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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 form '[Switching to systag]' to identify the
thread.
See “Stopping and starting multi-thread programs” (page 69), for more information
about how GDB behaves when you stop and start programs with multiple threads.
See “Killing the child process” (page 45), 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 44)). 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.
4.10 Debugging programs with multiple processes 49