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

NAME
     signal — simplified software signal facilities

SYNOPSIS
     #include <signal.h>

     void
     *signal(sig, func());

     void
     (*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(2) call (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	     system call given invalid
					     argument
     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 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 dis‐
     carded. If SIG_IGN is not used, further occurrences of the signal are
     automatically blocked and func is called.

     The handled signal is unblocked with 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).

     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.

RETURN VALUES
     The previous action is returned on a successful call.  Otherwise, -1 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), ptrace(2), kill(2), sigaction(2), sigaltstack(2),
     sigprocmask(2), sigsuspend(2), setjmp(3), tty(4)

HISTORY
     This signal facility appeared in 4.0BSD.

4th Berkeley Distribution	April 19, 1994	     4th Berkeley Distribution
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