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SIGNAL(3C)							    SIGNAL(3C)

NAME
       signal - simplified software signal facilities

SYNOPSIS
       #include <signal.h>

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

DESCRIPTION
       Signal  is  a  simplified  interface  to	 the more general sigaction(2)
       facility.

       A signal is generated by some abnormal event, initiated by a user at  a
       terminal (quit, interrupt, stop), by a program error (bus error, etc.),
       by request of another program (kill), or	 when  a  process  is  stopped
       because	it  wishes  to	access its control terminal while in the back‐
       ground (see tty(4)).  Signals are optionally generated when  a  process
       resumes	after  being  stopped,	when  the  status  of  child processes
       changes, or when input is ready at the control terminal.	 Most  signals
       cause  termination of the receiving process if no action is taken; some
       signals instead cause the process receiving them to be stopped, or  are
       simply  discarded  if  the process has not requested otherwise.	Except
       for the SIGKILL and SIGSTOP signals, the	 signal	 call  allows  signals
       either  to be ignored or to cause an interrupt to a specified location.
       The following is a list of all signals with names  as  in  the  include
       file <signal.h>:

       SIGHUP	 1    hangup
       SIGINT	 2    interrupt
       SIGQUIT	 3*   quit
       SIGILL	 4*   illegal instruction
       SIGTRAP	 5*   trace trap
       SIGABRT	 6*   abort() call (formerly SIGIOT)
       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 with no one to read it
       SIGALRM	 14   alarm clock
       SIGTERM	 15   software termination signal
       SIGURG	 16@  urgent condition present on socket
       SIGSTOP	 17|+'stop (cannot be caught or ignored)
       SIGTSTP	 18|+'stop signal generated from keyboard
       SIGCONT	 19@  continue after stop
       SIGCHLD	 20@  child status has changed
       SIGTTIN	 21|+'background read attempted from control terminal
       SIGTTOU	 22|+'background write attempted to control terminal
       SIGIO	 23@  i/o is possible on a descriptor (see fcntl(2))
       SIGXCPU	 24   cpu time limit exceeded (see setrlimit(2))
       SIGXFSZ	 25   file size limit exceeded (see setrlimit(2))
       SIGVTALRM 26   virtual time alarm (see setitimer(2))
       SIGPROF	 27   profiling timer alarm (see setitimer(2))
       SIGWINCH	 28@  Window size change
       SIGINFO	 29@  status request from keyboard
       SIGUSR1	 30   User defined signal 1
       SIGUSR2	 31   User defined signal 2

       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  (with a core image for starred signals)
       except for signals marked with @ or |+'.	 Signals  marked  with	@  are
       discarded  if  the action is SIG_DFL; signals marked with |+' cause the
       process to stop.	 If func is SIG_IGN the signal is subsequently ignored
       and pending instances of the signal are discarded.  Otherwise, when the
       signal occurs further  occurrences  of  the  signal  are	 automatically
       blocked and func is called.

       A  return  from	the function unblocks the handled signal and continues
       the process at the point it was interrupted.   Unlike  previous	signal
       facilities,  the handler func remains installed after a signal has been
       delivered.

       If a caught signal occurs during certain system calls, causing the call
       to terminate prematurely, the call is automatically restarted (the han‐
       dler is installed using the SA_RESTART flag  with  sigaction(2)).   The
       affected	  system   calls   include   read(2),	write(2),   sendto(2),
       recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a
       slow  device  (such as a terminal, but not a regular file) and during a
       wait(2) or ioctl(2).  However, calls that have  already	committed  are
       not  restarted,	but  instead  return a partial success (for example, a
       short read count).

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

       After  a fork(2) or vfork(2) the child inherits all signals.  Execve(2)
       resets all caught signals to the default action; ignored signals remain
       ignored.

RETURN VALUE
       The previous action is returned on a successful call.  Otherwise, -1 is
       returned and errno is set to indicate the error.

ERRORS
       Signal will fail and no action will take place if one of the  following
       occur:

       [EINVAL]	      Sig is not a valid signal number.

       [EINVAL]	      An  attempt  is  made  to ignore or supply a handler for
		      SIGKILL or SIGSTOP.

SEE ALSO
       kill(1),	 ptrace(2),  kill(2),  sigaction(2),  sigprocmask(2),  sigsus‐
       pend(2), sigstack(2), setjmp(3), tty(4)

NOTES  (VAX-11)
       The handler routine can be declared:

	   void handler(sig, code, scp)

       Here sig is the signal number, into which the hardware faults and traps
       are mapped as defined below.  Code is a parameter  which	 is  either  a
       constant	 as  given  below  or, for compatibility mode faults, the code
       provided by the hardware.  Scp is a pointer to  the  struct  sigcontext
       used  by the system to restore the process context from before the sig‐
       nal.  Compatibility mode faults are distinguished from the other SIGILL
       traps by having PSL_CM set in the psl.

       The  following  defines	the  mapping  of hardware traps to signals and
       codes.  All of these symbols are defined in <signal.h>:

	  Hardware condition		      Signal	   Code

       Arithmetic traps:
	  Integer overflow		      SIGFPE	   FPE_INTOVF_TRAP
	  Integer division by zero	      SIGFPE	   FPE_INTDIV_TRAP
	  Floating overflow trap	      SIGFPE	   FPE_FLTOVF_TRAP
	  Floating/decimal division by zero   SIGFPE	   FPE_FLTDIV_TRAP
	  Floating underflow trap	      SIGFPE	   FPE_FLTUND_TRAP
	  Decimal overflow trap		      SIGFPE	   FPE_DECOVF_TRAP
	  Subscript-range		      SIGFPE	   FPE_SUBRNG_TRAP
	  Floating overflow fault	      SIGFPE	   FPE_FLTOVF_FAULT
	  Floating divide by zero fault	      SIGFPE	   FPE_FLTDIV_FAULT
	  Floating underflow fault	      SIGFPE	   FPE_FLTUND_FAULT
       Length access control		      SIGSEGV
       Protection violation		      SIGBUS
       Reserved instruction		      SIGILL	   ILL_RESAD_FAULT
       Customer-reserved instr.		      SIGEMT
       Reserved operand			      SIGILL	   ILL_PRIVIN_FAULT
       Reserved addressing		      SIGILL	   ILL_RESOP_FAULT
       Trace pending			      SIGTRAP
       Bpt instruction			      SIGTRAP
       Compatibility-mode		      SIGILL	   hardware supplied code
       Chme				      SIGSEGV
       Chms				      SIGSEGV
       Chmu				      SIGSEGV

4th Berkeley Distribution	 July 1, 1990			    SIGNAL(3C)
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