zombie process remove without wait C

Question

I've got a child process which just exit(0). It became zombie. Is there way to remove it without wait or waitpid in parent process?

R+  ./server //parent
R+  ./server //child
Z+  (server) //child zombie 

Answer 1

You could catch the SIGCHLD signal (e.g. using sigaction(2) etc...). Be careful, very few functions are safely callable from a signal handler. Read several times signal(7) & signal-safety(7). A good thing to do inside a signal handler is to just set some volatile sigatomic_t flag (which you would test later, outside of the signal handler, e.g. in some event loop). Or you could, at initialization time, setup a pipe(7) (to your own process) and write a few bytes on it in your signal handler (and poll(2) the read end elsewhere), as suggested by Qt.

And waitpid(2) can be told to not stop with WNOHANG

If you never wait for your child process, it will become zombie.

Read Advanced Linux Programming. It has a good chapter on processes.

Answer 2

Simplest is to make SIGCHLD ignored in the parent:

signal(SIGCHLD, SIG_IGN);

After that, exiting child processes will just cleanly go away without needing to be waited on.

Note: This a bit of a quick&dirty approach, and assumes that you simply do not care about exit status of the child process, or about when it exits. If you do care, as you probably should in a real robust application, then see the other answer, about creating a proper signal handler function.

Addition: This is not universal in Unixes, but works at least on Linux. According to an UNIX FAQ, it is undefined behaviour in POSIX. This Mac OS X man page suggests that this behaviour was introduced in FreeBSD 5.0, and works on OS X, for example.