typedef void (*sighandler_t)(int);
sighandler_t sigset(int sig, sighandler_t disp);
int sighold(int sig);
int sigrelse(int sig);
int sigignore(int sig);
The sigset() function modifies the disposition of the signal sig. The disp argument can be the address of a signal handler function, or one of the following constants:
If disp specifies the address of a signal handler, then sig is added to the process's signal mask during execution of the handler.
If disp was specified as a value other than SIG_HOLD, then sig is removed from the process's signal mask.
The dispositions for SIGKILL and SIGSTOP cannot be changed.
The sighold() function adds sig to the calling process's signal mask.
The sigrelse() function removes sig from the calling process's signal mask.
The sigignore() function sets the disposition of sig to SIG_IGN.
The sighold(), sigrelse(), and sigignore() functions return 0 on success; on error, these functions return -1 and set errno to indicate the error.
For sighold() and sigrelse() see the ERRORS under sigprocmask(2).
For sigignore(), see the errors under sigaction(2).
The sighandler_t type is a GNU extension; it is only used on this page to make the sigset() prototype more easily readable.
The sigset() function provides reliable signal handling semantics (as when calling sigaction() with sa_mask equal to 0).
On System V, the signal() function provides unreliable semantics (as when calling sigaction() with sa_mask equal to SA_RESETHAND | SA_NODEFER). On BSD, signal() provides reliable semantics. POSIX.1-2001 leaves these aspects of signal() unspecified. See signal(2) for further details.
In order to wait for a signal, BSD and System V both provided a function named sigpause(), but this function has a different argument on the two systems. See sigpause(3) for details.
In all versions of glibc up to and including 2.3.5, sigset() does not correctly return the previous disposition of the signal in two cases. First, if disp is specified as SIG_HOLD, then a successful sigset() always returns SIG_HOLD. Instead, it should return the previous disposition of the signal (unless the signal was blocked, in which case SIG_HOLD should be returned). Second, if the signal is currently blocked, then the return value of a successful sigset() should be SIG_HOLD. Instead, the previous disposition of the signal is returned.