signal

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  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.


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 Sys-
       tem  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 seman-
       tics. 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 sig-
       nal.  (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 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),  sig-
       nal(7), sigsetops(3), sigvec(2), alarm(2)



Linux 2.2                         2000-04-28                         SIGNAL(2)