signal(2) — Linux manual page


signal(2)                  System Calls Manual                 signal(2)

NAME         top

       signal - ANSI C signal handling

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

DESCRIPTION         top

       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 Portability

       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

       *  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

       The signals SIGKILL and SIGSTOP cannot be caught or ignored.

RETURN VALUE         top

       signal() returns the previous value of the signal handler.  On
       failure, it returns SIG_ERR, and errno is set to indicate the

ERRORS         top

       EINVAL signum is invalid.

VERSIONS         top

       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);

       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().

STANDARDS         top

       C11, POSIX.1-2008.

HISTORY         top

       C89, POSIX.1-2001.

       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

           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.

NOTES         top

       The effects of signal() in a multithreaded process are

       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.

SEE ALSO         top

       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)

Linux man-pages (unreleased)     (date)                        signal(2)

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