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 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
       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 signal (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 Portabil-
	  ity 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.

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, 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 divid-
       ing 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.  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);

   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.  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 recursive invocations of the handler.

       BSD improved on this situation by changing the semantics of signal handling (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 executing.

       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 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 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 var-
	 ious 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), sigqueue(2), sigsuspend(2),
       bsd_signal(3), raise(3), siginterrupt(3), sigsetops(3), sigvec(3), sysv_signal(3), feature_test_macros(7), signal(7)

COLOPHON

       This page is part of release 3.27 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)