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 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
10 .\" Modified 2004-11-19, mtk:
11 .\" added pointer to sigaction.2 for details of ignoring SIGCHLD
12 .\" 2007-06-03, mtk: strengthened portability warning, and rewrote
14 .\" 2008-07-11, mtk: rewrote and expanded portability discussion.
16 .TH signal 2 (date) "Linux man-pages (unreleased)"
18 signal \- ANSI C signal handling
21 .RI ( libc ", " \-lc )
24 .B #include <signal.h>
26 .B typedef void (*sighandler_t)(int);
28 .BI "sighandler_t signal(int " signum ", sighandler_t " handler );
34 varies across UNIX versions,
35 and has also varied historically across different versions of Linux.
36 \fBAvoid its use\fP: use
39 See \fIPortability\fP below.
42 sets the disposition of the signal
49 or the address of a programmer-defined function (a "signal handler").
53 is delivered to the process, then one of the following happens:
56 If the disposition is set to
58 then the signal is ignored.
61 If the disposition is set to
63 then the default action associated with the signal (see
68 If the disposition is set to a function,
69 then first either the disposition is reset to
71 or the signal is blocked (see \fIPortability\fP below), and then
73 is called with argument
75 If invocation of the handler caused the signal to be blocked,
76 then the signal is unblocked upon return from the handler.
82 cannot be caught or ignored.
85 returns the previous value of the signal handler.
86 On failure, it returns
90 is set to indicate the error.
99 is a GNU extension, exposed if
102 .\" libc4 and libc5 define
103 .\" .IR SignalHandler ;
104 glibc also defines (the BSD-derived)
108 (glibc 2.19 and earlier)
111 (glibc 2.19 and later)
113 Without use of such a type, the declaration of
115 is the somewhat harder to read:
119 .BI "void ( *" signal "(int " signum ", void (*" handler ")(int)) ) (int);"
123 The only portable use of
125 is to set a signal's disposition to
129 The semantics when using
131 to establish a signal handler vary across systems
132 (and POSIX.1 explicitly permits this variation);
133 .B do not use it for this purpose.
135 POSIX.1 solved the portability mess by specifying
137 which provides explicit control of the semantics when a
138 signal handler is invoked; use that interface instead of
145 In the original UNIX systems, when a handler that was established using
147 was invoked by the delivery of a signal,
148 the disposition of the signal would be reset to
150 and the system did not block delivery of further instances of the signal.
151 This is equivalent to calling
153 with the following flags:
157 sa.sa_flags = SA_RESETHAND | SA_NODEFER;
161 System\ V also provides these semantics for
163 This was bad because the signal might be delivered again
164 before the handler had a chance to reestablish itself.
165 Furthermore, rapid deliveries of the same signal could
166 result in recursive invocations of the handler.
168 BSD improved on this situation, but unfortunately also
169 changed the semantics of the existing
171 interface while doing so.
172 On BSD, when a signal handler is invoked,
173 the signal disposition is not reset,
174 and further instances of the signal are blocked from
175 being delivered while the handler is executing.
176 Furthermore, certain blocking system calls are automatically
177 restarted if interrupted by a signal handler (see
179 The BSD semantics are equivalent to calling
181 with the following flags:
185 sa.sa_flags = SA_RESTART;
189 The situation on Linux is as follows:
193 system call provides System\ V semantics.
195 By default, in glibc 2 and later, the
197 wrapper function does not invoke the kernel system call.
200 using flags that supply BSD semantics.
201 This default behavior is provided as long as a suitable
202 feature test macro is defined:
204 on glibc 2.19 and earlier or
206 in glibc 2.19 and later.
207 (By default, these macros are defined; see
208 .BR feature_test_macros (7)
210 If such a feature test macro is not defined, then
212 provides System\ V semantics.
214 .\" System V semantics are also provided if one uses the separate
215 .\" .BR sysv_signal (3)
220 .\" function in Linux libc4 and libc5 provide System\ V semantics.
221 .\" If one on a libc5 system includes
222 .\" .I <bsd/signal.h>
227 .\" provides BSD semantics.
231 in a multithreaded process are unspecified.
233 According to POSIX, the behavior of a process is undefined after it
239 signal that was not generated by
243 Integer division by zero has undefined result.
244 On some architectures it will generate a
247 (Also dividing the most negative integer by \-1 may generate
249 Ignoring this signal might lead to an endless loop.
253 for details on what happens when the disposition
259 .BR signal\-safety (7)
260 for a list of the async-signal-safe functions that can be
261 safely called from inside a signal handler.
275 .BR siginterrupt (3),