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SIGNAL(2)		   Linux Programmers Manual		    SIGNAL(2)

       signal - ANSI C signal handling


       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

       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.

       signal()  returns  the previous value of the signal handler, or SIG_ERR
       on error.

       EINVAL signum is invalid.

       C89, C99, POSIX.1-2001.

       The effects of signal() in a multi-threaded process are unspecified.

       According to POSIX, the behavior of a process  is  undefined  after  it
       ignores	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

       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.  Various versions of libc
       predefine this  type;  libc4  and  libc5  define  SignalHandler,  glibc
       defines	sig_t  and,  when  _GNU_SOURCE	is defined, also sighandler_t.
       Without use of such a type, the declaration of signal() is the somewhat
       harder to read:

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

       The  only  portable use of signal() is to set a signals 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.	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.
       Furthermore, rapid deliveries of the same signal could result in recur
       sive invocations of the handler.

       BSD improved on this situation by changing the semantics of signal han
       dling (but, unfortunately, silently changed the semantics  when	estab
       lishing	a  handler  with  signal()).  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 exe

       The situation on Linux is as follows:

       * The kernels 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
	 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  instead
	 of , then signal() provides BSD semantics.

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

       This  page  is  part of release 3.05 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				  2008-07-11			     SIGNAL(2)

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