I have a supervisor program that generally wants to receive SIGCHLD events. However, after it receives SIGTERM or similar signal, it would be safe and desirable to ignore SIGCHLD events to prevent zombies. The issue is that I'm not sure if it's legal to call signal(2) when already inside a signal handler, so that e.g. SIGTERM calls signal(SIGCHLD, SIG_IGN). It doesn't appear to cause problems, but I know there are many restrictions placed on what you can do in a signal handler, and I'm unable to figure out whether calling signal is in the forbidden zone.
So: is this legal, reliably? If not, is there an alternative way to make sure that no children still running at the time of the SIGTERM can possibly zombie, even if they aren't properly waited on?
For Linux, look at the manpage for signal(7). There is a list of async-signal-safe functions. It says:
POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2) requires an implementation to guarantee that the following functions can be safely called inside a signal handler:
signal() is in the list. So it's safe.