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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 varied historically
       across different versions of Linux.  Avoid its use: use sigaction(2) instead.  See  Porta-
       bility 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 function (a "signal handler").

       If the signal signum is delivered to the process, then one of the following 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 associated with the  sig-
	  nal (see signal(7)) occurs.

       *  If  the disposition is set to a function, then first either the disposition is reset to
	  SIG_DFL, or the signal is blocked (see Portability 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 ignores a SIGFPE, SIG-
       ILL, 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 invoked; 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 delivery of further instances of the  signal.   This
       is equivalent to calling sigaction(2) with the following flags:

	   sa.sa_flags = SA_RESETHAND | SA_NODEFER;

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

       BSD improved on this situation, but unfortunately also changed the semantics of the exist-
       ing signal() interface while doing so.  On BSD, when a signal handler is invoked, the sig-
       nal 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 sigaction(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 ker-
	 nel system call.  Instead, it calls sigaction(2) using flags that supply BSD  semantics.
	 This  default	behavior  is  provided	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 sig-
	 nal() provides System V semantics.  (The default implicit 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).)

       * The  signal() function in Linux libc4 and libc5 provide System V semantics.  If one on a
	 libc5 system includes <bsd/signal.h> instead of <signal.h>, then signal()  provides  BSD
	 semantics.

SEE ALSO
       kill(1), alarm(2), kill(2), killpg(2), pause(2), sigaction(2), signalfd(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 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-04-19					SIGNAL(2)
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