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  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 pro-
       grammer-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.

ERRORS
       EINVAL signum is invalid.

CONFORMING TO
       C89, C99, POSIX.1-2001.

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 gen-
       erate  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 SIGCHLD is set to SIG_IGN.

       See signal(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.  Various versions of libc predefine this type; libc4 and libc5 define Signal-
       Handler; glibc defines sig_t and, when _GNU_SOURCE is defined, also sighandler_t.  Without use of such a type, the decla-
       ration 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  sig-
       nal()  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.   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 by changing the semantics of signal handling (but, unfortunately, silently changed the
       semantics when establishing a handler with signal()).  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.

       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 the _BSD_SOURCE
         feature  test  macro  is  defined.   By  default,  _BSD_SOURCE is defined; it is also implicitly defined if one defines
         _GNU_SOURCE, and can of course be explicitly defined.

         On glibc 2 and later, if the _BSD_SOURCE feature test macro is not defined, then signal() provides System V  semantics.
         (The  default  implicit  definition  of  _BSD_SOURCE is not provided if one invokes gcc(1) in one of its standard modes
         (-std=xxx or -ansi) or defines various other feature test macros such as _POSIX_SOURCE, _XOPEN_SOURCE, or _SVID_SOURCE;
         see feature_test_macros(7).)

       * The signal() function in Linux libc4 and libc5 provide System V semantics.  If one on a libc5 system includes <bsd/sig-
         nal.h> instead of <signal.h>, then signal() provides BSD semantics.

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

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-07-11                                                  SIGNAL(2)