#include <signal.h>
     struct sigaction {
             void     (*sa_handler)();
             sigset_t sa_mask;
             int      sa_flags;
     };

     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 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 pro-
     cess 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 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 (normal-
     ly empty).  It may be changed with a sigprocmask(2) call, or when a sig-
     nal 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 re-
     sume 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
     action for the signal is to discard the signal, and if a signal is pend-
     ing, 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 signal
     stack, specified with sigstack(2).

     Finally, the SA_SIGINFO option causes the 2nd argument for the signal
     handler to be a pointer to a siginfo_t as described in <sys/siginfo.h>.
     The siginfo_t is a part of . and provides much more information about the
     causes and attributes of the signal that is being delivered.

     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
                                             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, sip, scp)
           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
     is NULL.  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.