signal(2) — Linux manual page


SIGNAL(2)                 Linux Programmer's Manual                SIGNAL(2)

NAME         top

       signal - ANSI C signal handling

SYNOPSIS         top

       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

DESCRIPTION         top

       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

       The signals SIGKILL and SIGSTOP cannot be caught or ignored.

RETURN VALUE         top

       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

ERRORS         top

       EINVAL signum is invalid.

CONFORMING TO         top

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

NOTES         top

       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);

       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 disposi‐
       tion of the signal would be reset to SIG_DFL, and the system did not
       block delivery of further instances of the signal.  This is equiva‐
       lent 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         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)

COLOPHON         top

       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

Linux                            2017-09-15                        SIGNAL(2)

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