signal

SIGNAL(2)                   Linux Programmer's Manual                  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
       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 the disposition 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 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 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 5.08 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/.



Linux                              2017-09-15                          SIGNAL(2)