What is the difference between sigaction and signal?

Question

I was about to add an extra signal handler to an app we have here and I noticed that the author had used sigaction() to set up the other signal handlers. I was

Answer 1

To me, this below line was enough to decide:

The sigaction() function provides a more comprehensive and reliable mechanism for controlling signals; new applications should use sigaction() rather than signal()

http://pubs.opengroup.org/onlinepubs/009695399/functions/signal.html#tag_03_690_07

Whether you're starting from scratch or modifying an old program, sigaction should be the right option.

Answer 2

signal() is standard C, sigaction() is not.

If you're able to use either (that is, you're on a POSIX system), then use sigaction(); it's unspecified whether signal() resets the handler, meaning that to be portable you have to call signal() again inside the handler. What's worse is that there's a race: if you get two signals in quick succession, and the second is delivered before you reinstall the handler, you'll have the default action, which is probably going to be to kill your process. sigaction(), on the other hand, is guaranteed to use “reliable” signal semantics. You need not reinstall the handler, because it will never be reset. With SA_RESTART, you can also get some system calls to automatically restart (so you don't have to manually check for EINTR). sigaction() has more options and is reliable, so its use is encouraged.

Psst... don't tell anyone I told you this, but POSIX currently has a function bsd_signal() which acts like signal() but gives BSD semantics, which means it's reliable. Its main use is for porting old applications that assumed reliable signals, and POSIX does not recommend using it.

Answer 3

From the signal(3) man page:

DESCRIPTION

 This signal() facility is a simplified interface to the more
 general sigaction(2) facility.

Both invoke the same underlying facility. You should presumably not manipulate the response the a single signal with both, but mixing them shouldn't cause anything to break...