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


NAME
     sigaction - software signal facilities

SYNOPSIS
     #include <signal.h>

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

     sigaction(sig, act, oact)
     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 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 execute with the signal that caused their
     invocation blocked, but other signals may yet occur.  A glo-
     bal 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 sig-
     nals 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


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


     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 call is made).  This mask is formed
     by taking the union of the current signal mask set, the sig-
     nal to be delivered, and the signal mask associated 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 han-
     dling 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-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
     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.

     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 sig-
     nal 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


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


     (such as a terminal, but not a regular file) and during a
     wait(2) or ioctl(2).  However, calls that have already com-
     mitted 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 inher-
     ited 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	     Action	 Description
	  SIGHUP     terminate	 terminal line hangup
	  SIGINT     terminate	 interrupt program
	  SIGQUIT    core	 quit program
	  SIGILL     core	 illegal instruction
	  SIGTRAP    core	 trace trap
	  SIGIOT     core	 abort(2) call (same as SIGABRT)
	  SIGEMT     core	 emulate instruction executed
	  SIGFPE     core	 floating-point exception
	  SIGKILL    terminate	 kill program
	  SIGBUS     core	 bus error
	  SIGSEGV    core	 segmentation violation
	  SIGSYS     core	 system call given invalid argu-
	  ment
	  SIGPIPE    terminate	 write on a pipe with no reader
	  SIGALRM    terminate	 real-time timer expired
	  SIGTERM    terminate	 software termination signal
	  SIGURG     discard	 urgent condition present on
	  socket
	  SIGSTOP    stop	 stop (cannot be caught or
	  ignored)
	  SIGTSTP    stop	 stop generated from keyboard
	  SIGCONT    discard	 continue after stop
	  SIGCHLD    discard	 child status has changed
	  SIGTTIN    stop	 background read attempted on
	  control terminal
	  SIGTTOU    stop	 background write attemped to
	  control terminal
	  SIGIO      discard	 I/O is possible on a descriptor
	  (see fcntl(2))
	  SIGXCPU    terminate	 cpu time limit exceeded (see
	  setrlimit(2))
	  SIGXFSZ    terminate	 file size limit exceeded (see
	  setrlimit(2))


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


	  SIGVTALRM  terminate	 virtual time alarm (see setiti-
	  mer(2))
	  SIGPROF    terminate	 profiling timer alarm (see seti-
	  timer(2))
	  SIGWINCH   discard	 Window size change
	  SIGINFO    discard	 status request from keyboard
	  SIGUSR1    terminate	 User defined signal 1
	  SIGUSR2    terminate	 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 indi-
     cated the reason.

EXAMPLE
     The handler routine can be declared:

     int 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 Std1003.1-1988
     (``POSIX'').  The SA_ONSTACK and SA_RESTART flags are Berke-
     ley 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.


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


BUGS
     The networking related syscalls are not properly restarted
     in 2.11BSD.  The SIGINFO signal is not implemented in
     2.11BSD.

SEE ALSO
     kill(1), fcntl(2), ptrace(2), kill(2), setitimer(2),
     setrlimit(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)


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