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SIGACTION(2)		  OpenBSD Programmer's Manual		  SIGACTION(2)

NAME
     sigaction - software signal facilities

SYNOPSIS
     #include <signal.h>

     struct sigaction {
	     union {	     /* signal handler */
		     void    (*__sa_handler)(int);
		     void    (*__sa_sigaction)(int, siginfo_t *, void *);
	     } __sigaction_u;
	     sigset_t sa_mask;		/* signal mask to apply */
	     int      sa_flags;		/* see signal options below */
     };

     #define sa_handler	 __sigaction_u.__sa_handler
     #define sa_sigaction     __sigaction_u.__sa_sigaction

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

DESCRIPTION
     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 process
     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, in
     which case its delivery is postponed 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 process.  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
     invocation blocked, but other signals may yet occur.  A global signal
     mask defines the set of signals currently blocked from delivery to a
     process.  The signal mask for a process is initialized from that of its
     parent (normally empty).  It may be changed with a sigprocmask(2) call,
     or when a signal is delivered to the process.

     When a signal condition arises for a process, the signal is added to a
     set of signals pending for the process.  If the signal is not currently
     blocked by the process then it is delivered to the process.  Signals may
     be delivered any time a process 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) function.	 When a caught
     signal is delivered, the current state of the process 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 normally the process will resume execution in
     the context from before the signal's delivery.  If the process wishes to
     resume in a different context, then it must arrange to restore the
     previous context itself.

     When a signal is delivered to a process a new signal mask is installed
     for the duration of the process' signal handler (or until a
     sigprocmask(2) 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 sa_mask associated with the handler to be invoked, but always
     excluding SIGKILL and SIGSTOP.

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

     Once a signal handler is installed, it normally remains installed until
     another sigaction() call is made, or an execve(2) is performed.  The
     value of sa_handler (or, if the SA_SIGINFO flag is set, the value of
     sa_sigaction instead) indicates what action should be performed when a
     signal arrives.  A signal-specific default action may be reset by setting
     sa_handler to SIG_DFL.  Alternately, if the SA_RESETHAND flag is set the
     default action will be reinstated when the signal is first posted.	 The
     defaults are process termination, possibly with core dump; no action;
     stopping the process; or continuing the process.  See the signal 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 signal is masked.
     If sa_handler is set to SIG_IGN, current and pending instances of the
     signal are ignored and discarded.	If sig is SIGCHLD and sa_handler is
     set to SIG_IGN, the SA_NOCLDWAIT flag (described below) is implied.

     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
			   function 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.

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

	   SA_NODEFER	   If this bit is set, further occurrences of the
			   delivered signal are not masked during the
			   execution 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_SIGINFO	   If this bit is set, the 2nd argument of the handler
			   is set to be a pointer to a siginfo_t structure as
			   described in <sys/siginfo.h>.  The siginfo_t
			   structure is a part of IEEE Std 1003.1b
			   (``POSIX'').	 It provides much more information
			   about the causes and attributes of the signal that
			   is being delivered.

	   SA_RESTART	   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.  Restarting of pending calls is
			   requested by setting the SA_RESTART bit in
			   sa_flags.  The affected system calls include
			   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 fork(2) or vfork(2), all signals, the signal mask, the signal
     stack, and the restart/interrupt flags are inherited by the child.

     execve(2) 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
     <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

RETURN VALUES
     A 0 value indicates that the call succeeded.  A -1 return value indicates
     an error occurred and errno is set to indicate the reason.

EXAMPLES
     The handler routine can be declared:

	   void
	   handler(int sig)

     If the SA_SIGINFO option is enabled, the canonical way to declare it is:

	   void
	   handler(int sig, siginfo_t *sip, struct sigcontext *scp)

     Here sig is the signal number, into which the hardware faults and traps
     are mapped.  If the SA_SIGINFO option is set, sip is a pointer to a
     siginfo_t as described in <sys/siginfo.h>.	 If SA_SIGINFO is not set,
     this pointer will be NULL instead.	 The function specified in
     sa_sigaction will be called instead of the function specified by
     sa_handler (Note that in some implementations these are in fact the
     same).  scp is a pointer to the sigcontext structure (defined in
     <signal.h>), used to restore the context from before the signal.

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

     [EFAULT]	   Either act or oact points to memory that is not a valid
		   part of the process address space.

     [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), kill(2), ptrace(2), sigaltstack(2), sigprocmask(2),
     sigsuspend(2), wait(2), setjmp(3), sigblock(3), sigpause(3),
     sigsetops(3), sigvec(3), tty(4)

STANDARDS
     The sigaction() function conforms to IEEE Std 1003.1-1990 (``POSIX'').
     The SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as are the
     signals SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ,
     SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO.	 These 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, SA_NOCLDWAIT, and SA_SIGINFO flags are options commonly
     found in other operating systems.	The following functions are either
     reentrant or not interruptible by signals and are async-signal safe.
     Therefore applications may invoke them, without restriction, from signal-
     catching functions:

     Base Interfaces:

     _Exit(), _exit(), accept(), access(), alarm(), bind(), cfgetispeed(),
     cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(), chmod(), chown(),
     clock_gettime(), close(), connect(), creat(), dup(), dup2(), execl(),
     execle(), execv(), execve(), fchdir(), fchmod(), fchown(), fcntl(),
     fork(), fpathconf(), fstat(), fsync(), ftruncate(), futimes(), getegid(),
     geteuid(), getgid(), getgroups(), getpeername(), getpgrp(), getpid(),
     getppid(), getsockname(), getsockopt(), getuid(), kill(), link(),
     listen(), lseek(), lstate(), mkdir(), mkfifo(), mknod(), open(),
     pathconf(), pause(), pipe(), poll(), raise(), read(), readlink(), recv(),
     recvfrom(), recvmsg(), rename(), rmdir(), select(), send(), sendmsg(),
     sendto(), setgid(), setpgid(), setsid(), setsockopt(), setuid(),
     shutdown(), sigaction(), sigaddset(), sigdelset(), sigemptyset(),
     sigfillset(), sigismember(), signal(), sigpause(), sigpending(),
     sigprocmask(), sigsuspend(), sleep(), socket(), socketpair(), stat(),
     symlink(), sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(),
     tcgetpgrp(), tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(),
     umask(), uname(), unlink(), utime(), utimes(), wait(), waitpid(),
     write().

     ANSI C Interfaces:

     strcat(), strcpy(), strncat(), strncpy(), and perhaps some others.

     Extension Interfaces:

     chflags(), fchflags(), getresgid(), getresuid(), 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
     modification 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:

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

OpenBSD 4.9		       February 25, 2010		   OpenBSD 4.9
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