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     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
     either by user at a typewriter (quit, interrupt), by a pro-
     gram error (bus error, etc.), or by request of another pro-
     gram (kill).  Normally all signals cause termination of the
     receiving process, but this call allows them either to be
     ignored 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
     label 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
     ignored.  If func is non-zero and even, it is assumed to be
     the address of a function entry point.  When the signal
     occurs, the function will be called.  A return from the
     function will continue the process at the point it was
     interrupted.  As in the assembler call, signal must in gen-
     eral 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
     during a read or write on a slow device (like a typewriter;
     but not a file); and during or wait.  When such a signal
     occurs, 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-
     execute 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.