signal(2)

NAME

   signal - ANSI C signal handling

SYNOPSIS

   #include <signal.h>

   typedef void (*sighandler_t)(int);

   sighandler_t signal(int signum, sighandler_t handler);

DESCRIPTION

   The  behavior  of  signal()  varies  across UNIX versions, and has also
   varied historically across different versions of Linux.  Avoid its use:
   use sigaction(2) instead.  See Portability below.

   signal() sets the disposition of the signal signum to handler, which is
   either  SIG_IGN,  SIG_DFL,  or  the  address  of  a  programmer-defined
   function (a "signal handler").

   If  the  signal  signum  is  delivered  to the process, then one of the
   following happens:

   *  If the disposition is set to SIG_IGN, then the signal is ignored.

   *  If the disposition is  set  to  SIG_DFL,  then  the  default  action
      associated with the signal (see signal(7)) occurs.

   *  If  the  disposition  is  set  to  a function, then first either the
      disposition is reset to SIG_DFL,  or  the  signal  is  blocked  (see
      Portability below), and then handler is called with argument signum.
      If invocation of the handler caused the signal to be  blocked,  then
      the signal is unblocked upon return from the handler.

   The signals SIGKILL and SIGSTOP cannot be caught or ignored.

RETURN VALUE

   signal()  returns  the previous value of the signal handler, or SIG_ERR
   on error.  In the event of an error,  errno  is  set  to  indicate  the
   cause.

ERRORS

   EINVAL signum is invalid.

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, C89, C99.

NOTES

   The effects of signal() in a multithreaded process are unspecified.

   According  to  POSIX,  the  behavior of a process is undefined after it
   ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not  generated  by
   kill(2)  or  raise(3).   Integer division by zero has undefined result.
   On some architectures it will generate a SIGFPE signal.  (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  the  disposition
   SIGCHLD is set to SIG_IGN.

   See signal(7) for a list of the async-signal-safe functions that can be
   safely called from inside a signal handler.

   The use of sighandler_t is a GNU extension, exposed if  _GNU_SOURCE  is
   defined;  glibc  also  defines  (the  BSD-derived) sig_t if _BSD_SOURCE
   (glibc 2.19 and earlier) or _DEFAULT_SOURCE (glibc 2.19 and  later)  is
   defined.   Without  use  of such a type, the declaration of signal() is
   the somewhat harder to read:

       void ( *signal(int signum, void (*handler)(int)) ) (int);

   Portability
   The only portable use of signal() is to set a signal's  disposition  to
   SIG_DFL  or  SIG_IGN.  The semantics when using signal() to establish a
   signal handler vary across systems (and POSIX.1 explicitly permits this
   variation); do not use it for this purpose.

   POSIX.1  solved  the portability mess by specifying sigaction(2), which
   provides explicit control of the semantics when  a  signal  handler  is
   invoked; use that interface instead of signal().

   In the original UNIX systems, when a handler that was established using
   signal() was invoked by the delivery of a signal,  the  disposition  of
   the  signal  would  be  reset  to SIG_DFL, and the system did not block
   delivery of further instances of the signal.   This  is  equivalent  to
   calling sigaction(2) with the following flags:

       sa.sa_flags = SA_RESETHAND | SA_NODEFER;

   System V  also  provides  these  semantics  for signal().  This was bad
   because the signal might be delivered again before the  handler  had  a
   chance  to  reestablish  itself.   Furthermore, rapid deliveries of the
   same signal could result in recursive invocations of the handler.

   BSD improved on this situation,  but  unfortunately  also  changed  the
   semantics  of  the existing signal() interface while doing so.  On BSD,
   when a signal handler is invoked, the signal disposition is not  reset,
   and  further  instances  of the signal are blocked from being delivered
   while the handler is executing.  Furthermore, certain  blocking  system
   calls  are  automatically  restarted if interrupted by a signal handler
   (see  signal(7)).   The  BSD  semantics  are  equivalent   to   calling
   sigaction(2) with the following flags:

       sa.sa_flags = SA_RESTART;

   The situation on Linux is as follows:

   * The kernel's signal() system call provides System V semantics.

   * By  default, in glibc 2 and later, the signal() wrapper function does
     not invoke the kernel system call.  Instead,  it  calls  sigaction(2)
     using  flags  that  supply  BSD  semantics.  This default behavior is
     provided as long  as  a  suitable  feature  test  macro  is  defined:
     _BSD_SOURCE  on  glibc  2.19  and earlier or _DEFAULT_SOURCE in glibc
     2.19  and  later.   (By  default,  these  macros  are  defined;   see
     feature_test_macros(7) for details.)  If such a feature test macro is
     not defined, then signal() provides System V semantics.

SEE ALSO

   kill(1),  alarm(2),  kill(2),  pause(2),   sigaction(2),   signalfd(2),
   sigpending(2), sigprocmask(2), sigsuspend(2), bsd_signal(3), killpg(3),
   raise(3),  siginterrupt(3),   sigqueue(3),   sigsetops(3),   sigvec(3),
   sysv_signal(3), signal(7)

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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