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

NAME
     sigaction — software signal facilities

SYNOPSIS
     #include <signal.h>

     struct sigaction {
	     void     (*sa_handler)();
	     sigset_t sa_mask;
	     int      sa_flags;
     };

     sigaction(int sig, 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 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 sys‐
     tem 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 han‐
     dlers 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 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 (nor‐
     mally 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 sig‐
     nals are ready to be delivered at the same time, any signals that could
     be caused by traps are delivered first.  Additional signals may be pro‐
     cessed 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 han‐
     dling 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 previ‐
     ous 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
     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 associ‐
     ated with the handler to be invoked.

     Sigaction() assigns an action for a specific signal.  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 remains installed until another
     sigaction() call is made, or an execve(2) is performed.  A signal-spe‐
     cific 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 discarded even if the signal is masked.
     If sa_handler is set to SIG_IGN current and pending instances of the sig‐
     nal are ignored and discarded.

     Options may be specified by setting sa_flags.  If the SA_NOCLDSTOP 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.  Further, if the SA_ONSTACK bit is set in
     sa_flags, the system will deliver the signal to the process on a signal
     stack, specified with sigstack(2).

     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 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(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

NOTE
     The mask specified in act is not allowed to block SIGKILL or SIGSTOP.
     This is done silently by the system.

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

EXAMPLE
     The handler routine can be declared:

	   void handler(sig, code, scp)
	   int sig, code;
	   struct sigcontext *scp;

     Here sig is the signal number, into which the hardware faults and traps
     are mapped.  Code is a parameter that is either a constant or the code
     provided by the hardware.	Scp is a pointer to the sigcontext structure
     (defined in ⟨signal.h⟩), used to restore the context from before the sig‐
     nal.

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.

STANDARDS
     The sigaction function is defined by IEEE Std 1003.1-1988 (“POSIX.1”).
     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.  Those signals are
     available on most BSD-derived systems.

SEE ALSO
     kill(1), ptrace(2), kill(2), sigaction(2), sigprocmask(2), sigsuspend(2),
     sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), sigvec(2),
     setjmp(3), siginterrupt(3), sigsetops(3), tty(4)

BSD				 April 3, 1994				   BSD
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