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           #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 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.


           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.


           C89, C99, POSIX.1-2001.


           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.
           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).)

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