1 .\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
2 .\" and Copyright (c) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
3 .\" and Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
4 .\" <mtk.manpages@gmail.com>
5 .\" based on work by Rik Faith <faith@cs.unc.edu>
6 .\" and Mike Battersby <mike@starbug.apana.org.au>.
8 .\" %%%LICENSE_START(VERBATIM)
9 .\" Permission is granted to make and distribute verbatim copies of this
10 .\" manual provided the copyright notice and this permission notice are
11 .\" preserved on all copies.
13 .\" Permission is granted to copy and distribute modified versions of this
14 .\" manual under the conditions for verbatim copying, provided that the
15 .\" entire resulting derived work is distributed under the terms of a
16 .\" permission notice identical to this one.
18 .\" Since the Linux kernel and libraries are constantly changing, this
19 .\" manual page may be incorrect or out-of-date. The author(s) assume no
20 .\" responsibility for errors or omissions, or for damages resulting from
21 .\" the use of the information contained herein. The author(s) may not
22 .\" have taken the same level of care in the production of this manual,
23 .\" which is licensed free of charge, as they might when working
26 .\" Formatted or processed versions of this manual, if unaccompanied by
27 .\" the source, must acknowledge the copyright and authors of this work.
30 .\" Modified 2004-11-19, mtk:
31 .\" added pointer to sigaction.2 for details of ignoring SIGCHLD
32 .\" 2007-06-03, mtk: strengthened portability warning, and rewrote
34 .\" 2008-07-11, mtk: rewrote and expanded portability discussion.
36 .TH SIGNAL 2 2013-04-19 "Linux" "Linux Programmer's Manual"
38 signal \- ANSI C signal handling
40 .B #include <signal.h>
42 .B typedef void (*sighandler_t)(int);
44 .BI "sighandler_t signal(int " signum ", sighandler_t " handler );
48 varies across UNIX versions,
49 and has also varied historically across different versions of Linux.
50 \fBAvoid its use\fP: use
53 See \fIPortability\fP below.
56 sets the disposition of the signal
63 or the address of a programmer-defined function (a "signal handler").
67 is delivered to the process, then one of the following happens:
70 If the disposition is set to
72 then the signal is ignored.
75 If the disposition is set to
77 then the default action associated with the signal (see
82 If the disposition is set to a function,
83 then first either the disposition is reset to
85 or the signal is blocked (see \fIPortability\fP below), and then
87 is called with argument
89 If invocation of the handler caused the signal to be blocked,
90 then the signal is unblocked upon return from the handler.
96 cannot be caught or ignored.
99 returns the previous value of the signal handler, or
102 In the event of an error,
104 is set to indicate the cause.
111 C89, C99, POSIX.1-2001.
115 in a multithreaded process are unspecified.
117 According to POSIX, the behavior of a process is undefined after it
123 signal that was not generated by
127 Integer division by zero has undefined result.
128 On some architectures it will generate a
131 (Also dividing the most negative integer by \-1 may generate
133 Ignoring this signal might lead to an endless loop.
137 for details on what happens when
144 for a list of the async-signal-safe functions that can be
145 safely called from inside a signal handler.
149 is a GNU extension, exposed if
152 .\" libc4 and libc5 define
153 .\" .IR SignalHandler ;
154 glibc also defines (the BSD-derived)
159 Without use of such a type, the declaration of
161 is the somewhat harder to read:
165 .BI "void ( *" signal "(int " signum ", void (*" handler ")(int)) ) (int);"
169 The only portable use of
171 is to set a signal's disposition to
175 The semantics when using
177 to establish a signal handler vary across systems
178 (and POSIX.1 explicitly permits this variation);
179 .B do not use it for this purpose.
181 POSIX.1 solved the portability mess by specifying
183 which provides explicit control of the semantics when a
184 signal handler is invoked; use that interface instead of
187 In the original UNIX systems, when a handler that was established using
189 was invoked by the delivery of a signal,
190 the disposition of the signal would be reset to
192 and the system did not block delivery of further instances of the signal.
193 This is equivalent to calling
195 with the following flags:
197 sa.sa_flags = SA_RESETHAND | SA_NODEFER;
199 System\ V also provides these semantics for
201 This was bad because the signal might be delivered again
202 before the handler had a chance to reestablish itself.
203 Furthermore, rapid deliveries of the same signal could
204 result in recursive invocations of the handler.
206 BSD improved on this situation, but unfortunately also
207 changed the semantics of the existing
209 interface while doing so.
210 On BSD, when a signal handler is invoked,
211 the signal disposition is not reset,
212 and further instances of the signal are blocked from
213 being delivered while the handler is executing.
214 Furthermore, certain blocking system calls are automatically
215 restarted if interrupted by a signal handler (see
217 The BSD semantics are equivalent to calling
219 with the following flags:
221 sa.sa_flags = SA_RESTART;
223 The situation on Linux is as follows:
227 system call provides System\ V semantics.
229 By default, in glibc 2 and later, the
231 wrapper function does not invoke the kernel system call.
234 using flags that supply BSD semantics.
235 This default behavior is provided as long as the
237 feature test macro is defined.
241 it is also implicitly defined if one defines
243 and can of course be explicitly defined.
245 On glibc 2 and later, if the
247 feature test macro is not defined, then
249 provides System\ V semantics.
250 (The default implicit definition of
252 is not provided if one invokes
254 in one of its standard modes
255 .RI ( -std=xxx " or " -ansi )
256 or defines various other feature test macros such as
262 .BR feature_test_macros (7).)
264 .\" System V semantics are also provided if one uses the separate
265 .\" .BR sysv_signal (3)
270 function in Linux libc4 and libc5 provide System\ V semantics.
271 If one on a libc5 system includes
277 provides BSD semantics.
291 .BR siginterrupt (3),
298 This page is part of release 3.67 of the Linux
301 A description of the project,
302 information about reporting bugs,
303 and the latest version of this page,
305 \%http://www.kernel.org/doc/man\-pages/.