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SIGACTION(2)		  FreeBSD System Calls Manual		  SIGACTION(2)

     sigaction -- software signal facilities

     Standard C	Library	(libc, -lc)

     #include <signal.h>

     struct  sigaction {
	     void    (*sa_handler)(int);
	     void    (*sa_sigaction)(int, siginfo_t *, void *);
	     int     sa_flags;		     /*	see signal options below */
	     sigset_t sa_mask;		     /*	signal mask to apply */

     sigaction(int sig,	const struct sigaction * restrict act,
	 struct	sigaction * restrict oact);

     The system	defines	a set of signals that may be delivered to a process.
     Signal delivery resembles the occurrence of a hardware interrupt: the
     signal is normally	blocked	from further occurrence, the current thread
     context is	saved, and a new one is	built.	A process may specify a
     handler to	which a	signal is delivered, or	specify	that a signal is to be
     ignored.  A process may also specify that a default action	is to be taken
     by	the system when	a signal occurs.  A signal may also be blocked for a
     thread, in	which case it will not be delivered to that thread until it is
     unblocked.	 The action to be taken	on delivery is determined at the time
     of	delivery.  Normally, signal handlers execute on	the current stack of
     the thread.  This may be changed, on a per-handler	basis, so that signals
     are taken on a special signal stack.

     Signal routines normally execute with the signal that caused their	invo-
     cation blocked, but other signals may yet occur.  A global	signal mask
     defines the set of	signals	currently blocked from delivery	to a thread.
     The signal	mask for a thread is initialized from that of its parent (nor-
     mally empty).  It may be changed with a sigprocmask(2) or
     pthread_sigmask(3)	call, or when a	signal is delivered to the thread.

     When a signal condition arises for	a process or thread, the signal	is
     added to a	set of signals pending for the process or thread.  Whether the
     signal is directed	at the process in general or at	a specific thread
     depends on	how it is generated.  For signals directed at a	specific
     thread, if	the signal is not currently blocked by the thread then it is
     delivered to the thread.  For signals directed at the process, if the
     signal is not currently blocked by	all threads then it is delivered to
     one thread	that does not have it blocked (the selection of	which is
     unspecified).  Signals may	be delivered any time a	thread enters the
     operating system (e.g., during a system call, page	fault or trap, or
     clock interrupt).	If multiple signals are	ready to be delivered at the
     same time,	any signals that could be caused by traps are delivered	first.
     Additional	signals	may be processed at the	same time, with	each appearing
     to	interrupt the handlers for the previous	signals	before their first
     instructions.  The	set of pending signals is returned by the
     sigpending(2) system call.	 When a	caught signal is delivered, the	cur-
     rent state	of the thread is saved,	a new signal mask is calculated	(as
     described below), and the signal handler is invoked.  The call to the
     handler is	arranged so that if the	signal handling	routine	returns	nor-
     mally the thread will resume execution in the context from	before the
     signal's delivery.	 If the	thread wishes to resume	in a different con-
     text, then	it must	arrange	to restore the previous	context	itself.

     When a signal is delivered	to a thread a new signal mask is installed for
     the duration of the process' signal handler (or until a sigprocmask(2)
     system call is made).  This mask is formed	by taking the union of the
     current signal mask set, the signal to be delivered, and the signal mask
     associated	with the handler to be invoked.

     The sigaction() system call assigns an action for a signal	specified by
     sig.  If act is non-NULL, it specifies an action (SIG_DFL,	SIG_IGN, or a
     handler routine) and mask to be used when delivering the specified	sig-
     nal.  If oact is non-NULL,	the previous handling information for the sig-
     nal is returned to	the user.

     The above declaration of struct sigaction is not literal.	It is provided
     only to list the accessible members.  See <sys/signal.h> for the actual
     definition.  In particular, the storage occupied by sa_handler and
     sa_sigaction overlaps, and	it is nonsensical for an application to
     attempt to	use both simultaneously.

     Once a signal handler is installed, it normally remains installed until
     another sigaction() system	call is	made, or an execve(2) is performed.  A
     signal-specific default action may	be reset by setting sa_handler to
     SIG_DFL.  The defaults are	process	termination, possibly with core	dump;
     no	action;	stopping the process; or continuing the	process.  See the sig-
     nal list below for	each signal's default action.  If sa_handler is
     SIG_DFL, the default action for the signal	is to discard the signal, and
     if	a signal is pending, the pending signal	is discarded even if the sig-
     nal is masked.  If	sa_handler is set to SIG_IGN current and pending
     instances of the signal are ignored and discarded.

     Options may be specified by setting sa_flags.  The	meaning	of the various
     bits is as	follows:

	   SA_NOCLDSTOP	   If this bit is set when installing a	catching func-
			   tion	for the	SIGCHLD	signal,	the SIGCHLD signal
			   will	be generated only when a child process exits,
			   not when a child process stops.

	   SA_NOCLDWAIT	   If this bit is set when calling sigaction() 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.  The	same effect of avoiding	zombie cre-
			   ation can also be achieved by setting sa_handler
			   for SIGCHLD to SIG_IGN.

	   SA_ONSTACK	   If this bit is set, the system will deliver the
			   signal to the process on a signal stack, specified
			   by each thread with sigaltstack(2).

	   SA_NODEFER	   If this bit is set, further occurrences of the
			   delivered signal are	not masked during the execu-
			   tion	of the handler.

	   SA_RESETHAND	   If this bit is set, the handler is reset back to
			   SIG_DFL at the moment the signal is delivered.

	   SA_RESTART	   See paragraph below.

	   SA_SIGINFO	   If this bit is set, the handler function is assumed
			   to be pointed to by the sa_sigaction	member of
			   struct sigaction and	should match the prototype
			   shown above or as below in EXAMPLES.	 This bit
			   should not be set when assigning SIG_DFL or

     If	a signal is caught during the system calls listed below, the call may
     be	forced to terminate with the error EINTR, the call may return with a
     data transfer shorter than	requested, or the call may be restarted.
     Restart of	pending	calls is requested by setting the SA_RESTART bit in
     sa_flags.	The affected system calls include open(2), read(2), write(2),
     sendto(2),	recvfrom(2), sendmsg(2)	and recvmsg(2) on a communications
     channel or	a slow device (such as a terminal, but not a regular file) and
     during a wait(2) or ioctl(2).  However, calls that	have already committed
     are not restarted,	but instead return a partial success (for example, a
     short read	count).

     After a pthread_create(3) the signal mask is inherited by the new thread
     and the set of pending signals and	the signal stack for the new thread
     are empty.

     After a fork(2) or	vfork(2) all signals, the signal mask, the signal
     stack, and	the restart/interrupt flags are	inherited by the child.

     The execve(2) system call reinstates the default action for all signals
     which were	caught and resets all signals to be caught on the user stack.
     Ignored signals remain ignored; the signal	mask remains the same; signals
     that restart pending system calls continue	to do so.

     The following is a	list of	all signals with names as in the include file

     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
     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
     SIGSTOP	     stop process	     stop (cannot be caught or
     SIGTSTP	     stop process	     stop signal generated from
     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
     SIGXFSZ	     terminate process	     file size limit exceeded (see
     SIGVTALRM	     terminate process	     virtual time alarm	(see
     SIGPROF	     terminate process	     profiling timer alarm (see
     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 sa_mask field specified in act	is not allowed to block	SIGKILL	or
     SIGSTOP.  Any attempt to do so will be silently 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 or from a child
     process after calling fork(2) in a	multi-threaded process:

     Base Interfaces:

     _Exit(), _exit(), accept(), access(), alarm(), bind(), cfgetispeed(),
     cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(), chmod(), chown(),
     close(), connect(), creat(), dup(), dup2(), execl(), execle(), execv(),
     execve(), faccessat(), fchdir(), fchmod(),	fchmodat(), fchown(),
     fchownat(), fcntl(), fork(), fstat(), fstatat(), fsync(), ftruncate(),
     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(), 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(), sigpending(), sigprocmask(), sigsuspend(), sleep(),
     sockatmark(), socket(), socketpair(), stat(), symlink(), symlinkat(),
     tcdrain(),	tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(),
     tcsetattr(), tcsetpgrp(), time(), times(),	umask(), uname(), unlink(),
     unlinkat(), utime(), wait(), waitpid(), write().

     X/Open Systems Interfaces:

     sigpause(), sigset(), utimes().

     Realtime Interfaces:

     aio_error(), clock_gettime(), timer_getoverrun(), aio_return(),
     fdatasync(), sigqueue(), timer_gettime(), aio_suspend(), sem_post(),

     Base Interfaces not specified as async-signal safe	by POSIX:

     fpathconf(), pathconf(), sysconf().

     Base Interfaces not specified as async-signal safe	by POSIX, but planned
     to	be:

     ffs(), htonl(), htons(), memccpy(), memchr(), memcmp(), memcpy(),
     memmove(),	memset(), ntohl(), ntohs(), stpcpy(), stpncpy(), strcat(),
     strchr(), strcmp(), strcpy(), strcspn(), strlen(),	strncat(), strncmp(),
     strncpy(),	strnlen(), strpbrk(), strrchr(), strspn(), strstr(),
     strtok_r(), wcpcpy(), wcpncpy(), wcscat(),	wcschr(), wcscmp(), wcscpy(),
     wcscspn(),	wcslen(), wcsncat(), wcsncmp(),	wcsncpy(), wcsnlen(),
     wcspbrk(),	wcsrchr(), wcsspn(), wcsstr(), wcstok(), wmemchr(), wmemcmp(),
     wmemcpy(),	wmemmove(), wmemset().

     Extension Interfaces:

     accept4(),	bindat(), closefrom(), connectat(), eaccess(), ffsl(),
     ffsll(), flock(), fls(), flsl(), flsll(), futimesat(), pipe2(),
     strlcat().	 strlcpy(), strsep().

     In	addition, reading or writing errno is async-signal safe.

     All functions not in the above lists are considered to be unsafe with
     respect to	signals.  That is to say, the behaviour	of such	functions is
     undefined when they are called from a signal handler that interrupted an
     unsafe function.  In general though, signal handlers should do little
     more than set a flag; most	other actions are not safe.

     Also, it is good practice to make a copy of the global variable errno and
     restore it	before returning from the signal handler.  This	protects
     against the side effect of	errno being set	by functions called from
     inside the	signal handler.

     The sigaction() function returns the value	0 if successful; otherwise the
     value -1 is returned and the global variable errno	is set to indicate the

     There are three possible prototypes the handler may match:

	   ANSI	C:
		  void handler(int);

	   Traditional BSD style:
		  void handler(int, int	code, struct sigcontext	*scp);

		  void handler(int, siginfo_t *info, ucontext_t	*uap);

     The handler function should match the SA_SIGINFO prototype	if the
     SA_SIGINFO	bit is set in sa_flags.	 It then should	be pointed to by the
     sa_sigaction member of struct sigaction.  Note that you should not	assign
     SIG_DFL or	SIG_IGN	this way.

     If	the SA_SIGINFO flag is not set,	the handler function should match
     either the	ANSI C or traditional BSD prototype and	be pointed to by the
     sa_handler	member of struct sigaction.  In	practice, FreeBSD always sends
     the three arguments of the	latter and since the ANSI C prototype is a
     subset, both will work.  The sa_handler member declaration	in FreeBSD
     include files is that of ANSI C (as required by POSIX), so	a function
     pointer of	a BSD-style function needs to be casted	to compile without
     warning.  The traditional BSD style is not	portable and since its capa-
     bilities are a full subset	of a SA_SIGINFO	handler, its use is depre-

     The sig argument is the signal number, one	of the SIG... values from

     The code argument of the BSD-style	handler	and the	si_code	member of the
     info argument to a	SA_SIGINFO handler contain a numeric code explaining
     the cause of the signal, usually one of the SI_...	values from
     <sys/signal.h> or codes specific to a signal, i.e., one of	the FPE_...
     values for	SIGFPE.

     The scp argument to a BSD-style handler points to an instance of struct

     The uap argument to a POSIX SA_SIGINFO handler points to an instance of

     The sigaction() system call will fail and no new signal handler will be
     installed if one of the following occurs:

     [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), setitimer(2),	setrlimit(2), sigaltstack(2),
     sigpending(2), sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3),
     setjmp(3),	siginfo(3), siginterrupt(3), sigsetops(3), ucontext(3),	tty(4)

     The sigaction() system call is expected to	conform	to ISO/IEC 9945-1:1990
     (``POSIX.1'').  The SA_ONSTACK and	SA_RESTART flags are Berkeley exten-
     sions, as are the signals,	SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG,
     Those signals are available on most BSD-derived systems.  The SA_NODEFER
     and SA_RESETHAND flags are	intended for backwards compatibility with
     other operating systems.  The SA_NOCLDSTOP, and SA_NOCLDWAIT flags	are
     featuring options commonly	found in other operating systems.  The flags
     are approved by Version 2 of the Single UNIX Specification	(``SUSv2''),
     along with	the option to avoid zombie creation by ignoring	SIGCHLD.

FreeBSD	Ports 11.2		 June 28, 2018		    FreeBSD Ports 11.2


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