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SIGNAL(2)		   Linux Programmer's Manual		     SIGNAL(2)

NAME
       signal -	ANSI C signal handling

SYNOPSIS
       #include	<signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

DESCRIPTION
       The behavior of signal()	varies across UNIX versions, and has also var-
       ied historically	across different versions of Linux.   Avoid  its  use:
       use sigaction(2)	instead.  See Portability below.

       signal()	sets the disposition of	the signal signum to handler, which is
       either SIG_IGN, SIG_DFL,	or the address of a  programmer-defined	 func-
       tion (a "signal handler").

       If  the signal signum is	delivered to the process, then one of the fol-
       lowing happens:

       *  If the disposition is	set to SIG_IGN,	then the signal	is ignored.

       *  If the disposition is	set to SIG_DFL,	then the default action	 asso-
	  ciated with the signal (see signal(7)) occurs.

       *  If  the disposition is set to	a function, then first either the dis-
	  position is reset to SIG_DFL,	or the signal is blocked  (see	Porta-
	  bility  below), and then handler is called with argument signum.  If
	  invocation of	the handler caused the signal to be blocked, then  the
	  signal is unblocked upon return from the handler.

       The signals SIGKILL and SIGSTOP cannot be caught	or ignored.

RETURN VALUE
       signal()	 returns  the previous value of	the signal handler, or SIG_ERR
       on error.  In the event of an error,  errno  is	set  to	 indicate  the
       cause.

ERRORS
       EINVAL signum is	invalid.

CONFORMING TO
       C89, C99, POSIX.1-2001.

NOTES
       The effects of signal() in a multithreaded process are unspecified.

       According to POSIX, the behavior	of a process is	undefined after	it ig-
       nores a SIGFPE, SIGILL, or SIGSEGV signal that  was  not	 generated  by
       kill(2)	or  raise(3).	Integer	division by zero has undefined result.
       On some architectures it	will generate a	SIGFPE signal.	(Also dividing
       the  most  negative  integer by -1 may generate SIGFPE.)	 Ignoring this
       signal might lead to an endless loop.

       See sigaction(2)	for details on what happens when  SIGCHLD  is  set  to
       SIG_IGN.

       See signal(7) for a list	of the async-signal-safe functions that	can be
       safely called from inside a signal handler.

       The use of sighandler_t is a GNU	extension, exposed if  _GNU_SOURCE  is
       defined;	 glibc	also defines (the BSD-derived) sig_t if	_BSD_SOURCE is
       defined.	 Without use of	such a type, the declaration  of  signal()  is
       the somewhat harder to read:

	   void	( *signal(int signum, void (*handler)(int)) ) (int);

   Portability
       The  only  portable use of signal() is to set a signal's	disposition to
       SIG_DFL or SIG_IGN.  The	semantics when using signal() to  establish  a
       signal handler vary across systems (and POSIX.1 explicitly permits this
       variation); do not use it for this purpose.

       POSIX.1 solved the portability mess by specifying  sigaction(2),	 which
       provides	explicit control of the	semantics when a signal	handler	is in-
       voked; use that interface instead of signal().

       In the original UNIX systems, when a handler that was established using
       signal()	 was  invoked  by the delivery of a signal, the	disposition of
       the signal would	be reset to SIG_DFL, and the system did	not block  de-
       livery of further instances of the signal.  This	is equivalent to call-
       ing sigaction(2)	with the following flags:

	   sa.sa_flags = SA_RESETHAND |	SA_NODEFER;

       System V	also provides these semantics for signal().  This was bad  be-
       cause  the  signal  might  be  delivered	again before the handler had a
       chance to reestablish itself.  Furthermore,  rapid  deliveries  of  the
       same signal could result	in recursive invocations of the	handler.

       BSD  improved on	this situation,	but unfortunately also changed the se-
       mantics of the existing signal()	interface while	 doing	so.   On  BSD,
       when  a signal handler is invoked, the signal disposition is not	reset,
       and further instances of	the signal are blocked	from  being  delivered
       while  the  handler is executing.  Furthermore, certain blocking	system
       calls are automatically restarted if interrupted	by  a  signal  handler
       (see  signal(7)).   The	BSD semantics are equivalent to	calling	sigac-
       tion(2) with the	following flags:

	   sa.sa_flags = SA_RESTART;

       The situation on	Linux is as follows:

       * The kernel's signal() system call provides System V semantics.

       * By default, in	glibc 2	and later, the signal()	wrapper	function  does
	 not  invoke  the  kernel system call.	Instead, it calls sigaction(2)
	 using flags that supply BSD semantics.	 This default behavior is pro-
	 vided	as  long as the	_BSD_SOURCE feature test macro is defined.  By
	 default, _BSD_SOURCE is defined; it is	also implicitly	defined	if one
	 defines _GNU_SOURCE, and can of course	be explicitly defined.

       * On  glibc  2  and later, if the _BSD_SOURCE feature test macro	is not
	 defined, then signal()	provides System	V semantics.  (The default im-
	 plicit	 definition  of	 _BSD_SOURCE  is  not  provided	if one invokes
	 gcc(1)	in one of its standard modes (-std=xxx or  -ansi)  or  defines
	 various   other   feature   test   macros   such   as	_POSIX_SOURCE,
	 _XOPEN_SOURCE,	or _SVID_SOURCE; see feature_test_macros(7).)

SEE ALSO
       kill(1),	alarm(2), kill(2),  killpg(2),	pause(2),  sigaction(2),  sig-
       nalfd(2),  sigpending(2), sigprocmask(2), sigsuspend(2),	bsd_signal(3),
       raise(3),  siginterrupt(3),   sigqueue(3),   sigsetops(3),   sigvec(3),
       sysv_signal(3), signal(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-08-19			     SIGNAL(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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