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155 lines
8.1 KiB
HTML
155 lines
8.1 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>Context management (Debugging with GDB)</title>
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<meta name="description" content="Context management (Debugging with GDB)">
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<meta name="keywords" content="Context management (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="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design" rel="up" title="GDB/MI General Design">
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<link href="Asynchronous-and-non_002dstop-modes.html#Asynchronous-and-non_002dstop-modes" rel="next" title="Asynchronous and non-stop modes">
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<link href="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design" rel="prev" title="GDB/MI General Design">
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<style type="text/css">
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</style>
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</head>
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<body lang="en">
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<a name="Context-management"></a>
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<div class="header">
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<p>
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Next: <a href="Asynchronous-and-non_002dstop-modes.html#Asynchronous-and-non_002dstop-modes" accesskey="n" rel="next">Asynchronous and non-stop modes</a>, Up: <a href="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design" accesskey="u" rel="up">GDB/MI General Design</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="Context-management-1"></a>
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<h4 class="subsection">27.1.1 Context management</h4>
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<a name="Threads-and-Frames"></a>
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<h4 class="subsubsection">27.1.1.1 Threads and Frames</h4>
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<p>In most cases when <small>GDB</small> accesses the target, this access is
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done in context of a specific thread and frame (see <a href="Frames.html#Frames">Frames</a>).
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Often, even when accessing global data, the target requires that a thread
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be specified. The CLI interface maintains the selected thread and frame,
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and supplies them to target on each command. This is convenient,
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because a command line user would not want to specify that information
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explicitly on each command, and because user interacts with
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<small>GDB</small> via a single terminal, so no confusion is possible as
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to what thread and frame are the current ones.
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</p>
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<p>In the case of MI, the concept of selected thread and frame is less
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useful. First, a frontend can easily remember this information
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itself. Second, a graphical frontend can have more than one window,
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each one used for debugging a different thread, and the frontend might
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want to access additional threads for internal purposes. This
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increases the risk that by relying on implicitly selected thread, the
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frontend may be operating on a wrong one. Therefore, each MI command
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should explicitly specify which thread and frame to operate on. To
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make it possible, each MI command accepts the ‘<samp>--thread</samp>’ and
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‘<samp>--frame</samp>’ options, the value to each is <small>GDB</small> global
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identifier for thread and frame to operate on.
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</p>
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<p>Usually, each top-level window in a frontend allows the user to select
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a thread and a frame, and remembers the user selection for further
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operations. However, in some cases <small>GDB</small> may suggest that the
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current thread or frame be changed. For example, when stopping on a
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breakpoint it is reasonable to switch to the thread where breakpoint is
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hit. For another example, if the user issues the CLI ‘<samp>thread</samp>’ or
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‘<samp>frame</samp>’ commands via the frontend, it is desirable to change the
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frontend’s selection to the one specified by user. <small>GDB</small>
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communicates the suggestion to change current thread and frame using the
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‘<samp>=thread-selected</samp>’ notification.
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</p>
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<p>Note that historically, MI shares the selected thread with CLI, so
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frontends used the <code>-thread-select</code> to execute commands in the
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right context. However, getting this to work right is cumbersome. The
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simplest way is for frontend to emit <code>-thread-select</code> command
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before every command. This doubles the number of commands that need
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to be sent. The alternative approach is to suppress <code>-thread-select</code>
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if the selected thread in <small>GDB</small> is supposed to be identical to the
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thread the frontend wants to operate on. However, getting this
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optimization right can be tricky. In particular, if the frontend
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sends several commands to <small>GDB</small>, and one of the commands changes the
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selected thread, then the behaviour of subsequent commands will
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change. So, a frontend should either wait for response from such
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problematic commands, or explicitly add <code>-thread-select</code> for
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all subsequent commands. No frontend is known to do this exactly
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right, so it is suggested to just always pass the ‘<samp>--thread</samp>’ and
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‘<samp>--frame</samp>’ options.
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</p>
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<a name="Language"></a>
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<h4 class="subsubsection">27.1.1.2 Language</h4>
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<p>The execution of several commands depends on which language is selected.
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By default, the current language (see <a href="Show.html#show-language">show language</a>) is used.
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But for commands known to be language-sensitive, it is recommended
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to use the ‘<samp>--language</samp>’ option. This option takes one argument,
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which is the name of the language to use while executing the command.
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For instance:
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</p>
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<div class="smallexample">
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<pre class="smallexample">-data-evaluate-expression --language c "sizeof (void*)"
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^done,value="4"
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(gdb)
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</pre></div>
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<p>The valid language names are the same names accepted by the
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‘<samp>set language</samp>’ command (see <a href="Manually.html#Manually">Manually</a>), excluding ‘<samp>auto</samp>’,
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‘<samp>local</samp>’ or ‘<samp>unknown</samp>’.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Asynchronous-and-non_002dstop-modes.html#Asynchronous-and-non_002dstop-modes" accesskey="n" rel="next">Asynchronous and non-stop modes</a>, Up: <a href="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design" accesskey="u" rel="up">GDB/MI General Design</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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