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SIGNAL(3)		 BSD 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
     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>:

     No	   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
					     socket
     17	   SIGSTOP	stop process	     stop (cannot be caught or
					     ignored)
     18	   SIGTSTP	stop process	     stop signal generated from
					     keyboard
     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
					     setrlimit(2))
     25	   SIGXFSZ	terminate process    file size limit exceeded (see
					     setrlimit(2))
     26	   SIGVTALRM	terminate process    virtual time alarm (see
					     setitimer(2))
     27	   SIGPROF	terminate process    profiling timer alarm (see
					     setitimer(2))
     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

     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
     receipt of a signal.  The affected system calls include read(2),
     write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communi‐
     cations 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 seman‐
     tics 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.

     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
     issues 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.

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
     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
			SIGKILL or SIGSTOP.

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

HISTORY
     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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