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       signal - ANSI C signal handling


       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);


       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.


       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


       EINVAL signum is invalid.


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


       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

       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);

       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).)


       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)


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