NAME

       signal - ANSI C signal handling

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

DESCRIPTION

       WARNING:  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.   On  failure,  it  returns
       SIG_ERR, and errno is set to indicate the error.

ERRORS

       EINVAL signum is invalid.

VERSIONS

       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().

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       C89, POSIX.1-2001.

       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.

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

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)