Name
signal — ANSI C signal handling
Synopsis
sighandler_t signal( |
int |
signum, |
| |
sighandler_t |
handler); |
DESCRIPTION
The behaviour 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 invokation 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.
ERRORS
- EINVAL
-
signum is
invalid.
CONFORMING TO
C89, C99, POSIX.1-2001.
NOTES
The effects of signal() in a
multi-threaded process are unspecified.
According to POSIX, the behaviour of a process is
undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by
kill(2) or the 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.
Portability
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 behaviour.
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 behaviour. This is not
recommended.
SEE ALSO
kill(1), alarm(2), kill(2), pause(2), sigaction(2), sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), bsd_signal(3), killpg(3), raise(3), siginterrupt(3), sigsetops(3), sigvec(3), sysv_signal(3), feature_test_macros(7),
signal(7)
Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
and Copyright (c) 2007 Michael Kerrisk <mtk-manpages@gmx.net>
based on work by Rik Faith <faith@cs.unc.edu>
and Mike Battersby <mike@starbug.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.
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