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177 lines
8.4 KiB
HTML
177 lines
8.4 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>All-Stop Mode (Debugging with GDB)</title>
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<meta name="description" content="All-Stop Mode (Debugging with GDB)">
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<meta name="keywords" content="All-Stop Mode (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="Non_002dStop-Mode.html#Non_002dStop-Mode" rel="next" title="Non-Stop Mode">
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<link href="Thread-Stops.html#Thread-Stops" rel="prev" title="Thread Stops">
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<style type="text/css">
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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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="All_002dStop-Mode"></a>
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<div class="header">
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<p>
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Next: <a href="Non_002dStop-Mode.html#Non_002dStop-Mode" accesskey="n" rel="next">Non-Stop Mode</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="All_002dStop-Mode-1"></a>
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<h4 class="subsection">5.5.1 All-Stop Mode</h4>
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<a name="index-all_002dstop-mode"></a>
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<p>In all-stop mode, whenever your program stops under <small>GDB</small> for any reason,
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<em>all</em> threads of execution stop, not just the current thread. This
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allows you to examine the overall state of the program, including
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switching between threads, without worrying that things may change
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underfoot.
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</p>
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<p>Conversely, whenever you restart the program, <em>all</em> threads start
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executing. <em>This is true even when single-stepping</em> with commands
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like <code>step</code> or <code>next</code>.
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</p>
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<p>In particular, <small>GDB</small> cannot single-step all threads in lockstep.
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Since thread scheduling is up to your debugging target’s operating
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system (not controlled by <small>GDB</small>), other threads may
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execute more than one statement while the current thread completes a
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single step. Moreover, in general other threads stop in the middle of a
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statement, rather than at a clean statement boundary, when the program
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stops.
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</p>
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<p>You might even find your program stopped in another thread after
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continuing or even single-stepping. This happens whenever some other
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thread runs into a breakpoint, a signal, or an exception before the
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first thread completes whatever you requested.
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</p>
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<a name="index-automatic-thread-selection"></a>
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<a name="index-switching-threads-automatically"></a>
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<a name="index-threads_002c-automatic-switching"></a>
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<p>Whenever <small>GDB</small> stops your program, due to a breakpoint or a
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signal, it automatically selects the thread where that breakpoint or
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signal happened. <small>GDB</small> alerts you to the context switch with a
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message such as ‘<samp>[Switching to Thread <var>n</var>]</samp>’ to identify the
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thread.
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</p>
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<p>On some OSes, you can modify <small>GDB</small>’s default behavior by
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locking the OS scheduler to allow only a single thread to run.
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</p>
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<dl compact="compact">
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<dt><code>set scheduler-locking <var>mode</var></code></dt>
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<dd><a name="index-scheduler-locking-mode"></a>
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<a name="index-lock-scheduler"></a>
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<p>Set the scheduler locking mode. It applies to normal execution,
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record mode, and replay mode. If it is <code>off</code>, then there is no
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locking and any thread may run at any time. If <code>on</code>, then only
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the current thread may run when the inferior is resumed. The
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<code>step</code> mode optimizes for single-stepping; it prevents other
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threads from preempting the current thread while you are stepping, so
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that the focus of debugging does not change unexpectedly. Other
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threads never get a chance to run when you step, and they are
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completely free to run when you use commands like ‘<samp>continue</samp>’,
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‘<samp>until</samp>’, or ‘<samp>finish</samp>’. However, unless another thread hits a
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breakpoint during its timeslice, <small>GDB</small> does not change the
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current thread away from the thread that you are debugging. The
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<code>replay</code> mode behaves like <code>off</code> in record mode and like
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<code>on</code> in replay mode.
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</p>
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</dd>
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<dt><code>show scheduler-locking</code></dt>
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<dd><p>Display the current scheduler locking mode.
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</p></dd>
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</dl>
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<a name="index-resume-threads-of-multiple-processes-simultaneously"></a>
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<p>By default, when you issue one of the execution commands such as
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<code>continue</code>, <code>next</code> or <code>step</code>, <small>GDB</small> allows only
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threads of the current inferior to run. For example, if <small>GDB</small>
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is attached to two inferiors, each with two threads, the
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<code>continue</code> command resumes only the two threads of the current
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inferior. This is useful, for example, when you debug a program that
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forks and you want to hold the parent stopped (so that, for instance,
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it doesn’t run to exit), while you debug the child. In other
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situations, you may not be interested in inspecting the current state
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of any of the processes <small>GDB</small> is attached to, and you may want
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to resume them all until some breakpoint is hit. In the latter case,
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you can instruct <small>GDB</small> to allow all threads of all the
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inferiors to run with the <code>set <span class="nolinebreak">schedule-multiple</span></code><!-- /@w --> command.
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</p>
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<dl compact="compact">
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<dd><a name="index-set-schedule_002dmultiple"></a>
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</dd>
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<dt><code>set schedule-multiple</code></dt>
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<dd><p>Set the mode for allowing threads of multiple processes to be resumed
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when an execution command is issued. When <code>on</code>, all threads of
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all processes are allowed to run. When <code>off</code>, only the threads
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of the current process are resumed. The default is <code>off</code>. The
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<code>scheduler-locking</code> mode takes precedence when set to <code>on</code>,
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or while you are stepping and set to <code>step</code>.
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</p>
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</dd>
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<dt><code>show schedule-multiple</code></dt>
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<dd><p>Display the current mode for resuming the execution of threads of
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multiple processes.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Non_002dStop-Mode.html#Non_002dStop-Mode" accesskey="n" rel="next">Non-Stop Mode</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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</body>
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</html>
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