What happens when a signal is received while already in a signal handler?

Question

I have a parent process spanning several child processes. I want to know when any child process exits by registering a SIGCHLD signal handler.

The question is, what happens if another SIGCHLD (or any other signal) is received, while the parent process is already in a signal handler?

I can think of the following outcomes:

• The signal is ignored
• The signal is queued, and will be processed as soon as the current handler returns
• The current handler is in turn interrupted, just like the main program

Which one is correct?

Answer 1

In your concrete example (the same signal being received), the signal is delivered after the signal handler has finished (so bullet point #2 is correct). Note, however, that you may "lose" signals.

The reason for that is that while a signal is being inside its handler, it is blocked. Blocked signals are set to pending, but not queued. The term "pending" means that the operating system remembers that there is a signal waiting to be delivered at the next opportunity, and "not queued" means that it does this by setting a flag somewhere, but not by keeping an exact record of how many signals have arrived.

Thus, you may receive 2 or 3 (or 10) more SIGCHLD while in your handler, but only see one (so in some cases, bullet point #1 can be correct, too).

Note that several flags that you can pass to sigaction can affect the default behaviour, such as SA_NODEFER (prevents blocking signal) and SA_NOCLDWAIT (may not generate signal at all on some systems).

Now of course, if you receive a different type of signal, there's no guarantee that it won't interrupt your handler. For that reason, one preferrably doesn't use non signal safe functions.