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

     either  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 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

     terminated  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 call.

     In C, if func is  0,  the  default  action  for  signal  sig

     (termination)  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

     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

     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


     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.