SIGNAL(II)                    8/5/73                   SIGNAL(II)

     signal - catch or ignore signals

     (signal = 48.)
     sys  signal; sig; label
     (old value in r0)

     signal(sig, func)
     int (*func)( );

     A  signal is generated by some abnormal event, initiated ei-
     ther by user at a typewriter (quit, interrupt), by a program
     error  (bus  error,  etc.), or by request of another program
     (kill).  Normally all signals cause termination of  the  re-
     ceiving  process, but this call allows them either to be ig-
     nored or to cause an  interrupt  to  a  specified  location.
     Here is the list of signals:

          1    hangup
          2    interrupt
          3*   quit
          4*   illegal instruction (not reset when caught)
          5*   trace trap (not reset when caught)
          6*   IOT instruction
          7*   EMT instruction
          8*   floating point exception
          9    kill (cannot be caught or ignored)
          10*  bus error
          11*  segmentation violation
          12*  bad argument to system call
          13   write on a pipe with no one to read it

     In  the assembler call, if label is 0, the process is termi-
     nated when the signal occurs; this is  the  default  action.
     If  label is odd, the signal is ignored.  Any other even la-
     bel specifies an address in the process where  an  interrupt
     is  simulated.   An  RTI or RTT instruction will return from
     the interrupt.  Except as indicated, a signal is reset to  0
     after  being  caught.   Thus if it is desired to catch every
     such signal, the catching routine must issue another  signal

     In  C, if func is 0, the default action for signal sig (ter-
     mination) is reinstated.  If func is 1, the  signal  is  ig-
     nored.   If  func  is non-zero and even, it is assumed to be
     the address of a function entry point.  When the signal  oc-
     curs,  the function will be called.  A return from the func-
     tion will continue the process at the point  it  was  inter-
     rupted.  As in the assembler call, signal must in general be
     called again to catch subsequent signals.

     When a caught signal occurs during certain system calls, the
     call  terminates  prematurely.  In particular this can occur

                              - 1 -

SIGNAL(II)                    8/5/73                   SIGNAL(II)

     during a read or write on a slow device (like a  typewriter;
     but not a file); and during or wait.  When such a signal oc-
     curs, the saved user status is arranged in such a  way  that
     when  return  from  the signal-catching takes place, it will
     appear that the system call returned a characteristic  error
     status.   The user's program may then, if it wishes, re-exe-
     cute the call.

     The starred signals in the list above cause a core image  if
     not caught or ignored.

     The value of the call is the old action defined for the sig-

     After a fork(II) the child inherits all signals.  Exec (II)
     resets all caught signals to default action.

     kill(I), kill(II), ptrace(II), reset(III)

     The  error  bit (c-bit) is set if the given signal is out of
     range.  In C, a -1 indicates an error; 0 indicates  success.


                              - 2 -