SIGNAL

Section: Linux Programmer's Manual (2)
Updated: 2000-04-28
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NAME

signal - ANSI C signal handling

SYNOPSIS

#include <signal.h>

typedef void (*sighandler_t)(int);

sighandler_t signal(int signum, sighandler_t handler);

DESCRIPTION

The signal() system call installs a new signal handler for the signal with number signum. The signal handler is set to sighandler which may be a user specified function, or either SIG_IGN or SIG_DFL.

Upon arrival of a signal with number signum the following happens. If the corresponding handler is set to SIG_IGN, then the signal is ignored. If the handler is set to SIG_DFL, then the default action associated to the signal (see signal(7)) occurs. Finally, if the handler is set to a function sighandler then the signal is blocked and sighandler is called with argument signum. The signal remains blocked during the execution of the signal handler.

Using a signal handler function for a signal is called "catching the signal". The signals SIGKILL and SIGSTOP cannot be caught or ignored.

RETURN VALUE

The signal() function returns the previous value of the signal handler, or SIG_ERR on error.

PORTABILITY

The original Unix signal() would reset the handler to SIG_DFL, and System V (and the Linux kernel and libc4,5) does the same. On the other hand, BSD does not reset the handler, but blocks new instances of this signal from occurring during a call of the handler. The glibc2 library follows the BSD behaviour.

In a libc5 system including <bsd/signal.h> instead of <signal.h> makes signal to be redefined as __bsd_signal and signal has the BSD semantics. This is not recommended.

In a glibc2 system defining a feature test macro such as _XOPEN_SOURCE or using a separate sysv_signal function, the classical behaviour is obtained. This is not recommended.

Trying to change the semantics of this call using defines and includes is not a good idea. It is better to avoid signal altogether, and use sigaction(2) instead.

NOTES

According to POSIX, the behaviour of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by the kill(2) or the raise(3) functions. Integer division by zero has undefined result. On some architectures it will generate a SIGFPE signal. (Also dividing the most negative integer by -1 may generate SIGFPE.) Ignoring this signal might lead to an endless loop.

According to POSIX (3.3.1.3) it is unspecified what happens when SIGCHLD is set to SIG_IGN. Here the BSD and SYSV behaviours differ, causing BSD software that sets the action for SIGCHLD to SIG_IGN to fail on Linux.

The use of the sighandler_t is a GNU extension. Various versions of libc predefine this type; libc4 and libc5 define SignalHandler, glibc defines sig_t and, when _GNU_SOURCE is defined, also sighandler_t.

CONFORMING TO

ANSI C

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Time: 16:38:48 GMT, January 15, 2025