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SIGNAL(3)		 BSD Library Functions Manual		     SIGNAL(3)

     signal -- simplified software signal facilities

     Standard C	Library	(libc, -lc)

     #include <signal.h>

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

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

     typedef void (*sig_t) (int);

     signal(int	sig, sig_t func);

     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
     termination of a process and those	that do	not.  Signals which cause ter-
     mination 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>:

     Num   Name		Default	Action	     Description
     1	   SIGHUP	terminate process    terminal line hangup
     2	   SIGINT	terminate process    interrupt program
     3	   SIGQUIT	create core image    quit program
     4	   SIGILL	create core image    illegal instruction
     5	   SIGTRAP	create core image    trace trap
     6	   SIGABRT	create core image    abort program (formerly SIGIOT)
     7	   SIGEMT	create core image    emulate instruction executed
     8	   SIGFPE	create core image    floating-point exception
     9	   SIGKILL	terminate process    kill program
     10	   SIGBUS	create core image    bus error
     11	   SIGSEGV	create core image    segmentation violation
     12	   SIGSYS	create core image    non-existent system call invoked
     13	   SIGPIPE	terminate process    write on a	pipe with no reader
     14	   SIGALRM	terminate process    real-time timer expired
     15	   SIGTERM	terminate process    software termination signal
     16	   SIGURG	discard	signal	     urgent condition present on
     17	   SIGSTOP	stop process	     stop (cannot be caught or
     18	   SIGTSTP	stop process	     stop signal generated from
     19	   SIGCONT	discard	signal	     continue after stop
     20	   SIGCHLD	discard	signal	     child status has changed
     21	   SIGTTIN	stop process	     background	read attempted from
					     control terminal
     22	   SIGTTOU	stop process	     background	write attempted	to
					     control terminal
     23	   SIGIO	discard	signal	     I/O is possible on	a descriptor
					     (see fcntl(2))
     24	   SIGXCPU	terminate process    cpu time limit exceeded (see
     25	   SIGXFSZ	terminate process    file size limit exceeded (see
     26	   SIGVTALRM	terminate process    virtual time alarm	(see
     27	   SIGPROF	terminate process    profiling timer alarm (see
     28	   SIGWINCH	discard	signal	     Window size change
     29	   SIGINFO	discard	signal	     status request from keyboard
     30	   SIGUSR1	terminate process    User defined signal 1
     31	   SIGUSR2	terminate process    User defined signal 2
     32	   SIGTHR	terminate process    thread interrupt
     33	   SIGLIBRT	terminate process    real-time library interrupt

     The sig argument specifies	which signal was received.  The	func procedure
     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.	 Any handler installed with signal(3) will have	the SA_RESTART
     flag set, meaning that any	restartable system call	will not return	on re-
     ceipt of a	signal.	 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 re-
     main ignored.

     If	a process explicitly specifies SIG_IGN as the action for the signal
     SIGCHLD, the system will not create zombie	processes when children	of the
     calling process exit.  As a consequence, the system will discard the exit
     status from the child processes.  If the calling process subsequently is-
     sues a call to wait(2) or equivalent, it will block until all of the
     calling process's children	terminate, and then return a value of -1 with
     errno set to ECHILD.

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

     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.

     The signal() function will	fail and no action will	take place if one of
     the following occur:

     [EINVAL]		The sig	argument is not	a valid	signal number.

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

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

     The signal	facility appeared in 4.0BSD.  The option to avoid the creation
     of	child zombies through ignoring SIGCHLD appeared	in FreeBSD 5.0.

BSD				 June 7, 2004				   BSD


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