signal(2) — SYSTEM CALLS

NAME

signal, sigset, sighold, sigrelse, sigignore, sigpause − simplified signal management

SYNOPSIS

#include <signal.h>
void (∗signal(int sig, void (∗disp)(int)))(int);
void (∗sigset(int sig, void (∗disp)(int)))(int);
int sighold(int sig);
int sigrelse(int sig);
int sigignore(int sig);
int sigpause(int sig);

DESCRIPTION

These functions provide simplified signal management for application processes. See signal(5) for an explanation of general signal concepts.

signal and sigset are used to modify signal dispositions. sig specifies the signal, which may be any signal except SIGKILL and SIGSTOP. disp specifies the signal’s disposition, which may be SIG_DFL, SIG_IGN, or the address of a signal handler. If signal is used, disp is the address of a signal handler, and sig is not SIGILL, SIGTRAP, or SIGPWR, the system first sets the signal’s disposition to SIG_DFL before executing the signal handler. If sigset is used and disp is the address of a signal handler, the system adds sig to the calling process’s signal mask before executing the signal handler; when the signal handler returns, the system restores the calling process’s signal mask to its state prior to the delivery of the signal. In addition, if sigset is used and disp is equal to SIG_HOLD, sig is added to the calling process’s signal mask and the signal’s disposition remains unchanged.

sighold adds sig to the calling process’s signal mask.

sigrelse removes sig from the calling process’s signal mask.

sigignore sets the disposition of sig to SIG_IGN.

sigpause removes sig from the calling process’s signal mask and suspends the calling process until a signal is received.

These functions fail if any of the following are true.

EINVAL The value of the sig argument is not a valid signal or is equal to SIGKILL or SIGSTOP.

EINTR A signal was caught during the system call sigpause.

NOTES

sighold in conjunction with sigrelse or sigpause may be used to establish critical regions of code that require the delivery of a signal to be temporarily deferred.

Signals resulting from machine exceptions cannot be blocked or ignored. The system silently ignores attempts to do so. The signals that result from machine exceptions are SIGSEGV, SIGILL, SIGBUS, SIGTRAP, and SIGFPE. The behavior of a process that generates a machine exception (e.g. floating point exception or bus error) that gives rise to a signal that is blocked or ignored is specifically undefined and not portable. The behavior may change in future releases or on different processor boards.

If signal or sigset is used to set SIGCHLD’s disposition to a signal handler, SIGCHLD will not be sent when the calling process’s children are stopped or continued.

If any of the above functions are used to set SIGCHLD’s disposition to SIG_IGN, the calling process’s child processes will not create zombie processes when they terminate [see exit(2)]. If the calling process subsequently waits for its children, it blocks until all of its children terminate; it then returns a value of −1 with errno set to ECHILD [see wait(2), waitid(2)].

When a signal handler installed by the signal interface is invoked by the arrival of a signal, the system resets the action for that signal to SIG_DFL; to cause the handler to remain installed until explicitly changed, use sigset or sigaction instead of signal to install the handler.

DIAGNOSTICS

On success, signal returns the signal’s previous disposition. On failure, it returns SIG_ERR and sets errno to indicate the error.

On success, sigset returns SIG_HOLD if the signal had been blocked or the signal’s previous disposition if it had not been blocked. On failure, it returns SIG_ERR and sets errno to indicate the error.

All other functions return zero on success. On failure, they return −1 and set errno to indicate the error.

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