7.3 sigaction

If your program needs to specify its own handler, the proper method is to use sigaction.

This function has three parameters:

• int signum: the first parameter specifies the signal number that sigaction applies to.
• const struct sigaction *act: the second parameter is a pointer to specify the new handler of the signal.
• struct sigaction *oldact: the third parameter is a pointer to storage to save the current handler. If this parameter is NULL, then the previous handler is lost.

struct sigaction is a structure that consists of many attributes:

• sa_handler and sa_sigaction should be mutually exclusive.
• sa_mask: This field specifies signals that should be allowed to interrupt when the signal handler is executing. The functions sigaddset, sigdelset and sigemptyset should be used to specify the mask. If a signal handler is to block all other signals, then sigemptyset should be used.
• sa_flags: Specify SA_SIGINFO in this field to use sa_sigaction (instead of sa_handler). It can also contain other flags (bitwise-ored if multiple are specified).
• sa_restorer: this attribute is said to be obsolete and should not be used.

7.3.1 Why the complication?

Compared to signal, sigaction is a bit more complex to use. However, this is not without reasons.

First of all, the sa_mask field of struct sigaction allows the specification of whether the handling of a particular signal can block other signals (including another instance of itself!) from interrupting it. This is very important when it comes to variable consistency.