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 .