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.
The effects of signal() in a
multi-threaded 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 inside from inside a signal handler.
The use of sighandler_t is a GNU
extension. Various versions of libc predefine this type;
libc4 and libc5 define SignalHandler, glibc defines
sig_t and, when _GNU_SOURCE is defined, also sighandler_t.
The original Unix signal()
would reset the handler to SIG_DFL, and System V (and the Linux
kernel and libc4,5) does the same. On the other hand, BSD
does not reset the handler, but blocks new instances of
this signal from occurring during a call of the handler.
The glibc2 library follows the BSD behavior.
If one on a libc5 system includes <bsd/signal.h> instead of <signal.h>
then signal() is redefined as
__bsd_signal() and
signal() has the BSD
semantics. This is not recommended.
If one on a glibc2 system defines a feature test macro
such as _XOPEN_SOURCE or uses
a separate sysv_signal(3) function,
one obtains classical behavior. This is not
recommended.
kill(1), alarm(2), kill(2), killpg(2), pause(2), sigaction(2), signalfd(2), sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), bsd_signal(3), raise(3), siginterrupt(3), sigsetops(3), sigvec(3), sysv_signal(3), feature_test_macros(7), signal(7)
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Copyright (c) 2000 Andries Brouwer <aebcwi.nl> and Copyright (c) 2007 Michael Kerrisk <mtk.manpagesgmail.com> based on work by Rik Faith <faithcs.unc.edu> and Mike Battersby <mikestarbug.apana.org.au>. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. Modified 2004-11-19, mtk: added pointer to sigaction.2 for details of ignoring SIGCHLD 2007-06-03, mtk: strengthened portability warning, and rerote various sections. |