SIGNAL(3)                  POSIX Programmer's Manual                 SIGNAL(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       signal -- signal management

SYNOPSIS
       #include <signal.h>

       void (*signal(int sig, void (*func)(int)))(int);

DESCRIPTION
       The functionality described on this reference page is aligned with the
       ISO C standard. Any conflict between the requirements described here
       and the ISO C standard is unintentional. This volume of POSIX.1-2017
       defers to the ISO C standard.

       The signal() function chooses one of three ways in which receipt of the
       signal number sig is to be subsequently handled. If the value of func
       is SIG_DFL, default handling for that signal shall occur.  If the value
       of func is SIG_IGN, the signal shall be ignored.  Otherwise, the
       application shall ensure that func points to a function to be called
       when that signal occurs. An invocation of such a function because of a
       signal, or (recursively) of any further functions called by that
       invocation (other than functions in the standard library), is called a
       ``signal handler''.

       When a signal occurs, and func points to a function, it is
       implementation-defined whether the equivalent of a:


           signal(sig, SIG_DFL);

       is executed or the implementation prevents some implementation-defined
       set of signals (at least including sig) from occurring until the
       current signal handling has completed. (If the value of sig is SIGILL,
       the implementation may alternatively define that no action is taken.)
       Next the equivalent of:


           (*func)(sig);

       is executed. If and when the function returns, if the value of sig was
       SIGFPE, SIGILL, or SIGSEGV or any other implementation-defined value
       corresponding to a computational exception, the behavior is undefined.
       Otherwise, the program shall resume execution at the point it was
       interrupted. The ISO C standard places a restriction on applications
       relating to the use of raise() from signal handlers.  This restriction
       does not apply to POSIX applications, as POSIX.1-2008 requires raise()
       to be async-signal-safe (see Section 2.4.3, Signal Actions).

       If the process is multi-threaded, or if the process is single-threaded
       and a signal handler is executed other than as the result of:

        *  The process calling abort(), raise(), kill(), pthread__kill(), or
           sigqueue() to generate a signal that is not blocked

        *  A pending signal being unblocked and being delivered before the
           call that unblocked it returns

       the behavior is undefined if the signal handler refers to any object
       other than errno with static storage duration other than by assigning a
       value to an object declared as volatile sig_atomic_t, or if the signal
       handler calls any function defined in this standard other than one of
       the functions listed in Section 2.4, Signal Concepts.

       At program start-up, the equivalent of:


           signal(sig, SIG_IGN);

       is executed for some signals, and the equivalent of:


           signal(sig, SIG_DFL);

       is executed for all other signals (see exec).

       The signal() function shall not change the setting of errno if
       successful.

RETURN VALUE
       If the request can be honored, signal() shall return the value of func
       for the most recent call to signal() for the specified signal sig.
       Otherwise, SIG_ERR shall be returned and a positive value shall be
       stored in errno.

ERRORS
       The signal() function shall fail if:

       EINVAL The sig argument is not a valid signal number or an attempt is
              made to catch a signal that cannot be caught or ignore a signal
              that cannot be ignored.

       The signal() function may fail if:

       EINVAL An attempt was made to set the action to SIG_DFL for a signal
              that cannot be caught or ignored (or both).

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       The sigaction() function provides a more comprehensive and reliable
       mechanism for controlling signals; new applications should use
       sigaction() rather than signal().

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.4, Signal Concepts, exec, pause(), raise(), sigaction(),
       sigsuspend(), waitid()

       The Base Definitions volume of POSIX.1-2017, <signal.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                            SIGNAL(3)
