setsid() without forking OR debugging forked process with gdb

ta0kira · 08-07-2007, 07:57 AM

I am trying to debug a server which must setsid() in order to run. This is necessary because it forks an arbitrary number of processes which raise(SIGSTOP) so that the server can set up shared memory, pipes, and make all forks before continuing. The server then calls kill(0, SIGCONT) to restart all of the processes. Normally I use a daemon call, but gdb loses track of the real process when it forks. Calling setsid without daemonizing doesn't work when running from a terminal, and hence from gdb. Is there a way around this? Thanks.
ta0kira

jtshaw · 08-08-2007, 08:29 PM

What I would do.... and this is after two seconds of thought so there may be a better way...

Put in an infinite loop in your program after the fork. Run it. Attach to it with the debugger (after it forks of course). Break the loop and debug away.

Hko · 08-09-2007, 04:16 AM

To deamonize properly, a program forks() twice. After each fork(), the parent exits. By default, gdb followes the parent process when forking. I think this is why you lose control over you parent in gdb. You can tell gdb to follow child after a fork with the gdb-command "set follow-fork-mode child".

So I think it might help to set a breakpoint after the daemon() call, tell gdb to follow the child and run you program.

Some doc's about this:
http://www.unixguide.net/unix/programming/1.7.shtml
http://sourceware.org/gdb/current/on...b_5.html#SEC28
http://sourceware.org/gdb/current/on...b_5.html#SEC25
(the last one may help you try jtshaw's idea)

ta0kira · 08-09-2007, 06:22 AM

Quote:

Originally Posted by Hko

So I think it might help to set a breakpoint after the daemon() call, tell gdb to follow the child and run you program.

Thanks. I think the breakpoint will have to be right before the daemon call, though, since it won't work after. Then I'll need one after so it doesn't follow the additional forks later on. I'll look into that.

In the meantime, I've solved all of my problems the way I have been for years: with printf/std::cout. That never seems to fail me. Plus, the server has quite a few logging points with extensive output.
ta0kira