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119 lines
5.1 KiB
HTML
119 lines
5.1 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Interrupted System Calls (Debugging with GDB)</title>
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<meta name="description" content="Interrupted System Calls (Debugging with GDB)">
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<meta name="keywords" content="Interrupted System Calls (Debugging with GDB)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Thread-Stops.html#Thread-Stops" rel="up" title="Thread Stops">
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<link href="Observer-Mode.html#Observer-Mode" rel="next" title="Observer Mode">
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<link href="Thread_002dSpecific-Breakpoints.html#Thread_002dSpecific-Breakpoints" rel="prev" title="Thread-Specific Breakpoints">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Interrupted-System-Calls"></a>
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<div class="header">
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<p>
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Next: <a href="Observer-Mode.html#Observer-Mode" accesskey="n" rel="next">Observer Mode</a>, Previous: <a href="Thread_002dSpecific-Breakpoints.html#Thread_002dSpecific-Breakpoints" accesskey="p" rel="prev">Thread-Specific Breakpoints</a>, Up: <a href="Thread-Stops.html#Thread-Stops" accesskey="u" rel="up">Thread Stops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Interrupted-System-Calls-1"></a>
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<h4 class="subsection">5.5.5 Interrupted System Calls</h4>
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<a name="index-thread-breakpoints-and-system-calls"></a>
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<a name="index-system-calls-and-thread-breakpoints"></a>
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<a name="index-premature-return-from-system-calls"></a>
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<p>There is an unfortunate side effect when using <small>GDB</small> to debug
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multi-threaded programs. If one thread stops for a
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breakpoint, or for some other reason, and another thread is blocked in a
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system call, then the system call may return prematurely. This is a
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consequence of the interaction between multiple threads and the signals
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that <small>GDB</small> uses to implement breakpoints and other events that
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stop execution.
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</p>
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<p>To handle this problem, your program should check the return value of
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each system call and react appropriately. This is good programming
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style anyways.
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</p>
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<p>For example, do not write code like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> sleep (10);
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</pre></div>
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<p>The call to <code>sleep</code> will return early if a different thread stops
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at a breakpoint or for some other reason.
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</p>
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<p>Instead, write this:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> int unslept = 10;
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while (unslept > 0)
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unslept = sleep (unslept);
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</pre></div>
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<p>A system call is allowed to return early, so the system is still
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conforming to its specification. But <small>GDB</small> does cause your
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multi-threaded program to behave differently than it would without
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<small>GDB</small>.
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</p>
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<p>Also, <small>GDB</small> uses internal breakpoints in the thread library to
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monitor certain events such as thread creation and thread destruction.
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When such an event happens, a system call in another thread may return
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prematurely, even though your program does not appear to stop.
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</p>
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</body>
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</html>
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