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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  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.

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.  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
       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/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

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