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 (con-
       sult 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
       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  sig-
       nal  shall be ignored. Otherwise, the application shall ensure that func points to a function to be called when that sig-
       nal 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  alter-
       natively 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 implemen-
       tation-defined value corresponding to a computational exception, the behavior is undefined. Otherwise, the program  shall
       resume  execution  at  the  point it was interrupted. If the signal occurs as the result of calling the abort(), raise(),
       kill(), pthread_kill(), or sigqueue()  function, the signal handler shall not call the raise() function.

       If the signal occurs other than as the result of calling abort(), raise(),   kill(), pthread_kill(), or sigqueue(),   the
       behavior  is  undefined if the signal handler refers to any object 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 in the standard library
       other  than  one  of  the  functions listed in Signal Concepts . Furthermore, if such a call fails, the value of errno is
       unspecified.

       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).

RETURN VALUE
       If the request can be honored, signal() shall return the value of func for the most recent call to signal() for the spec-
       ified 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
       Signal   Concepts,   exec(),   pause(),   sigaction(),   sigsuspend(),   waitid(),   the   Base   Definitions  volume  of
       IEEE Std 1003.1-2001, <signal.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                    SIGNAL(3P)