DESCRIPTION

     The sigaction system call is used to change the action taken
     by a process on receipt of a specific signal.

     signum specifies the signal and  can  be  any  valid  signal
     except SIGKILL and SIGSTOP.

     If act is non-null, the new  action  for  signal  signum  is
     installed  from  act.   If  oldact is non-null, the previous
     action is saved in oldact.

     The sigaction structure is defined as

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

     sa_handler specifies the action to be associated with signum
     and may be SIG_DFL for the default action, SIG_IGN to ignore
     this signal, or a pointer to a signal handling function.

     sa_mask gives a mask of signals which should be blocked dur-
     ing  execution of the signal handler.  In addition, the sig-
     nal which triggered the handler will be blocked, unless  the
     SA_NODEFER or SA_NOMASK flags are used.

     sa_flags specifies a set of flags which modify the behaviour
     of  the signal handling process. It is formed by the bitwise
     OR of zero or more of the following:
          SA_NOCLDSTOP
               If signum is SIGCHLD, do not receive  notification
               when   child  processes  stop  (i.e.,  when  child
               processes receive one of SIGSTOP, SIGTSTP, SIGTTIN
               or SIGTTOU).

          SA_ONESHOT or SA_RESETHAND
               Restore the signal action  to  the  default  state
               once the signal handler has been called.  (This is
               the  default  behavior  of  the  signal(2)  system
               call.)

          SA_RESTART
               Provide  behaviour  compatible  with  BSD   signal
               semantics  by making certain system calls restart-
               able across signals.

          SA_NOMASK or SA_NODEFER
               Do not prevent the signal from being received from
               within its own signal handler.

     The sa_restorer element is obsolete and should not be used.

     The sigprocmask call is used to change the list of currently
     blocked  signals.  The behaviour of the call is dependent on
     the value of how, as follows.

          SIG_BLOCK
               The set of blocked signals is  the  union  of  the
               current set and the set argument.

          SIG_UNBLOCK
               The signals in set are removed  from  the  current
               set of blocked signals.  It is legal to attempt to
               unblock a signal which is not blocked.

          SIG_SETMASK
               The set of blocked signals is set to the  argument
               set.

     If oldset is non-null, the previous value of the signal mask
     is stored in oldset.

     The sigpending call allows the examination of  pending  sig-
     nals  (ones which have been raised while blocked).  The sig-
     nal mask of pending signals is stored in set.

     The sigsuspend call temporarily replaces the signal mask for
     the  process  with  that given by mask and then suspends the
     process until a signal is received.

RETURN VALUES

     sigaction, sigprocmask, sigpending and sigsuspend  return  0
     on success and -1 on error.

ERRORS

     EINVAL
          An invalid signal was specified.   This  will  also  be
          generated  if  an  attempt is made to change the action
          for SIGKILL or SIGSTOP, which cannot be caught.

     EFAULT
          act, oldact, set or oldset point to memory which is not
          a valid part of the process address space.

     EINTR
          System call was interrupted.

NOTES

     It is not possible to block SIGKILL or SIGSTOP with the sig-
     procmask call.  Attempts to do so will be silently ignored.

     According to POSIX, the behaviour of a process is  undefined
     after  it  ignores  a SIGFPE, SIGILL, or SIGSEGV signal that
     was not generated by the kill() or  the  raise()  functions.
     Integer  division  by  zero  has  undefined result.  On some
     architectures it  will  generate  a  SIGFPE  signal.   (Also
     dividing  the  most  negative  integer  by  -1  may generate
     SIGFPE.)  Ignoring this signal  might  lead  to  an  endless
     loop.

     POSIX (B.3.3.1.3) disallows setting the action  for  SIGCHLD
     to SIG_IGN.  The BSD and SYSV behaviours differ, causing BSD
     software that sets the action for SIGCHLD to SIG_IGN to fail
     on Linux.

     The POSIX spec only  defines  SA_NOCLDSTOP.   Use  of  other
     sa_flags is non-portable.

     The SA_RESETHAND flag is compatible with the  SVr4  flag  of
     the same name.

     The SA_NODEFER flag is compatible with the SVr4 flag of  the
     same  name  under kernels 1.3.9 and newer.  On older kernels
     the Linux implementation allowed the receipt of any  signal,
     not  just  the one we are installing (effectively overriding
     any sa_mask settings).

     The SA_RESETHAND and SA_NODEFER names for SVr4 compatibility
     are present only in library versions 3.0.9 and greater.

     sigaction can be called with a null second argument to query
     the  current  signal  handler.  It can also be used to check
     whether a given signal is valid for the current  machine  by
     calling it with null second and third arguments.

     See sigsetops(3) for details on manipulating signal sets.

CONFORMING TO

     POSIX, SVr4.  SVr4 does not document the EINTR condition.

SEE ALSO

     kill(1),  kill(2),  killpg(2),  siginterrupt(3),  signal(2),
     signal(7), sigvec(2)