signal(2)



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 var-
       ied 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  func-
       tion (a "signal handler").

       If  the signal signum is delivered to the process, then one of the fol-
       lowing 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  asso-
          ciated with the signal (see signal(7)) occurs.

       *  If  the disposition is set to a function, then first either the dis-
          position is reset to SIG_DFL, or the signal is blocked  (see  Porta-
          bility  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
       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 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 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 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 sigac-
       tion(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 pro-
         vided  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).)

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

COLOPHON
       This  page  is  part of release 3.71 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-08-19                         SIGNAL(2)

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