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.1‐2017 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(3POSIX)