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     SIGNAL(2)                                               SIGNAL(2)

     NAME
          signal - catch or ignore signals

     SYNOPSIS
          #include <signal.h>

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

     DESCRIPTION
          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 a signal call allows them either to
          be ignored or to cause an interrupt to a specified location.
          Here is the list of signals with names as in the include
          file.

          SIGHUP  1    hangup
          SIGINT  2    interrupt
          SIGQUIT 3*   quit
          SIGILL  4*   illegal instruction (not reset when caught)
          SIGTRAP 5*   trace trap (not reset when caught)
          SIGIOT  6*   IOT instruction
          SIGEMT  7*   EMT instruction
          SIGFPE  8*   floating point exception
          SIGKILL 9    kill (cannot be caught or ignored)
          SIGBUS  10*  bus error
          SIGSEGV 11*  segmentation violation
          SIGSYS  12*  bad argument to system call
          SIGPIPE 13   write on a pipe or link with no one to read it
          SIGALRM 14   alarm clock
          SIGTERM 15   software termination signal
                  16   unassigned

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

          If func is SIG_DFL, the default action for signal sig is
          reinstated; this default is termination, sometimes with a
          core image.  If func is SIG_IGN the signal is ignored.  Oth-
          erwise when the signal occurs func will be called with the
          signal number as argument.  A return from the function will
          continue the process at the point it was interrupted.
          Except as indicated, a signal is reset to SIG_DFL after
          being caught.  Thus if it is desired to catch every such
          signal, the catching routine must issue another signal call.

          When a caught signal occurs during certain system calls, the

     SIGNAL(2)                                               SIGNAL(2)

          call terminates prematurely.  In particular this can occur
          during a read or write(2) on a slow device (like a type-
          writer; but not a file); and during pause or wait(2). 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 an error
          status.  The user's program may then, if it wishes, re-
          execute the call.

          The value of signal is the previous (or initial) value of
          func for the particular signal.

          After a fork(2) the child inherits all signals.  Exec(2)
          resets all caught signals to default action.

     SEE ALSO
          kill(1), kill(2), ptrace(2), setjmp(3)

     DIAGNOSTICS
          The value (int)-1 is returned if the given signal is out of
          range.

     BUGS
          If a repeated signal arrives before the last one can be
          reset, there is no chance to catch it.

          The type specification of the routine and its func argument
          are problematical.

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

          If label is 0, default action is reinstated.  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.