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.   In  the
       event of an error, errno is set to indicate the cause.

ERRORS

       EINVAL signum is invalid.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, C89, C99.

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 sigaction(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 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).)

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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