418 lines
8.9 KiB
HTML
418 lines
8.9 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SIGNAL</TITLE>
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</HEAD><BODY>
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<H1>SIGNAL</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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signal - ANSI C signal handling
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/signal.h">signal.h</A>></B>
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<P>
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<B>typedef void (*sighandler_t)(int);</B>
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<P>
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<B>sighandler_t signal(int </B><I>signum</I><B>, sighandler_t </B><I>handler</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The behavior of
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<B>signal</B>()
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varies across UNIX versions,
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and has also varied historically across different versions of Linux.
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<B>Avoid its use</B>: use
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2)
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instead.
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See <I>Portability</I> below.
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<P>
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<B>signal</B>()
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sets the disposition of the signal
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<I>signum</I>
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to
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<I>handler</I>,
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which is either
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<B>SIG_IGN</B>,
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<B>SIG_DFL</B>,
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or the address of a programmer-defined function (a "signal handler").
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<P>
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If the signal
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<I>signum</I>
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is delivered to the process, then one of the following happens:
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<DL COMPACT>
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<DT id="1">*<DD>
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If the disposition is set to
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<B>SIG_IGN</B>,
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then the signal is ignored.
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<DT id="2">*<DD>
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If the disposition is set to
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<B>SIG_DFL</B>,
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then the default action associated with the signal (see
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<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7))
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occurs.
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<DT id="3">*<DD>
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If the disposition is set to a function,
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then first either the disposition is reset to
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<B>SIG_DFL</B>,
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or the signal is blocked (see <I>Portability</I> below), and then
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<I>handler</I>
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is called with argument
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<I>signum</I>.
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If invocation of the handler caused the signal to be blocked,
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then the signal is unblocked upon return from the handler.
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</DL>
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<P>
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The signals
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<B>SIGKILL</B>
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and
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<B>SIGSTOP</B>
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cannot be caught or ignored.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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<B>signal</B>()
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returns the previous value of the signal handler, or
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<B>SIG_ERR</B>
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on error.
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In the event of an error,
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<I>errno</I>
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is set to indicate the cause.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="4"><B>EINVAL</B>
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<DD>
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<I>signum</I>
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is invalid.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008, C89, C99.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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The effects of
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<B>signal</B>()
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in a multithreaded process are unspecified.
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<P>
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According to POSIX, the behavior of a process is undefined after it
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ignores a
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<B>SIGFPE</B>,
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<B>SIGILL</B>,
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or
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<B>SIGSEGV</B>
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signal that was not generated by
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<B><A HREF="/cgi-bin/man/man2html?2+kill">kill</A></B>(2)
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or
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<B><A HREF="/cgi-bin/man/man2html?3+raise">raise</A></B>(3).
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Integer division by zero has undefined result.
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On some architectures it will generate a
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<B>SIGFPE</B>
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signal.
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(Also dividing the most negative integer by -1 may generate
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<B>SIGFPE</B>.)
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Ignoring this signal might lead to an endless loop.
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<P>
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See
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2)
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for details on what happens when the disposition
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<B>SIGCHLD</B>
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is set to
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<B>SIG_IGN</B>.
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<P>
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See
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<B><A HREF="/cgi-bin/man/man2html?7+signal-safety">signal-safety</A></B>(7)
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for a list of the async-signal-safe functions that can be
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safely called from inside a signal handler.
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<P>
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The use of
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<I>sighandler_t</I>
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is a GNU extension, exposed if
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<B>_GNU_SOURCE</B>
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is defined;
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glibc also defines (the BSD-derived)
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<I>sig_t</I>
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if
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<B>_BSD_SOURCE</B>
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(glibc 2.19 and earlier)
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or
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<B>_DEFAULT_SOURCE</B>
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(glibc 2.19 and later)
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is defined.
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Without use of such a type, the declaration of
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<B>signal</B>()
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is the somewhat harder to read:
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<P>
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<B>void ( *</B><I>signal</I><B>(int </B><I>signum</I><B>, void (*</B><I>handler</I><B>)(int)) ) (int);</B>
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<A NAME="lbAI"> </A>
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<H3>Portability</H3>
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The only portable use of
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<B>signal</B>()
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is to set a signal's disposition to
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<B>SIG_DFL</B>
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or
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<B>SIG_IGN</B>.
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The semantics when using
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<B>signal</B>()
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to establish a signal handler vary across systems
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(and POSIX.1 explicitly permits this variation);
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<B>do not use it for this purpose.</B>
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<P>
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POSIX.1 solved the portability mess by specifying
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2),
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which provides explicit control of the semantics when a
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signal handler is invoked; use that interface instead of
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<B>signal</B>().
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<P>
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In the original UNIX systems, when a handler that was established using
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<B>signal</B>()
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was invoked by the delivery of a signal,
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the disposition of the signal would be reset to
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<B>SIG_DFL</B>,
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and the system did not block delivery of further instances of the signal.
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This is equivalent to calling
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2)
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with the following flags:
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<P>
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<BR> sa.sa_flags = SA_RESETHAND | SA_NODEFER;
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<P>
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System V also provides these semantics for
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<B>signal</B>().
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This was bad because the signal might be delivered again
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before the handler had a chance to reestablish itself.
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Furthermore, rapid deliveries of the same signal could
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result in recursive invocations of the handler.
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<P>
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BSD improved on this situation, but unfortunately also
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changed the semantics of the existing
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<B>signal</B>()
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interface while doing so.
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On BSD, when a signal handler is invoked,
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the signal disposition is not reset,
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and further instances of the signal are blocked from
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being delivered while the handler is executing.
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Furthermore, certain blocking system calls are automatically
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restarted if interrupted by a signal handler (see
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<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7)).
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The BSD semantics are equivalent to calling
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2)
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with the following flags:
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<P>
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<BR> sa.sa_flags = SA_RESTART;
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<P>
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The situation on Linux is as follows:
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<DL COMPACT>
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<DT id="5">*<DD>
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The kernel's
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<B>signal</B>()
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system call provides System V semantics.
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<DT id="6">*<DD>
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By default, in glibc 2 and later, the
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<B>signal</B>()
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wrapper function does not invoke the kernel system call.
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Instead, it calls
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2)
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using flags that supply BSD semantics.
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This default behavior is provided as long as a suitable
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feature test macro is defined:
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<B>_BSD_SOURCE</B>
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on glibc 2.19 and earlier or
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<B>_DEFAULT_SOURCE</B>
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in glibc 2.19 and later.
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(By default, these macros are defined; see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)
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for details.)
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If such a feature test macro is not defined, then
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<B>signal</B>()
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provides System V semantics.
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</DL>
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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?1+kill">kill</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?2+alarm">alarm</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+kill">kill</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+pause">pause</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+signalfd">signalfd</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+sigpending">sigpending</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+sigprocmask">sigprocmask</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+sigsuspend">sigsuspend</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+bsd_signal">bsd_signal</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+killpg">killpg</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+raise">raise</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+siginterrupt">siginterrupt</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sigqueue">sigqueue</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sigsetops">sigsetops</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sigvec">sigvec</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sysv_signal">sysv_signal</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7)
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<A NAME="lbAK"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="7"><A HREF="#lbAB">NAME</A><DD>
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<DT id="8"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="9"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="10"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="11"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="12"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="13"><A HREF="#lbAH">NOTES</A><DD>
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<DL>
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<DT id="14"><A HREF="#lbAI">Portability</A><DD>
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</DL>
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<DT id="15"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="16"><A HREF="#lbAK">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:34 GMT, March 31, 2021
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</BODY>
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</HTML>
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