Handling multiple SIGCHLD

Question

In a system running Linux 2.6.35+ my program creates many child processes and monitors them. If a child process dies I do some clean-up and spawn the process again. I use <

Answer 1

Actually the hassle-free way would be the waitfd functionally that would allow you to add a specific pid to poll()/epoll(). Unfortunately, it wasn't accepted to Linux years ago when it was proposed.

Answer 2

On Linux, multiple children terminating before you read a SIGCHLD with signalfd() will be compressed into a single SIGCHLD. This means that when you read the SIGCHLD signal, you have to clean up after all children that have terminated:

// Do this after you've read() a SIGCHLD from the signalfd file descriptor:
while (1) {
    int status;
    pid_t pid = waitpid(-1, &status, WNOHANG);
    if (pid <= 0) {
        break;
    }
    // something happened with child 'pid', do something about it...
    // Details are in 'status', see waitpid() manpage
}

I should note that I have in fact seen this signal compression when two child processed terminated at the same time. If I did only a single waitpid(), one of the children that terminated was not handled; and the above loop fixed it.

Corresponding documentation:

• http://man7.org/linux/man-pages/man7/signal.7.html "By contrast, if multiple instances of a standard signal are delivered while that signal is currently blocked, then only one instance is queued"
• http://man7.org/linux/man-pages/man3/sigwait.3p.html "If prior to the call to sigwait() there are multiple pending instances of a single signal number, it is implementation-defined whether upon successful return there are any remaining pending signals for that signal number."