite enough; in moderately complex data structures, the elements of interest may not actually be adjacent-for example, if you are interested in the values of pointers in an array. One useful work-around in this situation is to use a convenience variable (see "Convenience Variables") as a counter in an expression that prints the first interesting value, and then repeat that expression using Return. For instance, suppose you have an array, dtab, of pointers to structures, and you are interested in the values of a field, fv, in each structure. The following is an example of what you might type:

Output Formats

By default, GDB prints a value according to its data type. Sometimes this is not what you want. For example, you might want to print a number in hex, or a pointer in decimal. Or you might want to view data in memory at a certain address as a character string or as an instruction. To do these things, specify an output format when you print a value.

The simplest use of output formats is to say how to print a value already computed. This is done by starting the arguments of the print command with a slash and a format letter. The format letters supported are shown below.


Letter Value	Definition
x	Regard the bits of the value as an integer, and print the integer in hexadecimal.
d	Print as integer in signed decimal.
u	Print as integer in unsigned decimal.
o	Print as integer in octal.
t	Print as integer in binary. The letter `t' stands for "two" (`b' cannot be used because these format letters are also used with the x command, where `b' stands for "byte" (see "Examining Memory").
	(gdb) p/a 0x54320 $3 = 0x54320 <_initialize_vx+396
c	Regard as an integer and print it as a character constant.
f	Regard the bits of the value as a floating point number and print using typical floating point syntax.

For example, to print the program counter in hex (see "Registers"), type p/x $pc. No space is required before the slash because command names in GDB cannot contain a slash.

To reprint the last value in the value history with a different format, you can use the print command with just a format and no expression. For example, p/x reprints the last value in hex.

Examining Memory

You can use the x command (for "examine") to examine memory in any of several formats, independently of your program's data types.

n, f, and u are all optional parameters that specify how much memory to display and how to format it; addr is an expression giving the address where you want to start displaying memory. If you use defaults for nfu, you need not type the slash, `/'. Several commands set convenient defaults for addr.

The display format is one of the formats used by print, s (null-terminated string), or i (machine instruction). The default is x (hexadecimal) initially. The default changes each time you use either x or print.

The unit size is shown in the following table.

Unit Size

Type

Unit

b

Bytes

h

Half words (two bytes)

w

Words (four bytes); this is the initial default.

g

Giant words (eight bytes)

Unit Size
Type	Unit
b	Bytes
h	Half words (two bytes)
w	Words (four bytes); this is the initial default.
g	Giant words (eight bytes)

Each time you specify a unit size with x, that size becomes the default unit the next time you use x. (For the s and i formats, the unit size is ignored and is normally not written.)

addr is the address where you want GDB to begin displaying memory. The expression need not have a pointer value (though it may); it is always interpreted as an integer address of a byte of memory. See "Expressions". The default for addr is usually just after the last address examined-but several other commands also set the default address: info breakpoints (to the address of the last breakpoint listed), info line (to the starting address of a line), and print (if you use it to display a value from memory).

For example, x/3uh0x54320 is a request to display three half words (h) of memory, formatted as unsigned decimal integers (u), starting at address 0x54320.x/4xw$sp prints the four words (w) of memory above the stack pointer (here, $sp; (see "Registers") in hexadecimal (x).

Because the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works. The output specifications 4xw and 4wx mean exactly the same thing. (The count must come first; wx4 does not work.)

Even though the unit size u is ignored for the formats s and i, you might still want to use a count n. For example, 3i specifies that you want to see three machine instructions, including any operands. The disassemble command gives an alternative way of inspecting machine instructions; see "Source and Machine Code".

All the defaults for the arguments to x are designed to make it easy to continue scanning memory with minimal specifications each time you use x. For example, after you have inspected three machine instructions with x/3iaddr, you can inspect the next seven with just x/7. If you use Return to repeat the x command, the repeat count n is used again; the other arguments default as for successive uses of x.

The addresses and contents printed by the x command are not saved in the value history because there is often too much of them and they would get in the way. Instead, GDB makes these values available for subsequent use in expressions as values of the convenience variables $_ and $__. After an x command, the last address examined is available for use in expressions in the convenience variable $_. The contents of that address, as examined, are available in the convenience variable, $__.

If the x command has a repeat count, the address and contents saved are from the last memory unit printed; this is not the same as the last address printed if several units were printed on the last line of output.

Automatic Display

If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the automatic display list so that GDB prints its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like the following:

This display shows item numbers, expressions and their current values. As with displays you request manually, using x or print, you can specify the output format you prefer; in fact, display decides whether to use print or x depending on how elaborate your format specification is-it uses x if you specify a unit size, or one of the two formats (i and s) that are only supported by x; otherwise it uses print.

For fmt specifying only a display format and not a size or count, add the expression exp to the auto-display list but arrange to display it each time in the specified format, fmt (see "Output Formats").

For fmt i or s, or including a unit-size or a number of units, add the expression, addr, as a memory address to be examined each time your program stops. Examining means in effect doing
x/fmt addr (see "Examining Memory").

For example, display/i $pc can be helpful, to see the machine instruction about to be executed each time execution stops ($pc is a common name for the program counter; see "Registers").

Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values. This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available.

If a display expression refers to local variables, then it does not make sense outside the lexical context for which it was set up. Such an expression is disabled when execution enters a context where one of its variables is not defined. For example, if you give the command, display last_char, while inside a function with an argument, last_char, GDB displays this argument while your program continues to stop inside that function. When it stops elsewhere-where there is no variable, last_char, the display is disabled automatically. The next time your program stops where last_char is meaningful, you can enable the display expression once again.

Print Settings

GDB provides the following ways to control how arrays, structures, and symbols are printed. These settings are useful for debugging programs in any language:

GDB prints memory addresses showing the location of stack traces, structure values, pointer values, breakpoints, and so forth, even when it also displays the contents of those addresses. The default is on.

You can use set print address off to eliminate all machine dependent displays from the GDB interface. For example, with print address off, you should get the same text for backtraces on all machines-whether or not they involve pointer arguments.

When GDB prints a symbolic address, it normally prints the closest earlier symbol plus an offset. If that symbol does not uniquely identify the address (for example, it is a name whose scope is a single source file), you may need to clarify.

Alternately, you can set GDB to print the source file and line number when it prints a symbolic address:

Another situation where it is helpful to show symbol filenames and line numbers is when disassembling code; GDB shows you the line number and source file that corresponds to each instruction.

Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol:

Tell GDB to only display the symbolic form of an address if the offset between the closest earlier symbol and the address is less than max-offset. The default is 0, which tells GDB to always print the symbolic form of an address if any symbol precedes it.

If you have a pointer and you are not sure where it points, try
set print symbol-filename on. Then, you can determine the name and source file location of the variable where it points, using p/a pointer. This interprets the address in symbolic form. For instance, the following shows that a variable, ptt, points at another variable, t, defined in hi2.c:

(gdb) set print symbol-filename on
(gdb) p/a ptt
$4 = 0xe008 <t in hi2.c

Caution! For pointers that point to a local variable, p/a does not show the symbol name and filename of the referent, even with the appropriate set print options turned on.

	Caution! For pointers that point to a local variable, p/a does not show the symbol name and filename of the referent, even with the appropriate set print options turned on.

Set a limit on how many elements of an array GDB will print. If GDB is printing a large array, it stops printing after it has printed the number of elements set by the set print elements command. This limit also applies to the display of strings. Setting number-of-elements to zero means that the printing is unlimited.

Print using only seven-bit characters; if this option is set, GDB displays any eight-bit characters (in strings or character values) using the notation, \nnn. This setting is best if you are working in English (ASCII) and you use the high-order bit of characters as a marker or "meta" bit.

Decode using the algorithm in the C++ Annotated Reference Manual

Note: This setting alone is not sufficient to allow debugging cfront-generated executables. GDB would require further enhancement to permit that functionality.

.

Note: This setting alone is not sufficient to allow debugging cfront-generated executables. GDB would require further enhancement to permit that functionality.

Value History

Values printed by the print command are saved in the GDB value history. This allows you to refer to them in other expressions. Values are kept until the symbol table is reread or discarded (for example with the file or symbol-file commands). When the symbol table changes, the value history is discarded, becausebecause the values may contain pointers back to the types defined in the symbol table.

The values printed are given history numbers by which you can refer to them. These are successive integers starting with one. print shows you the history number assigned to a value by printing $num= before the value; num is the history number.

To refer to any previous value, use $ followed by the value's history number. The way print labels its output is designed to remind you of this. Just $ refers to the most recent value in the history, and $$ refers to the value before that. $$n refers to the nth value from the end; $$2 is the value just prior to $$, $$1 is equivalent to $$, and $$0 is equivalent to $.

For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure. It suffices to type p *$.

If you have a chain of structures where the component next points to the next one, you can print the contents of the next one with p *$.next. You can print successive links in the chain by repeating this command- which you can do by just using Return.

Note that the history records values, not expressions. Consider, for instance, if the value of x is 4 and you type the following example's commands.

Convenience Variables

GDB provides convenience variables that you can use within GDB to hold on to a value and refer to it later. These variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely.

Convenience variables are prefixed with $. Any name preceded by $ can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see "Registers"). Value history references, in contrast, are numbers preceded by $ (see "Value History").

You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example, set $foo = *object_ptr would save in $foo the value contained in the object pointed to by object_ptr.

Using a convenience variable for the first time creates it, but its value is void until you assign a new value. You can alter the value with another assignment at any time. Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value.

One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For instance, to print a field from successive elements of an array of structures, use the following as an example.

Some convenience variables are created automatically by GDB and given values likely to be useful.

The $_ variable is automatically set by the x command to the last address examined (see "Examining Memory"). Other commands which provide a default address for x to examine also set $_ to that address; these commands include info line and info breakpoint. The type of $_ is void* except when set by the x command, in which case it is a pointer to the type of $__.

Registers

You can refer to machine register contents, in expressions, as variables with names starting with $. The names of registers are different for each machine; use info registers to see the names used on your machine.

Print the relativized value of each specified register, regname. As discussed in the following, register values are normally relative to the selected stack frame. regname may be any register name valid on the machine you are using, with or without the initial $.

GDB has four "standard" register names that are available (in expressions) on most machines-whenever they do not conflict with an architecture's canonical mnemonics for registers. The register names $pc and $sp are used for the program counter register and the stack pointer. $fp is used for a register that contains a pointer to the current stack frame, and $ps is used for a register that contains the processor status. For example, you could print the program counter in hex with p/x $pc, or print the instruction to be executed next with x/i $pc, or add four to the stack pointer with
set $sp += 4. This is a way of removing one word from the stack, on machines where stacks grow downward in memory (most machines, nowadays). This assumes that the innermost stack frame is selected; setting $sp is not allowed when other stack frames are selected. To pop entire frames off the stack, regardless of machine architecture, use Return (see "Returning from a Function").

Whenever possible, these four standard register names are available on your machine even though the machine has different canonical mnemonics, so long as there is no conflict. The info registers command shows the canonical names. For example, on the SPARC, info registers displays the processor status register as $psr but you can also refer to it as $ps.

GDB always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with print/f $regname).

Some registers have distinct "raw" and "virtual" data formats. This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees. For example, the registers of the 68881 floating point coprocessor are always saved in "extended" (raw) format, but all C programs expect to work with "double" (virtual) format. In such cases, GDB normally works with the virtual format only (the format that makes sense for your program), but the info registers command prints the data in both formats.

Normally, register values are relative to the selected stack frame (see "Selecting a Frame"). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with frame 0).

However, GDB must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if GDB is unable to locate the saved registers, the selected stack frame makes no difference.

On AMD 29000 family processors, registers are saved in a separate "register stack". There is no way for GDB to determine the extent of this stack. Normally, GDB just assumes that the stack is "large enough". This may result in GDB referencing memory locations that do not exist. If necessary, you can get around this problem by specifying the ending address of the register stack with the set rstack_high_ address command. The argument should be an address, which you probably want to precede with 0x to specify in hexadecimal.

Floating Point Hardware

Depending on the configuration, GDB may be able to give you more information about the status of the floating point hardware.

Display hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip. Currently, info float is supported on the ARM and x86 machines.

Using GDB with Different Languages

Although programming languages generally have common aspects, they are rarely expressed in the same manner. For instance, in ANSI C, dereferencing a pointer, p, is accomplished by *p, but in Modula-2, it is accomplished by p^. Values can also be represented (and displayed) differently. Hex numbers in C appear as 0x1ae, while in Modula-2 they appear as 1AEH.

Language-specific information is built into GDB for some languages, allowing you to express operations like the previous in your program's native language, and allowing GDB to output values in a manner consistent with the syntax of your program's native language. The language you use to build expressions is called the working language.

See the following documentation for more specific discussion on languages that GDB accommodates.


Note: Although GDB is designed to support multiple languages, LynuxWorks currently supports only GDB for C, C++, and assembly languages.

Switching between Source Languages

There are two ways to control the working language-either have GDB set it automatically, or select it manually yourself. You can use the set language command for either purpose. On startup, GDB defaults to setting the language automatically. The working language is used to determine how expressions you type are interpreted, how values are printed, and so on.

In addition to the working language, every source file that GDB knows about has its own working language. For some object file formats, the compiler might indicate which language a particular source file is in. However, most of the time GDB infers the language from the name of the file. The language of a source file controls whether C++ names are demangled-this way backtrace can show each frame appropriately for its own language. There is no way to set the language of a source file from within GDB. This is most commonly a problem when you use a program, such as cfront or f2c, that generates C but is written in another language. In that case, make the program use #line directives in its C output; that way GDB will know the correct language of the source code of the original program, and will display that source code, not the generated C code.

List of Filename Extensions and Languages

If a source file name ends in one of the following extensions, then GDB infers that its language is the one indicated.

*Assembler source files behave almost like C, but GDB does not skip over function prologues when stepping.

Setting the Working Language

If you allow GDB to set the language automatically, expressions are interpreted the same way in your debugging session and your program. If you wish, you may set the language manually. To do this, issue the set language lang command, where lang is the name of a language, such as c or modula-2. For a list of the supported languages, type set language.

Setting the language manually prevents GDB from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages-but means different things. For instance, if the current source file were written in C, and GDB was parsing Modula-2, a command such as print a =b +c might not have the effect you intended. In C, this means to add b and c and place the result in a. The result printed would be the value of a. In Modula-2, this means to compare a to the result of b+c, yielding a Boolean value.

Having GDB Infer the Source Language

To have GDB set the working language automatically, use set language local or set language auto. GDB then infers the working language. That is, when your program stops in a frame (usually by encountering a breakpoint), GDB sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDB issues a warning.

This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using set language auto in this case frees you from having to set the working language manually.

Displaying the Language

The following commands help you find out which language is the working language, and also what language in which source files were written.

Display the source language for this frame. This language becomes the working language if you use an identifier from this frame. See "Information about a Frame" to identify the other information listed here.

Type and Range Checking

Caution! In this release, the GDB commands for type and range checking are included, but they do not yet have any effect. This section documents the intended facilities.

	Caution! In this release, the GDB commands for type and range checking are included, but they do not yet have any effect. This section documents the intended facilities.

Some languages are designed to guard against you making seemingly common errors through a series of compile and run-time checks. These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when your program is running.

GDB can check for conditions like the previous if you wish. Although GDB does not check the statements in your program, it can check expressions entered directly into GDB for evaluation via the print command, for example. As with the working language, GDB can also decide whether or not to check automatically based on your program's source language. See "Supported Languages," later in this chapter for the default settings of supported languages.

An Overview of Type Checking

Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. Consider the two following examples.

The second example fails because the CARDINAL 1 is not type-compatible with the REAL 2.3.

For the expressions you use in GDB commands, you can tell the GDB type checker to skip checking; to treat any mismatches as errors and abandon the expression; or to only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, GDB evaluates expressions like the second example, but also issues a warning.

Even if you turn type checking off, there may be other reasons related to type that prevent GDB from evaluating an expression. For instance, GDB does not know how to add an int and a struct foo. These particular type errors have nothing to do with the language in use, and usually arise from expressions, such as the one described which make little sense to evaluate anyway.

Each language defines to what degree it is strict about type. For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers. In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators. See "Supported Languages" for further details on specific languages.

GDB provides the following additional commands for controlling the type checker.

Set type checking on or off, overriding the default setting for the current working language. Issue a warning if the setting does not match the language default. If any type mismatches occur in evaluating an expression while type checking is on, GDB prints a message and aborts evaluation of the expression.

Cause the type checker to issue warnings, but to always attempt to evaluate the expression. Evaluating the expression may still be impossible for other reasons. For example, GDB cannot add numbers and structures.

An Overview of Range Checking

In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array. For expressions you use in GDB commands, you can tell GDB to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway. A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to "wrap around" to lower values-for example, if m is the largest integer value, and s is the smallest, then

This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. See "Supported Languages" for further details on specific languages. GDB provides some additional commands for controlling the range checker:

Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language default. If a range error occurs, then a message is printed and evaluation of the expression is aborted.

Output messages when the GDB range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many UNIX systems).

Supported Languages

GDB 4 supports C, C++, and Modula-2. Some GDB features may be used in expressions regardless of the language you use: the GDB @ and:: operators, and the {type}addr construct (see "Expressions") can be used with the constructs of any supported language. The following sections detail to what degree each source language is supported by GDB. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDB expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.

C and C++

Since C and C++ are so closely related, many features of GDB apply to both languages. Whenever this is the case, we discuss those languages together.

The C++ debugging facilities are jointly implemented by the GNU C++ compiler and GDB. Therefore, to debug your C++ code effectively, you must compile your C++ programs with the GNU C++ compiler, g++.

For best results when debugging C++ programs, use the stabs debugging format. You can select that format explicitly with the g++ command-line options -gstabs or -gstabs+. See "Options for Debugging Your Program or GNU CC" in Using GNU CC in GNUPro Compiler Tools for more information.

C and C++ Operators

Operators must be defined on values of specific types. For instance, + is defined on numbers and not on structures. Operators are often defined on groups of types. For the purposes of C and C++, the following definitions hold:

The following operators are supported, listed in order of increasing precedence:


,	The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.
=	Assignment. The value of an assignment expression is the value assigned. Defined on scalar types.
op=	Used in an expression of the form a op=b, and translated to a= a opb. op= and = have the same precedence. op is any one of the operators \|, ^, &, <<, >> , +, -, *, /, %.
?:	The ternary operator. a?b: c can be thought of as: if a, then b, else, c. a should be of an integral type.
\|\|	Logical OR. Defined on integral types.
&&	Logical AND. Defined on integral types.
\|	Bitwise OR. Defined on integral types.
^	Bitwise exclusive-OR. Defined on integral types.
&	Bitwise AND . Defined on integral types.
==, !=	Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.
<, >, <=, >=	Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.
<<, >>	Left shift, and right shift. Defined on integral types.
@	The GDB "artificial array" operator (see "Expressions" earlier in this chapter).
+, -	Addition and subtraction. Defined on integral types, floating-point types and pointer types.
*, /, %	Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types.
++, --	Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable's value is used before the operation takes place.
*	Pointer dereferencing. Defined on pointer types. Same precedence as ++.
&	Address operator. Defined on variables. Same precedence as ++.

For debugging C++, GDB implements a use of & beyond what is allowed in the C++ language itself: you can use &(&ref) (or, if you prefer, &&ref) to examine the address where a C++ reference variable (declared with &ref) is stored.


-	Negative. Defined on integral and floating-point types. Same precedence as ++.
!	Logical negation. Defined on integral types. Same precedence as ++.
~	Bitwise complement operator. Defined on integral types. Same precedence as ++.
., ->	Structure member, and pointer-to-structure member. For convenience, GDB regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on struct and union data.
[]	Array indexing. a[i] is defined as *(a+i). Same precedence as ->.
()	Function parameter list. Same precedence as ->.
::	C++ scope resolution operator. Defined on struct, union, and class types.
::	Doubled colons also represent the GDB scope operator ("Expressions"). Same precedence as ::.

C and C++ Constants

Integer constants are a sequence of digits. Octal constants are specified by a leading 0 (i.e., zero), and hexadecimal constants by a leading 0x or 0X. Constants may also end with a letter, l, specifying that the constant should be treated as a long value.

Floating point constants are a sequence of digits, followed by a decimal point, followed by a sequence of digits, and optionally followed by an exponent. An exponent is of the form: e[[+]|-]nnn, where nnn is another sequence of digits. The + is optional for positive exponents.

Enumerated constants consist of enumerated identifiers, or their integral equivalents.

Character constants are a single character surrounded by single quotes ('), or a number-the ordinal value of the corresponding character (usually its ASCII value). Within quotes, the single character may be represented by a letter or by escape sequences, which are of the form \nnn, where nnn is the octal representation of the character's ordinal value; or of the form \x, where x is a predefined special character-for example, \n for newline.

String constants are a sequence of character constants surrounded by double quotes (" ").

Pointer constants are an integral value. You can also write pointers to constants using the C operator, &.

Array constants are comma-separated lists surrounded by braces { and }; for example, {1,2,3} is a three-element array of integers, {{1,2}, {3,4}, {5,6}} is a three-by-two array, and {&"hi", &"there", &"fred"} is a three-element array of pointers.

C++ Expressions

GDB expression handling has a number of extensions to interpret a significant subset of C++ expressions.


	Caution! GDB can only debug C++ code if you compile with the GNU C++ compiler. Moreover, C++ debugging depends on the use of additional debugging information in the symbol table, and thus requires special support. GDB has this support only with the stabs debug format. In particular, if your compiler generates a.out, MIPS ECOFF, RS/6000 XCOFF, or ELF with stabs extensions to the symbol table, these facilities are all available. (With GNU CC, you can use the `-gstabs' option to request stabs debugging extensions explicitly.) Where the object code format is standard COFF or DWARF in ELF , on the other hand, most of the C++ support in GDB does not work.

While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, GDB allows implicit references to the class instance pointer, this, following the same rules as C++.

You can call overloaded functions; GDB resolves the function call to the right definition, with one restriction-you must use arguments of the type required by the function that you want to call. GDB does not perform conversions requiring constructors or user-defined type operators.

GDB understands variables declared as C++ references; you can use them in expressions just as you do in C++ source-they are automatically dereferenced.

In the parameter list shown when GDB displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified set print address off.

GDB supports the C++ name resolution operator ::-your expressions can use it just as expressions in your program do. Since one scope may be defined in another, you can use :: repeatedly if necessary, for example in an expression such as scope1::scope2::name. GDB also allows resolving name scope by reference to source files, in both C and C++ debugging (see "Program Variables" earlier in this chapter).

C and C++ Defaults

If you allow GDB to set type and range checking automatically, they both default to off whenever the working language changes to C or C++.

If you allow GDB to set the language automatically, it recognizes source files whose names end with .c, .C, or .cc, and when GDB enters code compiled from one of these files, it sets the working language to C or C++. See "Having GDB Infer the Source Language," earlier in this chapter, for further details.

C and C++ Type and Range Checks

By default, when GDB parses C or C++ expressions, type checking is not used. However, if you turn type checking on, GDB considers two variables type equivalent if:

Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.

GDB and C

The set print union and show print union commands apply to the union type. When set to on, any union that is inside a struct or class is also printed. Otherwise, it appears as {...}.

The @ operator aids in the debugging of dynamic arrays, formed with pointers and a memory allocation function (see "Expressions").

GDB features for C++

Some GDB commands are particularly useful with C++, and some are designed specifically for use with C++. The following is a summary:

When you want a breakpoint in a function whose name is overloaded, GDB breakpoint menus help you specify which function definition you want (see "Breakpoint Menus," earlier in this chapter).

set print demangle
show print demangle
set print asm-demangle
show print asm-demangle

Overloaded Symbol Names

You can specify a particular definition of an overloaded symbol, using the same notation that is used to declare such symbols in C++: type symbol(types) rather than just symbol. You can also use the GDB command-line word completion facilities to list the available choices, or to finish the type list for you. See "Command Completion," earlier in this chapter, for details on how to perform this function.

Examining the Symbol Table

The commands described in this section allow you to inquire about the symbols (names of variables, functions and types) defined in your program. This information is inherent in the text of your program and does not change as your program executes. GDB finds it in your program's symbol table, in the file indicated when you started GDB (see "Choosing Files," earlier in this chapter), or by one of the file-management commands (see "Commands to Specify Files," later in this chapter).

Occasionally, you may need to refer to symbols that contain unusual characters, which GDB ordinarily treats as word delimiters. The most frequent case is in referring to static variables in other source files (see "Program Variables," earlier in this chapter). File names are recorded in object files as debugging symbols, but GDB would ordinarily parse a typical file name, such as foo.c, as the three words foo , . , and c. To allow GDB to recognize foo.c as a single symbol, enclose it in single quotes; for example, p 'foo.c'::x looks up the value of x in the scope of the file `foo.c'.

Describe where the data for symbol is stored. For a register variable, this says which register it is kept in. For a non-register local variable, this prints the stack-frame offset at which the variable is always stored.

Note: The contrast with print &symbol does not work at all for a register variable, and for a stack local variable prints the exact address of the current instantiation of the variable.

Note: The contrast with print &symbol does not work at all for a register variable, and for a stack local variable prints the exact address of the current instantiation of the variable.

Print the data type of expression exp. exp is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place (see "Expressions").

Print a description of the type of expression exp. ptype differs from whatis by printing a detailed description, instead of just the name of the type. For instance, consider the following variable declaration example.

Print a brief description of all types whose name matches regexp (or all types in your program, if you supply no argument). Each complete typename is matched as though it were a complete line; thus, i type value gives information on all types in your program whose name includes the string value, but i type ^value$ gives information only on types whose complete name is value.

Print the names of all source files in your program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed.

Print the names and data types of all defined functions whose names contain a match for regular expression, regexp. Thus, info fun step finds all functions whose names include step; info fun ^step finds those whose names start with step.

Some systems allow individual object files that make up your program to be replaced without stopping and restarting your program. If you are running on one of these systems, you can allow GDB to reload the symbols for the following automatically relinked modules:

Do not replace symbol definitions when re-encountering object files of the same name. This is the default state; if you are not running on a system that permits automatically relinking modules, you should leave symbol-reloading off, since otherwise GDB may discard symbols when linking large programs, that may contain several modules (from different directories or libraries) with the same name.

maint print symbolsfilename
maint print psymbolsfilename
maint print msymbolsfilename

If you use maint print symbols, GDB includes all the symbols for which it has already collected full details: that is, filename reflects symbols for only those files whose symbols GDB has read.

You can use the info sources command to find out which files these are. If you use maint print psymbols instead, the dump shows information about symbols that GDB only knows partially-that is, symbols defined in files that GDB has skimmed, but not yet read completely.

Finally, maint print msymbols dumps just the minimal symbol information required for each object file from which GDB has read some symbols. See "Commands to Specify Files," later in this chapter for a discussion of how GDB reads symbols (in the description of symbol-file).

Altering Execution

Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run. You can find the answer by experiment, using the GDB features for altering execution of the program.

For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function.

Assignment to Variables

To alter the value of a variable, evaluate an assignment expression (see "Expressions"). For example, print x=4 stores the value 4 into the variable, x, and then prints the value of the assignment expression (which is 4). See "Using GDB with Different Languages"

If you are not interested in seeing the value of the assignment, use the set command instead of the print command. set is really the same as print except that the expression's value is not printed and is not put in the value history (see "Value History"). The expression is evaluated only for its effects.

If the beginning of the argument string of the set command appears identical to a set subcommand, use the set variable command instead of only set. This command is identical to set except for its lack of subcommands.

For example, if your program has a variable, width, you get an error if you try to set a new value with just set width=13, because GDB has the command set width:

The invalid expression, of course, is =47. In order to actually set the program's variable, width, use (gdb) set var width=47.

GDB allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and you can convert any structure to any other structure that is the same length or shorter. To store values into arbitrary places in memory, use the {...} construct to generate a value of specified type at a specified address (see "Expressions"). For example, {int}0x83040 refers to memory location 0x83040 as an integer (which implies a certain size and representation in memory), and set {int}0x83040 = 4 stores the value 4 into that memory location.

Continuing at a Different Address

Ordinarily, when you continue your program, you do so at the place where it stopped, with the continue command. You can instead continue at an address of your own choosing, with the following commands.

Resume execution at line, linespec. Execution stops again immediately if there is a breakpoint there. See "Printing Source Lines" for a description of the different forms of linespec.

The jump command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter. If line, linespec, is in a different function from the one currently executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables. For this reason, the jump command requests confirmation if the specified line is not in the function currently executing. However, even bizarre results are predictable if you are well acquainted with the machine-language code of your program.

You can get much the same effect as the jump command by storing a new value into the register, $pc. The difference is that this does not start your program running; it only changes the address of where it will run when you continue. For example, set $pc = 0x485 makes the next continue command or stepping command execute at address, 0x485, rather than at the address where your program stopped (see "Continuing and Stepping").

The most common occasion to use the jump command is to back up, perhaps with more breakpoints set, over a portion of a program that has already executed, in order to examine its execution in more detail.

Giving Your Program a Signal

Resume execution where your program stopped, but immediately give it the signal signal. signal can be the name or the number of a signal. For example, on many systems signal 2 and signal SIGINT are both ways of sending an interrupt signal.

Alternatively, if signal is zero, continue execution without giving a signal. This is useful when your program stopped on account of a signal and would ordinarily see the signal when resumed with the continue command; signal 0 causes it to resume without a signal.

signal does not repeat when you use Return a second time after executing the command.

Invoking the signal command is not the same as invoking the kill utility from the shell. Sending a signal with kill causes GDB to decide what to do with the signal, depending on the signal handling tables (see "Signals"). The signal command passes the signal directly to your program.

Returning from a Function

When you use return, GDB discards the selected stack frame (and all frames within it). You can think of this as making the discarded frame return prematurely. If you wish to specify a value to be returned, give that value as the argument to return.

This pops the selected stack frame (see "Selecting a Frame"), and any other frames inside of it, leaving its caller as the innermost remaining frame. That frame becomes selected. The specified value is stored in the registers used for returning values of functions.

The return command does not resume execution; it leaves the program stopped in the state that would exist if the function had just returned.

In contrast, the finish command (see "Continuing and Stepping") resumes execution until the selected stack frame returns naturally.

Calling Program Functions

You can use this variant of the print command if you want to execute a function from your program, but without cluttering the output with void returned values. If the result is not void, it is printed and saved in the value history.

A new user-controlled variable, call_scratch_address, specifies the location of a scratch area to be used when GDB calls a function in the target. This is necessary because the usual method of putting the scratch area on the stack does not work in systems that have separate instruction and data spaces.

Patching Programs

By default, GDB opens the file containing your program's executable code (or the corefile) read-only. This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program's binary.

If you'd like to be able to patch the binary, you can specify that explicitly with the set write command. For example, you might want to turn on internal debugging flags, or even to make emergency repairs.

If you specify set write on, GDB opens executable and core files for both reading and writing; if you specify set write off (the default), GDB opens them read-only. If you have already loaded a file, you must load it again (using the exec-file or core-file commands) after changing set write, for your new setting to take effect.

GDB Files

GDB needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program. To debug a core dump of a previous run, you must also tell GDB the name of the core dump file.

The following provides more details on command specification and symbol files with GDB.

Commands to Specify Files

You may want to specify executable and core dump file names. The usual way to do this is at start-up time, using the arguments to GDB's start-up commands (see "Getting In and Out of GDB").

Occasionally it is necessary to change to a different file during a GDB session. Or you may run GDB and forget to specify a file you want to use. In these situations the GDB commands to specify new files are useful.

Use filename as the program to be debugged. It is read for its symbols and for the contents of pure memory. It is also the program executed when you use the run command. If you do not specify a directory and the file is not found in the GDB working directory, GDB uses the environment variable, PATH, as a list of directories to search, just as the shell does when looking for a program to run. You can change the value of this variable, for both GDB and your program, using the path command.

Specify that the program to be run (but not the symbol table) is found in filename. GDB searches the environment variable, PATH, if necessary to locate your program. Omitting filename means to discard information on the executable file.

symbol-file with no argument clears out GDB information on your program's symbol table. The symbol-file command causes GDB to forget the contents of its convenience variables, the value history, and all breakpoints and auto-display expressions. This is because they may contain pointers to the internal data recording symbols and data types, which are part of the old symbol table data being discarded inside GDB.

When GDB is configured for a particular environment, it understands debugging information in whatever format is the standard generated for that environment; you may use either a GNU compiler, or other compilers that adhere to the local conventions. Best results are usually obtained from GNU compilers; for example, using gcc you can generate debugging information for optimized code.

On some kinds of object files, the symbol-file command does not normally read the symbol table in full right away. Instead, it scans the symbol table quickly to find which source files and which symbols are present. The details are read later, one source file at a time, as they are needed.

The purpose of this two-stage reading strategy is to make GDB start up faster. For the most part, it is invisible except for occasional pauses while the symbol table details for a particular source file are being read. (The set verbose command can turn these pauses into messages if desired, see "Optional Warnings and Messages".)

We have not implemented the two-stage strategy for COFF yet. When the symbol table is stored in COFF format, symbol-file reads the symbol table data in full right away.

You can override the GDB two-stage strategy for reading symbol tables by using the -readnow option with any of the commands that load symbol table information, if you want to be sure GDB has the entire symbol table available.

You can use both options together, to make sure the auxiliary symbol file has all the symbol information for your program. The auxiliary symbol file for a program called myprog is called myprog.syms. Once this file exists (so long as it is newer than the corresponding executable), GDB always attempts to use it when you debug myprog; no special options or commands are needed.

Specify the whereabouts of a core dump file to be used as the "contents of memory". Traditionally, core files contain only some parts of the address space of the process that generated them; GDB can access the executable file itself for other parts.


Note: The core file is ignored when your program is actually running under GDB. So, if you have been running your program and you wish to debug a core file instead, you must kill the subprocess in which the program is running. To do this, use the kill command (see "Killing the Child Process").

Depending on what remote debugging facilities are configured into GDB, the load command may be available. Where it exists, it is meant to make filename (an executable) available for debugging on the remote system-by downloading, or dynamic linking, e.g., load also records the filename symbol table in GDB, like the
add-symbol-file command.

If your GDB does not have a load command, attempting to execute it gets the error message "You can't do that when your target is...."

The file is loaded at whatever address is specified in the executable. For some object file formats, you can specify the load address when you link the program; for other formats, like a.out, the object file format specifies a fixed address.

add-symbol-filefilename address
add-symbol-filefilename address[-readnow][-mapped]

The add-symbol-file command reads additional symbol table information from the file, filename. You would use this command when filename has been dynamically loaded (by some other means) into the program that is running. address should be the memory address at which the file has been loaded; GDB cannot figure this out for itself. You can specify address as an expression.

The symbol table of the file, filename, is added to the symbol table originally read with the symbol-file command. You can use the command add-symbol-file any number of times; the new symbol data thus read keeps adding to the old. To discard all old symbol data instead, use the symbol-file command.

You can use the -readnow option, just as with the symbol-file command, to change how GDB manages the symbol table information for filename.

The section command changes the base address of section, SECTION, of the exec file to ADDR. This can be used if the exec file does not contain section addresses (such as in the a.out format), or when the addresses specified in the file itself are wrong. Each section must be changed separately. The info files command lists all the sections and their addresses.

info files and info target are synonymous; both print the current target (see "Specifying a Debugging Target"), including the names of the executable and core dump files currently in use by GDB, and the files from which symbols were loaded. The help target command lists all possible targets rather than current ones.

All file-specifying commands allow both absolute and relative file names as arguments. GDB always converts the file name to an absolute file name and remembers it that way.

Load shared object library symbols for files matching a UNIX regular expression. As with files loaded automatically, it only loads shared libraries required by your program for a core file or after using run. If regex is omitted, all shared libraries required by your program are loaded.

Errors Reading Symbol Files

While reading a symbol file, GDB occasionally encounters problems, such as symbol types it does not recognize, or known bugs in compiler output. By default, GDB does not notify you of such problems, since they are relatively common and primarily of interest to people debugging compilers.

If you are interested in seeing information about ill-constructed symbol tables, you can either ask GDB to print only one message about each such type of problem, no matter how many times the problem occurs; or you can ask GDB to print more messages, to see how many times the problems occur, with the set complaints command as shown in "Optional Warnings and Messages".

The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements). This error indicates that an inner scope block is not fully contained in its outer scope blocks.

GDB circumvents the problem by treating the inner block as if it had the same scope as the outer block. In the error message, symbol may be shown as "(don't know)" if the outer block is not a function.

GDB does not circumvent this problem, and has trouble locating symbols in the source file whose symbols it is reading. (You can often determine what source file is affected by specifying set verbose on. See "Optional Warnings and Messages".

The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line. This is known to occur in the SunOS 4.1.1 (and earlier) C compiler.

Symbol number n contains a pointer into the string table which is larger than the size of the string table. GDB circumvents the problem by considering the symbol to have the name, foo, which may cause other problems if many symbols end up with this name.

GDB circumvents the error by ignoring this symbol information. This usually allows you to debug your program, though certain symbols are not accessible. If you encounter such a problem and feel like debugging it, you can debug gdb with itself, breakpoint on complain, then go up to the function read_dbx_symtab and examine *bufp to see the symbol.

Specifying a Debugging Target

A target is the execution environment occupied by your program. Often, GDB runs in the same host environment as your program; in that case, the debugging target is specified as a side effect when you use the file or core commands. When you need more flexibility-for example, running GDB on a physically separate host, or controlling a standalone system over a serial port or a realtime system over a TCP/IP connection-you can use the target command to specify one of the target types configured for GDB.

Active Targets

There are three classes of targets: processes, core files, and executable files.

GDB can work concurrently on up to three active targets, one in each class. This allows you to (for example) start a process and inspect its activity without abandoning your work on a core file.

For example, if you execute gdb a.out, then the executable file, a.out, is the only active target. If you designate a core file as well-presumably from a prior run that crashed and coredumped-then GDB has two active targets and uses them in tandem, looking first in the corefile target, then in the executable file, to satisfy requests for memory addresses. (Typically, these two classes of target are complementary, since core files contain only a program's read-write memory-variables and so on-plus machine status, while executable files contain only the program text and initialized data.)

When you type run, your executable file becomes an active process target as well. When a process target is active, all GDB commands requesting memory addresses refer to that target; addresses in an active core file or executable file target are obscured while the process target is active.

Commands for Managing Targets

Connects the GDB host environment to a target machine or process. A target is typically a protocol for talking to debugging facilities. You use the argument, type, to specify the type or protocol of the target machine.

GDB uses its own library, BFD, to read your files. GDB knows whether it is reading an executable, a core, or a .o file; however you can specify the file format with the set gnutarget command.

Unlike most target commands, with gnutarget, the target refers to a program, not a machine.

Caution! To specify a file format with set gnutarget, you must know the actual BFD name. See "Commands to Specify Files".

	Caution! To specify a file format with set gnutarget, you must know the actual BFD name. See "Commands to Specify Files".

Use the show gnutarget command to display what file format
gnutarget is set to read. If you have not set gnutarget, GDB will determine the file format for each file automatically and show gnutarget displays this message:

The following are some common targets (available, or not, depending on the GDB configuration).

Remote serial target in GDB-specific protocol. The argument, dev, specifies what serial device to use for the connection (e.g., /dev/ttya); see "Remote Debugging". target remote now supports the load command. This is only useful if you have some other way of getting the stub to the target system, and you can put it somewhere in memory where it won't get clobbered by the download.

Different targets are available on different configurations of GDB; your configuration may have more or fewer targets.

Remote Debugging

If you are trying to debug a program running on a machine that cannot run GDB in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger.

Some configurations of GDB have special serial or TCP/IP interfaces to make this work with particular debugging targets. In addition, GDB comes with a generic serial protocol (specific to GDB, but not specific to any particular target system) which you can use if you write the remote stubs-the code that runs on the remote system to communicate with GDB.

Other remote targets may be available in your configuration of GDB; use help target to list them.

Using the gdbserver program

gdbserver is a control program for UNIX-like systems, which allows you to connect your program with a remote GDB via target remote-but without linking in the usual debugging stub.

GDB and gdbserver communicate via either a serial line or a TCP connection, using the standard GDB remote serial protocol.

On the Target Machine

You need to have a copy of the program you want to debug. gdbserver does not need your program's symbol table, so you can strip the program if necessary to save space. GDB on the host system does all the symbol handling. To use the server, you must tell it how to communicate with GDB; the name of your program; and the arguments for your program. The syntax is: target gdbserver comm program [args...].

comm is either a device name (to use a serial line) or a TCP hostname and portnumber. For example, to debug Emacs with the argument, foo.txt, and communicate with GDB over the serial port, /dev/com1, use the following:

gdbserver waits passively for the host GDB to communicate with it. To use a TCP connection instead of a serial line, use the following:

The only difference from the previous example is the first argument, specifying that you are communicating with the host GDB via TCP. The host:2345 argument means that gdbserver is to expect a TCP connection from machine host to local TCP port 2345. (Currently, the host part is ignored.) You can choose any number you want for the port number as long as it does not conflict with any TCP ports already in use on the target system. If you choose a port number that conflicts with another service, gdbserver prints an error message and exits.

On the GDB Host Machine

You need an unstripped copy of your program, since GDB needs symbols and debugging information.

Start up GDB as usual, using the name of the local copy of your program as the first argument. (You may also need the --baud option if the serial line is running at anything other than 9600 bps.)

Its argument is either a device name (usually a serial device like /dev/ttyb) or a TCP port descriptor in the form, host:port. For example, (gdb) target remote /dev/ttyb communicates with the server via serial line, /dev/ttyb.

(gdb) target remote target:2345 communicates via a TCP connection to port 2345 on host, target. For TCP connections, you must start up gdbserver prior to using the target remote command. Otherwise you may get an error whose text depends on the host system, but which usually looks something like "connection refused."

Stored Command Sequences

Aside from breakpoint commands (see "Breakpoint Command Lists"), GDB provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.

User-Defined Commands

A user-defined command is a sequence of GDB commands to which you assign a new name as a command. This is done with the define command. User commands may accept up to 10 arguments separated by whitespace. Arguments are accessed within the user command via
$arg0 ...$arg9. A trivial example is the following:


Note: The arguments are text substitutions, so they may reference variables, use complex expressions, or even perform inferior function calls.

The definition of the command is made up of other GDB command lines, which are given following the define command. The end of these commands is marked by a line containing end.

Takes a single argument, which is an expression to evaluate. It is followed by a series of commands that are executed only if the expression is true (non-zero). There can then optionally be a line else, followed by a series of commands that are only executed if the expression was false. The end of the list is marked by a line containing end.

The syntax is similar to if: the command takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an end. The commands are executed repeatedly as long as the expression evaluates to true.

Document the user-defined commandname command so that it can be accessed by help. The commandname command must already be defined. This command reads lines of documentation just as define reads the lines of the command definition, ending with end. After the document command is finished, help on command, commandname, displays the documentation you have written. You may use the document command again to change the documentation of a command. Redefining the command with define does not change the documentation.

When user-defined commands are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command. If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command. Many GDB commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.

User-Defined Command Hooks

You may define hooks, which are a special kind of user-defined command. Whenever you run the foo command, if the user-defined hook-foo command exists, it is executed (with no arguments) before that command. In addition, a pseudo-command, stop, exists. Defining hook-stop makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed. For example, to ignore SIGALRM signals while single-stepping, but treat them normally during normal execution, you could define the following debugging input.

You can define a hook for any single-word command in GDB, but not for command aliases; you should define a hook for the basic command name, e.g., backtrace rather than bt. If an error occurs during the execution of your hook, execution of GDB commands stops and GDB issues a prompt (before the command that you actually used had a chance to run).

If you try to define a hook which does not match any known command, you get a warning from the define command.

Command Files

Comments (lines starting with #) may also be included. An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal. When you start GDB, it automatically executes commands from its init files. These are files named .gdbinit. GDB reads the init file (if any) in your home directory, then processes command line options and operands, and then reads the init file (if any) in the current working directory. This is so the init file in your home directory can set options (such as set complaints) which affect the processing of the command line options and operands. The init files are not executed if you use the -nx option; see "Choosing Modes". You can also request the execution of a command file with the source command:

Commands that would ask for confirmation if used interactively proceed without asking when used in a command file. Many GDB commands that normally print messages to say what they are doing omit the messages when called from command files.

Commands for Controlled Output

During the execution of a command file or a user-defined command, normal GDB output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. The following documentation describes commands useful for generating exactly the output you want.

Print text. Non-printing characters can be included in text using C escape sequences, such as ` ' to print a newline.

Note: No newline is printed unless you specify one.

Note: No newline is printed unless you specify one.

In addition to the standard C escape sequences, a backslash followed by a space stands for a space. This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print and foo =, use the echo \ and foo = \ command. A backslash at the end of text can be used, as in C, to continue the command on to subsequent lines.

The previous example shows input that produces the same output as the following.

Print the value of expression and nothing but that value: no newlines, no $ nn= . The value is not entered in the value history either. See "Expressions" for more information on expressions.

Print the values of the expressions under the control of string. The expressions are separated by commas and may be either numbers or pointers. Their values are printed as specified by string, exactly as if your program were to execute the C subroutine, as in the following example.

The only backslash-escape sequences that you can use in the format string are the simple ones that consist of backslash followed by a letter.

Using GDB under GNU Emacs

A special interface allows you to use GNU Emacs to view (and edit) the source files for the program you are debugging with GDB.

To use this interface, use the command M-x gdb in Emacs. Give the executable file you want to debug as an argument. This command starts GDB as a subprocess of Emacs, with input and output through a newly created Emacs buffer.

Using GDB under Emacs is just like using GDB normally, except all "terminal" input and output goes through the Emacs buffer. This applies both to GDB commands and their output, and to the input and output done by the program you are debugging. This is useful because it means that you can copy the text of previous commands and input them again; you can even use parts of the output in this way. All the facilities of Emacs' Shell mode are available for interacting with your program. In particular, you can send signals the usual way-for example, Ctrlc, Ctrl-c for an interrupt, Ctrl-c, Ctrl-z for a stop. GDB displays source code through Emacs.

Each time GDB displays a stack frame, Emacs automatically finds the source file for that frame and puts an arrow (=>) at the left margin of the current line. Emacs uses a separate buffer for source display, and splits the screen to show both your GDB session and the source.

Explicit GDB list or search commands still produce output as usual, but you probably have no reason to use them from Emacs.


	Caution! If the directory where your program resides is not your current directory, it can be easy to confuse Emacs about the location of the source files, in which case the auxiliary display buffer does not appear to show your source.

GDB can find programs by searching your environment's PATH variable, so the GDB input and output session proceeds normally; but Emacs does not get enough information back from GDB to locate the source files in this situation.

To avoid this problem, either start GDB mode from the directory where your program resides, or specify an absolute file name when prompted for the M-x gdb argument.

A similar confusion can result if you use the GDB file command to switch to debugging a program in some other location, from an existing GDB buffer in Emacs.

By default, using the keystroke sequence, M-x gdb calls the program called gdb. If you need to call GDB by a different name (for example, if you keep several configurations around, with different names) you can set the Emacs variable gdbcommand-name.

For example, (setq gdb-command-name "mygdb")-which is preceded by using the keystroke sequence, Esc, Esc, or typed in the *scratch* buffer, or in your .emacs file-makes Emacs call the "mygdb" program instead.

In the GDB I/O buffer, you can use these special keystroke sequences of Emacs commands in addition to the standard Shell mode commands in Table: Shell Mode Commands.

Shell Mode Commands
Command	Description
C-h, m	Describe the features of Emacs' GDB Mode.
M-s	Execute to another source line, like the GDB step command; also update the display window to show the current file and location.
M-n	Execute to next source line in this function, skipping all function calls, like the GDB next command. Then update the display window to show the current file and location.
M-i	Execute one instruction, like the GDB stepi command; update display window accordingly.
M-x, gdb-nexti	Execute to next instruction, using the GDB nexti command; update display window accordingly.
Ctrl-c, Ctrl-f	Execute until exit from the selected stack frame, like the GDB finish command.
M-c	Continue execution of your program, like the GDB continue command. Note: In Emacs version 19, this command uses the Ctrl-c, Ctrl-p keystroke sequence.
M-u	Go up the number of frames indicated by the numeric argument (see "Numeric Arguments" in The GNU Emacs Manual), like the GDB up command. Note: In Emacs version 19, this command uses the Ctrl-c, Ctrl-u keystroke sequence.
M-d	Go down the number of frames indicated by the numeric argument, like the GDB down command. Note: In Emacs version 19, this command uses the Ctrl-c, Ctrl-d keystroke sequence.
Ctrl-x, &	Read the number where the cursor is positioned, and insert it at the end of the GDB I/O buffer. For example, if you wish to disassemble code around an address that was displayed earlier, type disassemble; then move the cursor to the address display, and pick up the argument for disassemble by using the Ctrl-x, & keystroke sequence. You can customize this further by defining elements of the list gdb-print-command; once it is defined, you can format or otherwise process numbers picked up by using the Ctrl-x, & keystroke sequence before they are inserted. A numeric argument to Ctrl-x, & indicates that you wish special formatting, and also acts as an index to pick an element of the list. If the list element is a string, the number to be inserted is formatted using the Emacs function format; otherwise the number is passed as an argument to the corresponding list element.

In any source file, the Emacs command using the C-x, Spacebar keystroke sequence and typing (gdb-break) tells GDB to set a breakpoint on the source line point.

If you accidentally delete the source-display buffer, an easy way to get it back is to type the command, f, in the GDB buffer, to request a frame display; when you run under Emacs, this recreates the source buffer if necessary to show you the context of the current frame.

The source files displayed in Emacs are in ordinary Emacs buffers which are visiting the source files in the usual way. You can edit the files with these buffers if you wish; but keep in mind that GDB communicates with Emacs in terms of line numbers.

If you add or delete lines from the text, the line numbers that GDB knows cease to correspond properly with the code.

Command Line Editing

Introduction to Line Editing

Note: The text Ctrl-K is read as "Control K" and describes the command to produce when using the Control and the K keys sequence. The text M-K is read as "Meta K" and describes the command to produce when using the meta key (if you have one, it may be the key with a diamond), and the K key. If you do not have a meta key, the identical keystroke can be generated by using the Esc key and then K. Either process is known as "meta-fying the K key." The text M-Ctrl-K is read as "Meta Control K" and describes the command to produce when asked to "meta-fy C K."

Note: The hyphen characters and the comma characters are not a part of the keystroke sequence to type.

Note: The hyphen characters and the comma characters are not a part of the keystroke sequence to type.

All uppercase letters require using the shift key, of course, since all commands are case sensitive.

In addition, several keys have their own names. Specifically, Delete, Esc, LFD (linefeed), Spacebar, Return, and Tab all stand for themselves when seen in this text or in an init file. See "Readline Init File" for more information.

Readline Interaction

Often during an interactive session you type in a long line of text, only to notice that the first word on the line is misspelled. The Readline library gives you a set of commands for manipulating the text as you type it in, allowing you to just fix your typo, and not forcing you to retype the majority of the line. Using these editing commands, you move the cursor to the place that needs correction, and delete or insert the text of the corrections. Then, when you are satisfied with the line, you simply use Return. You do not have to be at the end of the line to use Return; the entire line is accepted regardless of the location of the cursor within the line.

Readline Bare Essentials

In order to enter characters into the line, simply type them. The typed character appears where the cursor was, and then the cursor moves one space to the right. If you mistype a character, you can use Delete to back up, and delete the mistyped character.

Sometimes you may miss typing a character that you wanted to type, and not notice your error until you have typed several other characters. In that case, you can use Ctrl-B to move the cursor to the left, and then correct your mistake. Afterward, you can move the cursor to the right with Ctrl-F.

When you add text in the middle of a line, you will notice that characters to the right of the cursor get "pushed over" to make room for the text that you have inserted. Likewise, when you delete text behind the cursor, characters to the right of the cursor get "pulled back" to fill in the blank space created by the removal of the text. A list of the basic essentials for editing the text of an input line are in the following table.


Command	Action
Ctrl-B	Move back one character.
Ctrl-F	Move forward one character.
Delete	Delete the character to the left of the cursor.
Ctrl-D	Delete the character underneath the cursor.
Printing characters	Insert itself into the line at the cursor.
Ctrl-_	Undo the last thing that you did. You can undo all the way back to an empty line.

Readline Movement Commands

The previous commands are the most basic possible keystrokes that you need in order to do editing of the input line. For your convenience, many other commands have been added in addition to Ctrl-B, Ctrl-F, Ctrl-D, and Delete.


Command	Action
Ctrl-A	Move to the start of the line.
Ctrl-E	Move to the end of the line.
M-F	Move forward a word.
M-B	Move backward a word.
Ctrl-L	Clear the screen, reprinting the current line at the top.

Notice how Ctrl-F moves forward a character, while M-F moves forward a word. It is a loose convention that control keystrokes operate on characters while meta keystrokes operate on words.

Readline Killing Commands

Killing text means to delete the text from the line, but to save it away for later use, usually by yanking it back into the line. If the description for a command says that it "kills" text, then you can be sure that you can get the text back in a different (or the same) place later. Table 2-8 is a list of commands for killing text.


Command	Action
Ctrl-K	Kill the text from the current cursor position to the end of the line.
M-D	Kill from the cursor to the end of the current word, or if between words, to the end of the next word.
M-Delete	Kill from the cursor to the start of the previous word, or if between words, to the start of the previous word.
Ctrl-W	Kill from the cursor to the previous whitespace.
Ctrl-L	Clear the screen, reprinting the current line at the top. This is different than M-Delete because the word boundaries differ.


Command	Action
Ctrl-Y	Yank the most recently killed text back into the buffer at the cursor.Yank the most recently killed text back into the buffer at the cursor.
M-Y	Rotate the kill-ring, and yank the new top. You can only do this if the prior command is Ctrl-Y or M-Y.

When you use a kill command, the text is saved in a kill-ring. Any number of consecutive kills save all of the killed text together, so that when you yank it back, you get it in one clean sweep. The kill ring is not line specific; the text that you killed on a previously typed line is available to be yanked back later, when you are typing another line.

Readline Arguments

You can pass numeric arguments to Readline commands. Sometimes the arguments act as a repeat count, other times it is the sign of the argument that is significant. If you pass a negative argument to a command which normally acts in a forward direction, that command will act in a backward direction. For example, to kill text back to the start of the line, you might use M-- Ctrl-K.

The general way to pass numeric arguments to a command is to type meta digits before the command. If the first digit you type is a minus sign (-), then the sign of the argument will be negative. Once you have typed one meta digit to get the argument started, you can type the remainder of the digits, and then the command. For example, to give the Ctrl-D command an argument of 10, you could use the keystroke sequence, M-1, 0, Ctrl-D.

Readline Init File

Although the Readline library comes with a set of GNU Emacs-like keybindings, it is possible that you would like to use a different set of keybindings. You can customize programs that use Readline by putting commands in an init file in your home directory. The name of this file is ~/.inputrc.

When a program which uses the Readline library starts up, the ~/.inputrc file is read, and the keybindings are set.

In addition, the Ctrl-X, Ctrl-R command re-reads this init file, thus incorporating any changes that you might have made to it.

Readline Init Syntax

Variable Settings

You can change the state of a few variables in Readline. You do this by using the set command within the init file. Here is how you would specify that you wish to use vi line editing commands:

The editing-mode variable controls which editing mode you are using. By default, GNU Readline starts up in Emacs editing mode, where the keystrokes are most similar to Emacs. This variable can either be set to emacs or vi.

This variable can either be set to On or Off. Setting it to On means that the text of the lines that you edit will scroll horizontally on a single screen line when they are larger than the width of the screen, instead of wrapping onto a new screen line. By default, this variable is set to Off.

Key Bindings

The syntax for controlling keybindings in the ~/.inputrc file is simple. First you have to know the name of the command that you want to change. The following pages contain tables of the command name, the default keybinding, and a short description of what the command does.

Once you know the name of the command, simply place the name of the key you wish to bind the command to, a colon, and then the name of the command on a line in the ~/.inputrc file. The name of the key can be expressed in different ways, depending on which is most comfortable for you.

In the example, Ctrl-U is bound to the function, universal-argument, and Ctrl-O is bound to run the macro expressed on the right hand side (that is, to insert the text "&output" into the line).

In the example, Ctrl-U is bound to the function universal-argument (just as it was in the first example), Ctrl-X, Ctrl-R is bound to the function reread-init-file, and Esc-[, 1, 1, ~ is bound to insert the text Function Key 1. See the following table for additional information.


Command	Action
beginning-of-line (Ctrl-A)	Move to the start of the current line.
end-of-line (Ctrl-E)	Move to the end of the line.
forward-char (Ctrl-F)	Move forward a character.
backward-char (Ctrl-B)	Move back a character.
forward-word (M-F)	Move forward to end of the next word.
backward-word (M-B)	Move back to the start of this, or the previous, word.
clear-screen (Ctrl-L)	Clear the screen leaving the current line at the top of the screen.


Command	Action
accept-line (Newline, Return)	Accept the line regardless of where the cursor is. If this line is non-empty, add it to the history list. If this line was a history line, then restore the history line to its original state.
previous-history (Ctrl-P)	Move `up' through the history list.
next-history (Ctrl-N)	Move `down' through the history list.
beginning-of-history (M-<)	Move to the first line in the history.
end-of-history (M-)	Move to the end of the input history, i.e., the line you are entering.
reverse-search-history (Ctrl-R)	Search backward starting at the current line and moving `up' through the history as necessary. This is an incremental search.
forward-search-history (Ctrl-S)	Search forward starting at the current line and moving `down' through the the history as necessary.


Command	Action
delete-char (Ctrl-D)	Delete the character under the cursor. If the cursor is at the beginning of the line, and there are no characters in the line, and the last character typed was not Ctrl-D, then return EOF.
backward-delete-char (Rubout)	Delete the character behind the cursor. A numeric argument says to kill the characters instead of deleting them.
quoted-insert (Ctrl-Q, Ctrl-V)	Add the next character that you type to the line verbatim. This is how to insert things like Ctrl-Q, for example
tab-insert (M-Tab)	Insert a tab character.


Command	Action
self-insert (a, b, A, 1, !, ...)	Insert yourself.
transpose-chars (Ctrl-T)	Drag the character before point forward over the character at point. Point moves forward as well. If point is at the end of the line, then transpose the two characters before point. Negative arguments don't work.
transpose-words (M-T)	Drag the word behind the cursor past the word in front of the cursor moving the cursor over that word as well.
upcase-word (M-U)	Uppercase all letters in the current (or following) word. With a negative argument, do the previous word, but do not move point.
downcase-word (M-L)	Lowercase all letters in the current (or following) word. With a negative argument, do the previous word, but do not move point.
capitalize-word (M-C)	Uppercase the first letter in the current (or following) word. With a negative argument, do the previous word, but do not move point.
kill-line (Ctrl-K)	Kill the text from the current cursor position to the end of the line.
backward-kill-line ( )	Kill backward to the beginning of the line. This is normally unbound.
kill-word (M-D)	Kill from the cursor to the end of the current word, or if between words, to the end of the next word.
backward-kill-word (M-Delete)	Kill the word behind the cursor.
unix-line-discard (Ctrl-U)	Kill the whole line the way Ctrl-U used to in UNIX line input. The killed text is saved on the kill-ring.
unix-word-rubout (Ctrl-W)	Kill the word the way Ctrl-W used to in UNIX line input. The killed text is saved on the kill-ring. This is different than backward-kill-word because the word boundaries differ.
yank (Ctrl-Y)	Yank the top of the kill ring into the buffer at point.
yank-pop (M-Y)	Rotate the kill-ring, and yank the new top. You can only do this if the prior command is yank or yank-pop.


Command	Action
digit-argument (M-0, M-1, ... M--)	Add this digit to the argument already accumulating, or start a new argument. M-- starts a negative argument.
universal-argument ( )	Do what Ctrl-U does in GNU Emacs. By default, this is not bound.


Command	Action
complete (Tab)	Attempt to do completion on the text before point. This is implementation defined. Generally, if you are typing a filename argument, you can do filename completion; if you are typing a command, you can do command completion, if you are typing in a symbol to GDB, you can do symbol name completion, if you are typing in a variable to Bash, you can do variable name completion.
possible-completions (M-?)	List the possible completions of the text before point.


Command	Action
reread-init-file (Ctrl-X, Ctrl-R)	Read in the contents of your ~/.inputrc file, and incorporate any bindings found there.
abort (Ctrl-G)	Stop running the current editing command.
prefix-meta (Esc)	Make the next character that you type be metafied. This is for people without a meta key. Typing ESC F is equivalent to typing M-F.
undo (Ctrl-_)	Incremental undo, separately remembered for each line.
revert-line (M-R)	Undo all changes made to this line. This is like typing undo enough times to get back to the beginning.

Readline vi Mode

While the Readline library does not have a full set of vi editing functions, it does contain enough to allow simple editing of the line.

In order to switch interactively between GNU Emacs and vi editing modes, use the command M-Ctrl-J (toggle-editing-mode). When you enter a line in vi mode, you are already placed in insertion mode, as if you had typed an i. Using Esc switches you into edit mode, where you can edit the text of the line with the standard vi movement keys, move to previous history lines with k, and following lines with j, and so forth.

Using History Interactively

The following describes how to use the GNU History Library interactively, from a user's standpoint.

History Interaction

The History library provides a history expansion feature similar to the history expansion in csh. The following text describes the syntax you use to manipulate history information.

History expansion takes two parts. In the first part, determine which line from the previous history will be used for substitution. This line is called the event. In the second part, select portions of that line for inclusion into the current line. These portions are called words. GDB breaks the line into words in the same way that the Bash shell does, so that several English (or UNIX) words surrounded by quotes are considered one word.

Event Designators


!	Start a history substitution, except when followed by a space, tab, or the end of the line... = or (.
!!	Refer to the previous command. This is a synonym for !-1.
!n	Refer to command line n.
!-n	Refer to the command line n lines back.
!string	Refer to the most recent command starting with string.
!?string[?]	Refer to the most recent command containing string.

Word Designators

A: separates the event designator from the word designator. It can be omitted if the word designator begins with a ^, $, * or %. Words are numbered from the beginning of the line, with the first word being denoted by a 0 (zero).


0 (zero)	The zero'th word. For many applications, this is the command word.
n	The n'th word.
^	The first argument. that is, word 1.
T$	The last argument.
%	The word matched by the most recent ?string? search.
x-y	A range of words; -y abbreviates 0-y.
*	All of the words, excepting the zero'th. This is a synonym for 1-$. It is not an error to use * if there is just one word in the event. The empty string is returned in that case.

Modifiers

After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a :.


#	The entire command line typed so far. This means the current command, not the previous command.
h	Remove a trailing pathname component, leaving only the head.
r	Remove a trailing suffix of the form `.' suffix, leaving the basename.
e	Remove all but the suffix.
t	Remove all leading pathname components, leaving the tail.
p	Print the new command but do not execute it.

Total/db User's Guide

Debugging with GDB