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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),  sigsuspend(2),  bsd_signal(3),  raise(3),  siginterrupt(3),  sigqueue(3),
       sigsetops(3), sigvec(3), sysv_signal(3), signal(7)

COLOPHON

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