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. If you are doing native debugging, you may redirect your program's input and output, debug an already running process, or kill a child process.
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!).
Older versions of the GNU C compiler permitted a variant option `-gg' for debugging information. GDB no longer supports this format; if your GNU C compiler has this option, do not use it.
runcommand to start your program under GDB. You must first specify the program name (except on VxWorks) with an argument to GDB (see section Getting In and Out of GDB), or by using the
exec-filecommand (see section Commands to specify files).
If you are running your program in an execution environment that
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 four categories:
runcommand. 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
SHELLenvironment variable. See section Your program's arguments.
unset environmentto change parts of the environment that affect your program. See section Your program's environment.
cdcommand in GDB. See section Your program's working directory.
runcommand line, or you can use the
ttycommand to set a different device for your program. See section Your program's input and output. Warning: 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 section Stopping and Continuing, for discussion
of how to arrange for your program to stop. Once your program has
stopped, you may call functions in your program, using the
call commands. See section 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.
The arguments to your program can be specified by the arguments of the
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
the default shell (`/bin/sh' on Unix).
On non-Unix systems, the program is usually invoked directly by GDB, which emulates I/O redirection via the appropriate system calls, and the wildcard characters are expanded by the startup code of the program, not by the shell.
run with no arguments uses the same arguments used by the previous
run, or those set by the
set args command.
set argshas no arguments,
runexecutes your program with no arguments. Once you have run your program with arguments, using
set argsbefore the next
runis the only way to run it again without arguments.
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.
PATHenvironment variable (the search path for executables), for both GDB and your program. You may specify several directory names, separated by whitespace or by a system-dependent separator character (`:' on Unix, `;' on MS-DOS and MS-Windows). If directory is already in the path, it is moved to the front, so it is searched sooner. You can use the string `$cwd' to refer to whatever is the current working directory at the time GDB searches the path. If you use `.' instead, it refers to the directory where you executed the
pathcommand. GDB replaces `.' in the directory argument (with the current path) before adding directory to the search path.
show environment [varname]
set environment varname [=value]
set env USER = footells the debugged program, when subsequently run, that its user is named `foo'. (The spaces around `=' are used for clarity here; they are not actually required.)
unset environment varname
unset environmentremoves the variable from the environment, rather than assigning it an empty value.
Warning: On Unix systems, GDB runs your program using
the shell indicated
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
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
The GDB working directory also serves as a default for the commands that specify files for GDB to operate on. See section Commands to specify files.
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
tty command. This command accepts a file name as
argument, and causes this file to be the default for future
commands. It also resets the controlling terminal for the child
process, for future
run commands. For example,
directs that processes started with subsequent
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
effect on the input/output device, but not its effect on the controlling
When you use the
tty command or redirect input in the
command, only the input for your program is affected. The input
for GDB still comes from your terminal.
info filesshows 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
psutility, or with the `jobs -l' shell command.
attachdoes not repeat if you press RET a second time after executing the command.
attach, your program must be running in an environment
which supports processes; for example,
attach does not work for
programs on bare-board targets that lack an operating system. You must
also have permission to send the process a signal.
When you use
attach, the debugger finds the program running in
the process first by looking in the current working directory, then (if
the program is not found) by using the source file search path
(see section Specifying source directories). You can also use
file command to load the program. See section Commands to specify files.
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.
detachcommand to release it from GDB control. Detaching the process continues its execution. After the
detachcommand, that process and GDB become completely independent once more, and you are ready to
attachanother process or start one with
detachdoes not repeat if you press RET 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
confirm command (see section Optional warnings and messages).
This command is useful if you wish to debug a core dump instead of a running process. GDB ignores any core dump file while your program is running.
On some operating systems, a program cannot be executed outside GDB
while you have breakpoints set on it inside GDB. You can use the
kill command in this situation to permit running your program
outside the debugger.
kill command is also useful if you wish to recompile and
relink your program, since on many systems it is impossible to modify an
executable file while it is running in a process. In this case, when you
run, GDB notices that the file has changed, and
reads the symbol table again (while trying to preserve your current
In some operating systems, such as HP-UX and Solaris, 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 multi-thread programs:
Warning: These facilities are not yet available on every GDB configuration where the operating system supports threads. If your GDB does not support threads, these commands have no effect. For example, a system without thread support shows no output from `info threads', and always rejects the
threadcommand, like this:(gdb) info threads (gdb) thread 1 Thread ID 1 not known. Use the "info threads" command to see the IDs of currently known threads.
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 form `[New systag]'. systag 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.
(gdb) info threads 3 process 35 thread 27 0x34e5 in sigpause () 2 process 35 thread 23 0x34e5 in sigpause () * 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) at threadtest.c:68
On HP-UX systems:
For debugging purposes, GDB associates its own thread number--a small integer assigned in thread-creation order--with each thread in your program.
Whenever GDB detects a new thread in your program, it displays both GDB's thread number and the target system's identification for the thread with a message in the form `[New systag]'. systag is a thread identifier whose form varies depending on the particular system. For example, on HP-UX, you see
[New thread 2 (system thread 26594)]
when GDB notices a new thread.
(gdb) info threads * 3 system thread 26607 worker (wptr=0x7b09c318 "@") \
at quicksort.c:137 2 system thread 26606 0x7b0030d8 in __ksleep () \
from /usr/lib/libc.2 1 system thread 27905 0x7b003498 in _brk () \
(gdb) thread 2 [Switching to process 35 thread 23] 0x34e5 in sigpause ()As with the `[New ...]' message, the form of the text after `Switching to' depends on your system's conventions for identifying threads.
thread apply [threadno] [all] args
thread applycommand 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 allargs.
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 form `[Switching to systag]' to identify the thread.
See section Stopping and starting multi-thread programs, for more information about how GDB behaves when you stop and start programs with multiple threads.
See section Setting watchpoints, for information about watchpoints in programs with multiple threads.
On most systems, GDB has no special support for debugging
programs which create additional processes using the
function. When a program forks, GDB will continue to debug the
parent process and the child process will run unimpeded. If you have
set a breakpoint in any code which the child then executes, the child
will get a
SIGTRAP signal which (unless it catches the signal)
will cause it to terminate.
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 section Debugging an already-running process). From that point on you can debug
the child process just like any other process which you attached to.
On HP-UX (11.x and later only?), GDB provides support for
debugging programs that create additional processes using the
By default, when a program forks, GDB will continue to debug the parent process and the child process will run unimpeded.
If you want to follow the child process instead of the parent process,
use the command
set follow-fork-mode mode
vfork. A call to
vforkcreates a new process. The mode can be:
If you ask to debug a child process and a
vfork is followed by an
exec, GDB executes the new target up to the first
breakpoint in the new target. If you have a breakpoint set on
main in your original program, the breakpoint will also be set on
the child process's
When a child process is spawned by
vfork, you cannot debug the
child or parent until an
exec call completes.
If you issue a
run command to GDB after an
call executes, the new target restarts. To restart the parent process,
file command with the parent executable name as its
You can use the
catch command to make GDB stop whenever
exec call is made. See section Setting catchpoints.
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