SIGNAL(2)                 (2017-09-15)                  SIGNAL(2)

     NAME
          signal - ANSI C signal handling

     SYNOPSIS
          #include <signal.h>

          typedef void (*sighandler_t)(int);

          sighandler_t signal(int signum, sighandler_t handler);

     DESCRIPTION
          WARNING:
           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 Porta-
          bility 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.

     Page 1                        Linux             (printed 5/24/22)

     SIGNAL(2)                 (2017-09-15)                  SIGNAL(2)

     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 the dispo-
          sition SIGCHLD is set to SIG_IGN.

          See signal-safety(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 (glibc 2.19 and earlier) or
          _DEFAULT_SOURCE (glibc 2.19 and later) is defined.  Without
          use of such a type, the declaration of signal() is the some-
          what harder to read:

              void ( *signal(int signum, void (*handler

        Portability
          The only portable use of signal() is to set a signal's dis-
          position 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

          POSIX.1 solved the portability mess by specifying
          sigaction(2), which provides explicit control of the seman-
          tics when a signal handler is invoked; use that interface
          instead of signal().

          In the original UNIX systems, when a handler that was estab-
          lished using signal() was invoked by the delivery of a sig-
          nal, 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.

     Page 2                        Linux             (printed 5/24/22)

     SIGNAL(2)                 (2017-09-15)                  SIGNAL(2)

          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 seman-
            tics.

          * 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 seman-
            tics.  This default behavior is provided as long as a
            suitable feature test macro is defined: _BSD_SOURCE on
            glibc 2.19 and earlier or _DEFAULT_SOURCE in glibc 2.19
            and later.  (By default, these macros are defined; see
            feature_test_macros(7) for details.)  If such a feature
            test macro is not defined, then signal() provides System V
            semantics.

     SEE ALSO
          kill(1), alarm(2), kill(2), pause(2), sigaction(2),
          signalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2),
          bsd_signal(3), killpg(3), raise(3), siginterrupt(3),
          sigqueue(3), sigsetops(3), sigvec(3), sysv_signal(3),
          signal(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 3                        Linux             (printed 5/24/22)