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SIGNAL(3)	       FreeBSD Library Functions Manual		     SIGNAL(3)

NAME
     signal -- simplified software signal facilities

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <signal.h>

     void (*
     signal(int	sig, void (*func)(int)))(int);

     or	in FreeBSD's equivalent	but easier to read typedef'd version:

     typedef void (*sig_t) (int)

     sig_t
     signal(int	sig, sig_t func);

DESCRIPTION
     This signal() facility is a simplified interface to the more general
     sigaction(2) facility.

     Signals allow the manipulation of a process from outside its domain as
     well as allowing the process to manipulate	itself or copies of itself
     (children). There are two general types of	signals: those that cause ter-
     mination of a process and those that do not.  Signals which cause termi-
     nation of a program might result from an irrecoverable error or might be
     the result	of a user at a terminal	typing the `interrupt' character.
     Signals are used when a process is	stopped	because	it wishes to access
     its control terminal while	in the background (see tty(4)).	 Signals are
     optionally	generated when a process resumes after being stopped, when the
     status of child processes changes,	or when	input is ready at the control
     terminal.	Most signals result in the termination of the process receiv-
     ing them if no action is taken; some signals instead cause	the process
     receiving them to be stopped, or are simply discarded if the process has
     not requested otherwise.  Except for the SIGKILL and SIGSTOP signals, the
     signal() function allows for a signal to be caught, to be ignored,	or to
     generate an interrupt.  These signals are defined in the file <signal.h>:

     Name	     Default Action	     Description
     SIGHUP	     terminate process	     terminal line hangup
     SIGINT	     terminate process	     interrupt program
     SIGQUIT	     create core image	     quit program
     SIGILL	     create core image	     illegal instruction
     SIGTRAP	     create core image	     trace trap
     SIGABRT	     create core image	     abort program (formerly SIGIOT)
     SIGEMT	     create core image	     emulate instruction executed
     SIGFPE	     create core image	     floating-point exception
     SIGKILL	     terminate process	     kill program
     SIGBUS	     create core image	     bus error
     SIGSEGV	     create core image	     segmentation violation
     SIGSYS	     create core image	     non-existent system call invoked
     SIGPIPE	     terminate process	     write on a	pipe with no reader
     SIGALRM	     terminate process	     real-time timer expired
     SIGTERM	     terminate process	     software termination signal
     SIGURG	     discard signal	     urgent condition present on
					     socket
     SIGSTOP	     stop process	     stop (cannot be caught or
					     ignored)
     SIGTSTP	     stop process	     stop signal generated from
					     keyboard
     SIGCONT	     discard signal	     continue after stop
     SIGCHLD	     discard signal	     child status has changed
     SIGTTIN	     stop process	     background	read attempted from
					     control terminal
     SIGTTOU	     stop process	     background	write attempted	to
					     control terminal
     SIGIO	     discard signal	     I/O is possible on	a descriptor
					     (see fcntl(2))
     SIGXCPU	     terminate process	     cpu time limit exceeded (see
					     setrlimit(2))
     SIGXFSZ	     terminate process	     file size limit exceeded (see
					     setrlimit(2))
     SIGVTALRM	     terminate process	     virtual time alarm	(see
					     setitimer(2))
     SIGPROF	     terminate process	     profiling timer alarm (see
					     setitimer(2))
     SIGWINCH	     discard signal	     Window size change
     SIGINFO	     discard signal	     status request from keyboard
     SIGUSR1	     terminate process	     User defined signal 1
     SIGUSR2	     terminate process	     User defined signal 2

     The sig parameter specifies which signal was received.  The func proce-
     dure allows a user	to choose the action upon receipt of a signal.	To set
     the default action	of the signal to occur as listed above,	func should be
     SIG_DFL.  A SIG_DFL resets	the default action.  To	ignore the signal func
     should be SIG_IGN.	 This will cause subsequent instances of the signal to
     be	ignored	and pending instances to be discarded.	If SIG_IGN is not
     used, further occurrences of the signal are automatically blocked and
     func is called.

     The handled signal	is unblocked when the function returns and the process
     continues from where it left off when the signal occurred.	 Unlike	previ-
     ous signal	facilities, the	handler	func() remains installed after a sig-
     nal has been delivered.

     For some system calls, if a signal	is caught while	the call is executing
     and the call is prematurely terminated, the call is automatically
     restarted.	 (The handler is installed using the SA_RESTART	flag with
     sigaction(2).)  The affected system calls include read(2),	write(2),
     sendto(2),	recvfrom(2), sendmsg(2)	and recvmsg(2) on a communications
     channel or	a low speed device and during a	ioctl(2) or wait(2).  However,
     calls that	have already committed are not restarted, but instead return a
     partial success (for example, a short read	count).	 These semantics could
     be	changed	with siginterrupt(3).

     When a process which has installed	signal handlers	forks, the child
     process inherits the signals.  All	caught signals may be reset to their
     default action by a call to the execve(2) function; ignored signals
     remain ignored.

     See sigaction(2) for a list of functions that are considered safe for use
     in	signal handlers.

RETURN VALUES
     The previous action is returned on	a successful call.  Otherwise, SIG_ERR
     is	returned and  the global variable errno	is set to indicate the error.

ERRORS
     Signal() will fail	and no action will take	place if one of	the following
     occur:

     [EINVAL]		Sig is not a valid signal number.

     [EINVAL]		An attempt is made to ignore or	supply a handler for
			SIGKILL	or SIGSTOP.

SEE ALSO
     kill(1), kill(2), ptrace(2), sigaction(2),	sigaltstack(2),
     sigprocmask(2), sigsuspend(2), fpsetmask(3), setjmp(3), siginterrupt(3),
     tty(4)

HISTORY
     This signal() facility appeared in	4.0BSD.

FreeBSD	9.3			April 19, 1994			   FreeBSD 9.3

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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