Description

   When a process requests the sigaction function, the process
   can both examine and specify what action to perform when the
   specified signal is delivered. The arguments determine the
   behavior of the sigaction function as follows:

   o  Specifying the sig argument identifies the affected signal.
      Use any one of the signal values defined in the <signal.h>
      header file, except SIGKILL.

      If sig is SIGCHLD and the SA_NOCLDSTOP flag is not set in
      sa_flags, then a SIGCHLD signal is generated for the calling
      process whenever any of its child processes stop. If sig is
      SIGCHLD and the SA_NOCLDSTOP flag is set in sa_flags, then
      SIGCHLD signal is not generated in this way.

   o  Specifying the action argument, if not null, points to a
      sigaction structure that defines what action to perform when
      the signal is received. If the action argument is null, signal
      handling remains unchanged, so you can use the call to inquire
      about the current handling of the signal.

   o  Specifying the o_action argument, if not null, points to
      a sigaction structure that contains the action previously
      attached to the specified signal.

   The sigaction structure consists of the following members:

     void        (*sa_handler)(int);
     sigset_t    sa_mask;
     int         sa_flags;

   The sigaction structure members are defined as follows:

   sa_        This member can contain the following values:
   handler
              o  SIG_DFL - Specifies the default action taken when
                 the signal is delivered.

              o  SIG_IGN - Specifies that the signal has no effect on
                 the receiving process.

              o  Function pointer - Requests to catch the signal. The
                 signal causes the function call.

   sa_mask    This member can request that individual signals, in
              addition to those in the process signal mask, are
              blocked from delivery while the signal handler function
              specified by the sa_handler member is executing.
   sa_flags   This member can set the flags to enable further control
              over the actions taken when a signal is delivered.

   The sa_flags member of the sigaction structure has the following
   values:

   SA_ONSTACK     Setting this bit causes the system to run the
                  signal catching function on the signal stack
                  specified by the sigstack function. If this bit
                  is not set, the function runs on the stack of the
                  process where the signal is delivered.
   SA_RESETHAND   Setting this bit resets the signal to SIG_DFL. Be
                  aware that you cannot automatically reset SIGILL
                  and SIGTRAP.
   SA_NODEFER     Setting this bit does not automatically block the
                  signal as it is intercepted.
   SA_NOCLDSTOP   If this bit is set and the sig argument is equal
                  to SIGCHLD and a child process of the calling
                  process stops, then a SIGCHLD signal is sent to
                  the calling process only if SA_NOCLDSTOP is not
                  set for SIGCHLD.

   When a signal is intercepted by a signal-catching function
   installed by sigaction, a new signal mask is calculated and
   installed for the duration of the signal-catching function (or
   until a call to either sigprocmask or sigsuspend is made. This
   mask is formed by taking the union of the current signal mask and
   the value of the sa_mask for the signal being delivered unless
   SA_NODEFER or SA_RESETHAND is set, and then including the signal
   being delivered. If and when the user's signal handler returns
   normally, the original signal mask is restored.

   Once an action is installed for a specific signal, it remains
   installed until another action is explicitly requested (by
   another call to sigaction), until the SA_RESETHAND flag causes
   resetting of the handler, or until one of the exec functions is
   called.

   If the previous action for a specified signal had been
   established by signal, the values of the fields returned in
   the structure pointed to by the o_action argument of sigaction
   are unspecified, and in particular o_action->sa_handler is
   not necessarily the same value passed to signal. However, if a
   pointer to the same structure or a copy thereof is passed to a
   subsequent call to sigaction by means of the action argument
   of sigaction), the signal is handled as if the original call to
   signal were repeated.

   If sigaction fails, no new signal handler is installed.

   It is unspecified whether an attempt to set the action for a
   signal that cannot be intercepted or ignored to SIG_DFL is
   ignored or causes an error to be returned with errno set to
   EINVAL.

   See the "Error and Signal Handling" chapter of the VSI C RTL
   Reference Manual for more information on signal handling.

                                  NOTE

      The sigvec and signal functions are provided for
      compatibility to old UNIX systems; their function is a
      subset of that available with the sigaction function.

   See also sigvec, signal, wait, read, and write.