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signal - ANSI C signal handling




#include <signal.h>

typedef void (*sighandler_t)(int);

"sighandler_t signal(int "signum", sighandler_t "handler);


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 first either the handler is reset to SIG_DFL or an implementation-dependent blocking of the signal is performed and next sighandler is called with argument signum.

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


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


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.

If one on a libc5 system includes "<bsd/signal.h>" instead of "<signal.h>" then signal is redefined as __bsd_signal and signal has the BSD semantics. This is not recommended.

If one on a glibc2 system defines a feature test macro such as _XOPEN_SOURCE or uses a separate sysv_signal function, one obtains classical behaviour. 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.


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 ( 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 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.




kill(1), kill(2), killpg(2), pause(2), raise(3), sigaction(2), signal(7), sigsetops(3), sigvec(2), alarm(2)

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