#include <signal.h> typedef void (*sighandler_t)(int); int gsignal(int signum); sighandler_t ssignal(int signum, sighandler_t action);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Elsewhere, on System V-like systems, these functions implement software signaling, entirely independent of the classical signal(2) and kill(2) functions. The function ssignal() defines the action to take when the software signal with number signum is raised using the function gsignal(), and returns the previous such action or SIG_DFL. The function gsignal() does the following: if no action (or the action SIG_DFL) was specified for signum, then it does nothing and returns 0. If the action SIG_IGN was specified for signum, then it does nothing and returns 1. Otherwise, it resets the action to SIG_DFL and calls the action function with argument signum, and returns the value returned by that function. The range of possible values signum varies (often 1-15 or 1-17).