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 38.8 Signals
 ============
 
 Procedures to raise, handle and wait for signals.
 
  -- Scheme Procedure: kill pid sig
  -- C Function: scm_kill (pid, sig)
      Sends a signal to the specified process or group of processes.
 
      PID specifies the processes to which the signal is sent:
 
     PID greater than 0
           The process whose identifier is PID.
 
     PID equal to 0
           All processes in the current process group.
 
     PID less than -1
           The process group whose identifier is -PID
 
     PID equal to -1
           If the process is privileged, all processes except for some
           special system processes.  Otherwise, all processes with the
           current effective user ID.
 
      SIG should be specified using a variable corresponding to the Unix
      symbolic name, e.g.,
 
       -- Variable: SIGHUP
           Hang-up signal.
 
       -- Variable: SIGINT
           Interrupt signal.
 
  -- Scheme Procedure: raise sig
  -- C Function: scm_raise (sig)
      Sends a specified signal SIG to the current process, where SIG is
      as described for the kill procedure.
 
  -- Scheme Procedure: sigaction signum [handler [flags]]
  -- C Function: scm_sigaction (signum, handler, flags)
      Install or report the signal handler for a specified signal.
 
      SIGNUM is the signal number, which can be specified using the value
      of variables such as `SIGINT'.
 
      If ACTION is omitted, `sigaction' returns a pair: the CAR is the
      current signal hander, which will be either an integer with the
      value `SIG_DFL' (default action) or `SIG_IGN' (ignore), or the
      Scheme procedure which handles the signal, or `#f' if a non-Scheme
      procedure handles the signal.  The CDR contains the current
      `sigaction' flags for the handler.
 
      If ACTION is provided, it is installed as the new handler for
      SIGNUM.  ACTION can be a Scheme procedure taking one argument, or
      the value of `SIG_DFL' (default action) or `SIG_IGN' (ignore), or
      `#f' to restore whatever signal handler was installed before
      `sigaction' was first used.  Flags can optionally be specified for
      the new handler (`SA_RESTART' will always be added if it's
      available and the system is using restartable system calls.)  The
      return value is a pair with information about the old handler as
      described above.
 
      This interface does not provide access to the "signal blocking"
      facility.  Maybe this is not needed, since the thread support may
      provide solutions to the problem of consistent access to data
      structures.
 
  -- Scheme Procedure: restore-signals
  -- C Function: scm_restore_signals ()
      Return all signal handlers to the values they had before any call
      to `sigaction' was made.  The return value is unspecified.
 
  -- Scheme Procedure: alarm i
  -- C Function: scm_alarm (i)
      Set a timer to raise a `SIGALRM' signal after the specified number
      of seconds (an integer).  It's advisable to install a signal
      handler for `SIGALRM' beforehand, since the default action is to
      terminate the process.
 
      The return value indicates the time remaining for the previous
      alarm, if any.  The new value replaces the previous alarm.  If
      there was no previous alarm, the return value is zero.
 
  -- Scheme Procedure: pause
  -- C Function: scm_pause ()
      Pause the current process (thread?) until a signal arrives whose
      action is to either terminate the current process or invoke a
      handler procedure.  The return value is unspecified.
 
  -- Scheme Procedure: sleep i
  -- C Function: scm_sleep (i)
      Wait for the given number of seconds (an integer) or until a signal
      arrives.  The return value is zero if the time elapses or the
      number of seconds remaining otherwise.
 
  -- Scheme Procedure: usleep i
  -- C Function: scm_usleep (i)
      Sleep for I microseconds.  `usleep' is not available on all
      platforms.
 
  -- Scheme Procedure: setitimer which_timer interval_seconds
           interval_microseconds value_seconds value_microseconds
  -- C Function: scm_setitimer (which_timer, interval_seconds,
           interval_microseconds, value_seconds, value_microseconds)
      Set the timer specified by WHICH_TIMER according to the given
      INTERVAL_SECONDS, INTERVAL_MICROSECONDS, VALUE_SECONDS, and
      VALUE_MICROSECONDS values.
 
      Return information about the timer's previous setting.  Errors are
      handled as described in the guile info pages under "POSIX
      Interface Conventions".
 
      The timers available are: `ITIMER_REAL', `ITIMER_VIRTUAL', and
      `ITIMER_PROF'.
 
      The return value will be a list of two cons pairs representing the
      current state of the given timer.  The first pair is the seconds
      and microseconds of the timer `it_interval', and the second pair is
      the seconds and microseconds of the timer `it_value'.
 
  -- Scheme Procedure: getitimer which_timer
  -- C Function: scm_getitimer (which_timer)
      Return information about the timer specified by WHICH_TIMER Errors
      are handled as described in the guile info pages under "POSIX
      Interface Conventions".
 
      The timers available are: `ITIMER_REAL', `ITIMER_VIRTUAL', and
      `ITIMER_PROF'.
 
      The return value will be a list of two cons pairs representing the
      current state of the given timer.  The first pair is the seconds
      and microseconds of the timer `it_interval', and the second pair is
      the seconds and microseconds of the timer `it_value'.
 

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