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

NAME
     signal, bsd_signal	-- simplified software signal facilities

SYNOPSIS
     #include <signal.h>

     void
     (*signal(int sigcatch, void (*func)(int sigraised)))(int);

     void
     (*bsd_signal(int sigcatch,	void (*func)(int sigraised)))(int);

DESCRIPTION
     The signal() and bsd_signal() facilities are simplified interfaces	to the
     more general sigaction(2) facility.  The bsd_signal() interface is	pro-
     vided for source compatibility only.  It is mainly	used on	systems	where
     the standard signal() does	not have BSD semantics.	 On OpenBSD the	two
     interfaces	are identical.

     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 any 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(3)	call (formerly SIGIOT)
     SIGEMT	  create core image    emulate instruction executed
     SIGFPE	  create core image    floating-point exception
     SIGKILL	  terminate process    kill program (cannot be caught or
							       ignored)
     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
     SIGTHR	  discard signal       thread AST

     The func argument is a function to	be called as the action	upon receipt
     of	the signal sigcatch.  The function will	be called with one argument,
     sigraised,	which is the signal raised (thus the same function, func, can
     be	used by	more than one signal).	To set the default action of the sig-
     nal 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.

     If	the func is set	to SIG_IGN for the SIGCHLD signal, the system will not
     create zombie processes when children of the calling process exit.	 If
     the calling process subsequently issues a wait(2) (or equivalent),	it
     blocks until all of the calling process's child processes terminate, and
     then returns a value of -1	with errno set to ECHILD.  This	differs	from
     historical	BSD behavior but is consistent with AT&T System	V UNIX as well
     as	the X/Open Portability Guide Issue 4, Version 2	("XPG4.2").

     The handled signal	is unblocked when func returns and the process contin-
     ues from where it left off	when the signal	occurred.  Unlike previous
     signal facilities,	the handler func() remains installed after a signal
     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).	 The siginterrupt(3)
     function can be used to change the	system call restart behavior for a
     specific signal.

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

     The following functions are either	reentrant or not interruptible by sig-
     nals and are async-signal-safe.  Therefore	applications may invoke	them,
     without restriction, from signal-catching functions:

     Standard Interfaces:

     _exit(), _Exit(), abort(),	accept(), access(), alarm(), bind(),
     cfgetispeed(), cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(),
     chmod(), chown(), clock_gettime(),	close(), connect(), creat(), dup(),
     dup2(), execl(), execle(),	execv(), execve(), faccessat(),	fchdir(),
     fchmod(), fchmodat(), fchown(), fchownat(), fcntl(), fdatasync(), fork(),
     fpathconf(), fstat(), fstatat(), fsync(), ftruncate(), futimens(),
     futimes(),	getegid(), geteuid(), getgid(),	getgroups(), getpeername(),
     getpgrp(),	getpid(), getppid(), getsockname(), getsockopt(), getuid(),
     kill(), link(), linkat(), listen(), lseek(), lstat(), mkdir(), mkdirat(),
     mkfifo(), mkfifoat(), mknod(), mknodat(), open(), openat(), pathconf(),
     pause(), pipe(), poll(), pselect(), pthread_sigmask(), raise(), read(),
     readlink(), readlinkat(), recv(), recvfrom(), recvmsg(), rename(),
     renameat(), rmdir(), select(), send(), sendmsg(), sendto(), setgid(),
     setpgid(),	setsid(), setsockopt(),	setuid(), shutdown(), sigaction(),
     sigaddset(), sigdelset(), sigemptyset(), sigfillset(), sigismember(),
     signal(), sigpause(), sigpending(), sigprocmask(),	sigsuspend(), sleep(),
     sockatmark(), socket(), socketpair(), stat(), strcat(), strcpy(),
     strncat(),	strncpy(), symlink(), symlinkat(), sysconf(), tcdrain(),
     tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(),
     tcsetattr(), tcsetpgrp(), time(), times(),	umask(), uname(), unlink(),
     unlinkat(), utime(), utimensat(), utimes(), wait(), waitpid(), write(),
     and perhaps some others.

     Extension Interfaces:

     accept4(),	chflags(), chflagsat(),	dup3(),	fchflags(), getentropy(),
     getresgid(), getresuid(), pipe2(),	ppoll(), sendsyslog(), setresgid(),
     setresuid(), strlcat(), strlcpy(),	wait3(), wait4().

     In	addition, access and updates to	errno are guaranteed to	be safe.  Most
     functions not in the above	lists are considered to	be unsafe with respect
     to	signals.  That is to say, the behaviour	of such	functions when called
     from a signal handler is undefined.  In general though, signal handlers
     should do little more than	set a flag, ideally of type volatile
     sig_atomic_t; most	other actions are not safe.

     Additionally, it is advised that signal handlers guard against modifica-
     tion of the external symbol errno by the above functions, saving it at
     entry and restoring it on return, thus:

	   void
	   handler(int sig)
	   {
		   int save_errno = errno;

		   ...
		   errno = save_errno;
	   }

     The functions below are async-signal-safe in OpenBSD except when used
     with floating-point arguments or directives, but are probably unsafe on
     other systems:

	   dprintf()	 Safe.
	   vdprintf()	 Safe.
	   snprintf()	 Safe.
	   vsnprintf()	 Safe.
	   syslog_r()	 Safe if the syslog_data struct	is initialized as a
			 local variable.

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
     occurs:

     [EINVAL]		A specified signal 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), setjmp(3), siginterrupt(3),	tty(4)

HISTORY
     A signal()	system call first appeared in Version 4	AT&T UNIX.  In 4.2BSD,
     it	was reimplemented as a wrapper around the former sigvec() system call,
     and for 4.3BSD-Reno, it was rewritten to use sigaction(2) instead.

BSD				August 1, 2017				   BSD

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

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