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OpenDarwin 7.2.1 - man page for sigaction (opendarwin section 2)

SIGACTION(2)			     BSD System Calls Manual			     SIGACTION(2)

     sigaction -- software signal facilities

     Standard C Library (libc, -lc)

     #include <signal.h>

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

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

     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 fur-
     ther 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 pend-
     ing 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 instruc-
     tions.  The set of pending signals is returned by the sigpending(2) system call.  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) 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-
     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 informa-
     tion for the signal is returned to the user.

     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 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 dis-
     carded 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.

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

	   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 call-
			   ing 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_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 SIG_IGN.

     If a signal is caught during the system calls listed below, the call may be forced to termi-
     nate 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).  How-
     ever, calls that have already committed are not restarted, but instead return a partial suc-
     cess (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.

     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 <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
     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 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 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 signals and are async-
     signal safe.  Therefore applications may invoke them, without restriction, from signal-
     catching functions:

     Base Interfaces:

     _exit(), access(), alarm(), cfgetispeed(), cfgetospeed(), cfsetispeed(), cfsetospeed(),
     chdir(), chmod(), chown(), close(), creat(), dup(), dup2(), execle(), execve(), fcntl(),
     fork(), fpathconf(), fstat(), fsync(), getegid(), geteuid(), getgid(), getgroups(),
     getpgrp(), getpid(), getppid(), getuid(), kill(), link(), lseek(), mkdir(), mkfifo(),
     open(), pathconf(), pause(), pipe(), raise(), read(), rename(), rmdir(), setgid(),
     setpgid(), setsid(), setuid(), sigaction(), sigaddset(), sigdelset(), sigemptyset(),
     sigfillset(), sigismember(), signal(), sigpending(), sigprocmask(), sigsuspend(), sleep(),
     stat(), sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(),
     tcsetattr(), tcsetpgrp(), time(), times(), umask(), uname(), unlink(), utime(), wait(),
     waitpid(), write().

     Realtime Interfaces:

     aio_error(), clock_gettime(), sigpause(), timer_getoverrun(), aio_return(), fdatasync(),
     sigqueue(), timer_gettime(), aio_suspend(), sem_post(), sigset(), timer_settime().

     ANSI C Interfaces:

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

     Extension Interfaces:

     strlcpy(), strlcat().

     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 when called from a signal handler is unde-
     fined.  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 error.

     There are three possible prototypes the handler may match:

	   ANSI C:
		  void handler(int);

		  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
     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 func-
     tion needs to be casted to compile without warning.  The traditional BSD style is not porta-
     ble and since its capabilities 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 <signal.h>.

     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 uap argument to a POSIX SA_SIGINFO handler points to an instance of ucontext_t.

     The sigaction() system call 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]		The sig argument is not a valid signal number.

     [EINVAL]		An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

     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 extensions, as are the signals, SIGTRAP,
     SIGINFO.  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 oper-
     ating systems.

     kill(1), kill(2), ptrace(2), sigaltstack(2), sigblock(2), sigpause(2), sigpending(2),
     sigprocmask(2), sigsetmask(2), sigsuspend(2), sigvec(2), wait(2), fpsetmask(3), setjmp(3),
     siginterrupt(3), sigsetops(3), ucontext(3), tty(4)

BSD					  April 3, 1994 				      BSD

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