Debugging with GDB Manual (5900-1473; WDB 6.2; January 2011)

See “Stopping and starting multi-thread programs” (page 64), for more information about
how GDB behaves when you stop and start programs with multiple threads.
See “Killing the child process” (page 40), for information about watchpoints in programs
with multiple threads.
NOTE: On HP-UX 11.x, debugging a multi-thread process can cause a deadlock if the
process is waiting for an NFS-server response. A thread can be stopped while asleep in
this state, and NFS holds a lock on the rnode while asleep.
To prevent the thread from being interrupted while holding the rnode lock, make the NFS
mount non-interruptible with the '-nointr' flag. See mount(1).
4.10 Debugging programs with multiple processes
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 do not 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 a Running Process” (page 39)). 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 Set the debugger response to a program call of
fork or vfork. A call to fork or vfork creates
a new process. The mode 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.
44 Running Programs Under GDB