NAME

       signal - ANSI C signal handling

SYNOPSIS

       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

DESCRIPTION

       WARNING:
        the  behavior of signal() varies across UNIX versions, and has also varied historically across different
       versions of Linux.  Avoid its use: use sigaction(2) instead.  See Portability below.

       signal() sets the disposition of the signal signum to handler, which is either SIG_IGN, SIG_DFL,  or  the
       address of a programmer-defined function (a "signal handler").

       If the signal signum is delivered to the process, then one of the following happens:

       *  If the disposition is set to SIG_IGN, then the signal is ignored.

       *  If  the  disposition  is  set  to  SIG_DFL,  then  the  default action associated with the signal (see
          signal(7)) occurs.

       *  If the disposition is set to a function, then first either the disposition is reset to SIG_DFL, or the
          signal  is  blocked  (see  Portability  below),  and  then handler is called with argument signum.  If
          invocation of the handler caused the signal to be blocked, then the signal is  unblocked  upon  return
          from the handler.

       The signals SIGKILL and SIGSTOP cannot be caught or ignored.

RETURN VALUE

       signal()  returns  the  previous  value  of  the signal handler, or SIG_ERR on error.  In the event of an
       error, errno is set to indicate the cause.

ERRORS

       EINVAL signum is invalid.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, C89, C99.

NOTES

       The effects of signal() in a multithreaded process are unspecified.

       According to POSIX, the behavior of a process is undefined after it ignores a SIGFPE, SIGILL, or  SIGSEGV
       signal that was not generated by kill(2) or raise(3).  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.

       See sigaction(2) for details on what happens when the disposition SIGCHLD is set to SIG_IGN.

       See  signal-safety(7) for a list of the async-signal-safe functions that can be safely called from inside
       a signal handler.

       The use of sighandler_t is a GNU extension, exposed if _GNU_SOURCE is defined; glibc  also  defines  (the
       BSD-derived)  sig_t  if _BSD_SOURCE (glibc 2.19 and earlier) or _DEFAULT_SOURCE (glibc 2.19 and later) is
       defined.  Without use of such a type, the declaration of signal() is the somewhat harder to read:

           void ( *signal(int signum, void (*handler)(int)) ) (int);

   Portability
       The only portable use of signal() is to set a signal's disposition to SIG_DFL or SIG_IGN.  The  semantics
       when  using  signal()  to  establish a signal handler vary across systems (and POSIX.1 explicitly permits
       this variation); do not use it for this purpose.

       POSIX.1 solved the portability mess by specifying sigaction(2), which provides explicit  control  of  the
       semantics when a signal handler is invoked; use that interface instead of signal().

       In  the  original  UNIX  systems,  when  a handler that was established using signal() was invoked by the
       delivery of a signal, the disposition of the signal would be reset to SIG_DFL, and  the  system  did  not
       block  delivery  of further instances of the signal.  This is equivalent to calling sigaction(2) with the
       following flags:

           sa.sa_flags = SA_RESETHAND | SA_NODEFER;

       System V also provides these semantics for signal().  This was bad because the signal might be  delivered
       again  before  the handler had a chance to reestablish itself.  Furthermore, rapid deliveries of the same
       signal could result in recursive invocations of the handler.

       BSD improved on this situation, but unfortunately also changed the semantics  of  the  existing  signal()
       interface while doing so.  On BSD, when a signal handler is invoked, the signal disposition is not reset,
       and further instances of the signal are blocked from being delivered  while  the  handler  is  executing.
       Furthermore, certain blocking system calls are automatically restarted if interrupted by a signal handler
       (see signal(7)).  The BSD semantics are equivalent to calling sigaction(2) with the following flags:

           sa.sa_flags = SA_RESTART;

       The situation on Linux is as follows:

       * The kernel's signal() system call provides System V semantics.

       * By default, in glibc 2 and later, the signal() wrapper function does not invoke the kernel system call.
         Instead,  it  calls  sigaction(2)  using  flags  that  supply  BSD semantics.  This default behavior is
         provided as long as a suitable feature test macro is defined: _BSD_SOURCE on glibc 2.19 and earlier  or
         _DEFAULT_SOURCE   in   glibc   2.19   and   later.    (By   default,  these  macros  are  defined;  see
         feature_test_macros(7) for details.)  If such a feature  test  macro  is  not  defined,  then  signal()
         provides System V semantics.

SEE ALSO

       kill(1),   alarm(2),   kill(2),   pause(2),  sigaction(2),  signalfd(2),  sigpending(2),  sigprocmask(2),
       sigsuspend(2), bsd_signal(3), killpg(3), raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3),
       sysv_signal(3), signal(7)

COLOPHON

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