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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 ver-
       sions 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  sig-
          nal  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 archi-
       tectures 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 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 han-
       dler.

       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.  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 varia-
       tion); do not use it for this purpose.

       POSIX.1 solved the portability mess by specifying sigaction(2), which provides explicit control of  the  seman-
       tics 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/signal.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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-07-11                         SIGNAL(2)