| Debugging with GDB: Forks |
|---|
Next: Checkpoint/Restart, Previous: Threads, Up: Running [Contents][Index]
On most systems, 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. 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 Attach). From that point on you can debug
the child process just like any other process which you attached to.
On some systems, GDB provides support for debugging programs
that create additional processes using the fork or vfork
functions. On GNU/Linux platforms, this feature is supported
with kernel version 2.5.46 and later.
The fork debugging commands are supported in native mode and when
connected to gdbserver in either target remote mode or
target extended-remote mode.
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
.
set follow-fork-mode mode
Set the debugger response to a program call of fork or
vfork. A call to fork or vfork creates a new
process. The mode argument can be:
parent
The original process is debugged after a fork. The child process runs unimpeded. This is the default.
child
The new process is debugged after a fork. The parent process runs unimpeded.
show follow-fork-mode
Display the current debugger response to a fork or vfork call.
On Linux, if you want to debug both the parent and child processes, use the
command set detach-on-fork
.
set detach-on-fork mode
Tells gdb whether to detach one of the processes after a fork, or retain debugger control over them both.
on
The child process (or parent process, depending on the value of
follow-fork-mode) will be detached and allowed to run
independently. This is the default.
off
Both processes will be held under the control of GDB.
One process (child or parent, depending on the value of
follow-fork-mode) is debugged as usual, while the other
is held suspended.
show detach-on-fork
Show whether detach-on-fork mode is on/off.
If you choose to set ‘detach-on-fork’ mode off, then GDB
will retain control of all forked processes (including nested forks).
You can list the forked processes under the control of GDB by
using the info inferiors
command, and switch from one fork
to another by using the inferior command (see Debugging Multiple Inferiors and Programs).
To quit debugging one of the forked processes, you can either detach
from it by using the detach inferiors
command (allowing it
to run independently), or kill it using the kill inferiors
command. See Debugging Multiple Inferiors
and Programs.
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 main.
On some systems, 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 exec
call executes, the new target restarts. To restart the parent
process, use the file command with the parent executable name
as its argument. By default, after an exec call executes,
GDB discards the symbols of the previous executable image.
You can change this behaviour with the set follow-exec-mode
command.
set follow-exec-mode mode
Set debugger response to a program call of exec. An
exec call replaces the program image of a process.
follow-exec-mode can be:
new
GDB creates a new inferior and rebinds the process to this
new inferior. The program the process was running before the
exec call can be restarted afterwards by restarting the
original inferior.
For example:
(gdb) info inferiors (gdb) info inferior Id Description Executable * 1 <null> prog1 (gdb) run process 12020 is executing new program: prog2 Program exited normally. (gdb) info inferiors Id Description Executable 1 <null> prog1 * 2 <null> prog2
same
GDB keeps the process bound to the same inferior. The new
executable image replaces the previous executable loaded in the
inferior. Restarting the inferior after the exec call, with
e.g., the run command, restarts the executable the process was
running after the exec call. This is the default mode.
For example:
(gdb) info inferiors Id Description Executable * 1 <null> prog1 (gdb) run process 12020 is executing new program: prog2 Program exited normally. (gdb) info inferiors Id Description Executable * 1 <null> prog2
follow-exec-mode is supported in native mode and
target extended-remote mode.
You can use the catch command to make GDB stop whenever
a fork, vfork, or exec call is made. See Setting Catchpoints.
Next: Checkpoint/Restart, Previous: Threads, Up: Running [Contents][Index]