signal(2)



SIGNAL(2)                  Linux Programmer's Manual                 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 var-
       ied 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  func-
       tion (a "signal handler").

       If  the signal signum is delivered to the process, then one of the fol-
       lowing 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  asso-
          ciated with the signal (see signal(7)) occurs.

       *  If  the disposition is set to a function, then first either the dis-
          position is reset to SIG_DFL, or the signal is blocked  (see  Porta-
          bility  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  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  sigac-
       tion(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 pro-
         vided  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  fea-
         ture_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),  killpg(2),  pause(2),  sigaction(2),  sig-
       nalfd(2),  sigpending(2), sigprocmask(2), sigsuspend(2), bsd_signal(3),
       raise(3),  siginterrupt(3),   sigqueue(3),   sigsetops(3),   sigvec(3),
       sysv_signal(3), signal(7)

COLOPHON
       This  page  is  part of release 4.06 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/.

Linux                             2016-03-15                         SIGNAL(2)

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