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SIGNAL(3P)                 POSIX Programmer's Manual                SIGNAL(3P)
PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
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.1-2008
       defers to the ISO C standard.
       Use of this function is unspecified in a multi-threaded process.
       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 appli-
       cation  shall  ensure  that func points to a function to be called when
       that signal occurs. An invocation of such a function because of a  sig-
       nal,  or  (recursively) of any further functions called by that invoca-
       tion (other than functions in the standard library), is called a ``sig-
       nal handler''.
       When  a signal occurs, and func points to a function, it is implementa-
       tion-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 cur-
       rent 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 success-
       ful.
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  sigac-
       tion() rather than signal().
RATIONALE
       None.
FUTURE DIRECTIONS
       None.
SEE ALSO
       Section 2.4, Signal Concepts, exec, pause(), raise(), sigaction(), sig-
       suspend(), waitid()
       The Base Definitions volume of POSIX.1-2008, <signal.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                           SIGNAL(3P)