SIGNAL(2)						     Linux Programmer's Manual							 SIGNAL(2)

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 han-
       dler is set to a function sighandler then first either the handler is reset to SIG_DFL or an implementation-dependent blocking of the  sig-
       nal 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.

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.

       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.

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, caus-
       ing 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.

CONFORMING TO

       ANSI C

SEE ALSO

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

Linux 2.2							    2000-04-28								 SIGNAL(2)