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)

Linux man-pages 6.9.1                              2024-05-02                                          signal(2)