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SIGNAL(2)		   Linux Programmer's Manual		     SIGNAL(2)

       signal - ANSI C signal handling

       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

       The  signal()  system call installs a new signal handler for the signal
       with number signum.  The signal handler is set to sighandler which  may
       be a user specified function, or either SIG_IGN or SIG_DFL.

       Upon  arrival of a signal with number signum the following happens.  If
       the corresponding handler  is  set  to  SIG_IGN,	 then  the  signal  is
       ignored.	  If  the  handler  is set to SIG_DFL, then the default action
       associated with the signal (see signal(7))  occurs.   Finally,  if  the
       handler	is  set to a function sighandler then first either the handler
       is reset to SIG_DFL or an implementation-dependent blocking of the sig-
       nal is performed and next sighandler is called with argument signum.

       Using  a	 signal	 handler function for a signal is called "catching the
       signal".	 The signals SIGKILL and SIGSTOP cannot be caught or  ignored.

       The signal() function returns the previous value of the signal handler,
       or SIG_ERR on error.

       The original Unix signal() would reset the handler to SIG_DFL, and Sys-
       tem  V  (and the Linux kernel and libc4,5) does the same.  On the other
       hand, BSD does not reset the handler, but blocks new instances of  this
       signal from occurring during a call of the handler.  The glibc2 library
       follows the BSD behaviour.

       If one on a libc5 system includes <bsd/signal.h> instead of  <signal.h>
       then  signal()  is  redefined  as  __bsd_signal	and signal has the BSD
       semantics. This is not recommended.

       If one on a  glibc2  system  defines  a	feature	 test  macro  such  as
       _XOPEN_SOURCE  or  uses	a  separate  sysv_signal function, one obtains
       classical behaviour. This is not recommended.

       Trying to change the semantics of this call using defines and  includes
       is  not a good idea. It is better to avoid signal() altogether, and use
       sigaction(2) instead.

       The effects of this call in a multi-threaded process are unspecified.

       The routine handler must be very careful,  since	 processing  elsewhere
       was interrupted at some arbitrary point. POSIX has the concept of "safe
       function".  If a signal interrupts  an  unsafe  function,  and  handler
       calls  an  unsafe  function, then the behavior is undefined. Safe func-
       tions are listed explicitly in the various standards.  The POSIX.1-2003
       list is

       _Exit()	_exit()	 abort()  accept()  access()  aio_error() aio_return()
       aio_suspend() alarm() bind() cfgetispeed() cfgetospeed()	 cfsetispeed()
       cfsetospeed() chdir() chmod() chown() clock_gettime() close() connect()
       creat() dup() dup2() execle() execve() fchmod() fchown() fcntl() fdata-
       sync()	fork()	 fpathconf()  fstat()  fsync()	ftruncate()  getegid()
       geteuid() getgid() getgroups() getpeername() getpgrp()  getpid()	 getp-
       pid()   getsockname()  getsockopt()  getuid()  kill()  link()  listen()
       lseek() lstat()	mkdir()	 mkfifo()  open()  pathconf()  pause()	pipe()
       poll()  posix_trace_event()  pselect() raise() read() readlink() recv()
       recvfrom()  recvmsg()  rename()	rmdir()	 select()  sem_post()	send()
       sendmsg()  sendto()  setgid()  setpgid() setsid() setsockopt() setuid()
       shutdown()  sigaction()	sigaddset()  sigdelset()  sigemptyset()	  sig-
       fillset()  sigismember() signal() sigpause() sigpending() sigprocmask()
       sigqueue() sigset() sigsuspend() sleep() socket()  socketpair()	stat()
       symlink()  sysconf()  tcdrain()	tcflow() tcflush() tcgetattr() tcgetp-
       grp() tcsendbreak() tcsetattr() tcsetpgrp()  time()  timer_getoverrun()
       timer_gettime()	 timer_settime()   times()  umask()  uname()  unlink()
       utime() wait() waitpid() write().

       According to POSIX, the behaviour of a process is  undefined  after  it
       ignores	a  SIGFPE, SIGILL, or SIGSEGV signal that was not generated by
       the kill(2) or the raise(3) functions.  Integer division	 by  zero  has
       undefined result.  On some architectures it will generate a SIGFPE sig-
       nal.  (Also dividing the most  negative	integer	 by  -1	 may  generate
       SIGFPE.)	 Ignoring this signal might lead to an endless loop.

       See  sigaction(2)  for  details	on what happens when SIGCHLD is set to

       The use of sighandler_t is a GNU extension.  Various versions  of  libc
       predefine  this	type;  libc4  and  libc5  define  SignalHandler, glibc
       defines sig_t and, when _GNU_SOURCE is defined, also sighandler_t.

       C89, POSIX.1-2001.

       kill(1), alarm(2), kill(2), pause(2), sigaction(2), sigpending(2), sig-
       procmask(2),  sigqueue(2),  sigsuspend(2),  killpg(3), raise(3), sigse-
       tops(3), sigvec(3), feature_test_macros(7), signal(7)

Linux 2.2			  2000-04-28			     SIGNAL(2)
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