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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 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.
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.
EINVAL signum is invalid.
POSIX.1-2001, POSIX.1-2008, C89, C99.
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.
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.
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)
This page is part of release 4.12 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 2016-12-12 SIGNAL(2)
Pages that refer to this page: alarm(2), getitimer(2), kill(2), pause(2), prctl(2), sigaction(2), sigpending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), sigwaitinfo(2), syscalls(2), wait(2), wait4(2), bsd_signal(3), gsignal(3), killpg(3), profil(3), raise(3), siginterrupt(3), sigqueue(3), sigset(3), sigvec(3), sleep(3), sysv_signal(3), fifo(7), signal(7), signal-safety(7)