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 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
fork
or vfork
function.
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
fork
or
vfork
. A call to fork
or vfork
creates a new
process. The mode can be:
parent
child
ask
show follow-fork-mode
fork
or vfork
call.
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
.
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.
You can use the catch
command to make GDB stop whenever
a fork
, vfork
, or exec
call is made. See section Setting catchpoints.
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