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136 lines
7.4 KiB
HTML
136 lines
7.4 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>Agent Expressions (Debugging with GDB)</title>
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<meta name="description" content="Agent Expressions (Debugging with GDB)">
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<meta name="keywords" content="Agent Expressions (Debugging with GDB)">
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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="index.html#Top" rel="up" title="Top">
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<link href="General-Bytecode-Design.html#General-Bytecode-Design" rel="next" title="General Bytecode Design">
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<link href="Branch-Trace-Configuration-Format.html#Branch-Trace-Configuration-Format" rel="prev" title="Branch Trace Configuration Format">
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</style>
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</head>
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<body lang="en">
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<a name="Agent-Expressions"></a>
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<div class="header">
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<p>
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Next: <a href="Target-Descriptions.html#Target-Descriptions" accesskey="n" rel="next">Target Descriptions</a>, Previous: <a href="Remote-Protocol.html#Remote-Protocol" accesskey="p" rel="prev">Remote Protocol</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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="The-GDB-Agent-Expression-Mechanism"></a>
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<h2 class="appendix">Appendix F The GDB Agent Expression Mechanism</h2>
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<p>In some applications, it is not feasible for the debugger to interrupt
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the program’s execution long enough for the developer to learn anything
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helpful about its behavior. If the program’s correctness depends on its
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real-time behavior, delays introduced by a debugger might cause the
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program to fail, even when the code itself is correct. It is useful to
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be able to observe the program’s behavior without interrupting it.
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</p>
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<p>Using GDB’s <code>trace</code> and <code>collect</code> commands, the user can
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specify locations in the program, and arbitrary expressions to evaluate
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when those locations are reached. Later, using the <code>tfind</code>
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command, she can examine the values those expressions had when the
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program hit the trace points. The expressions may also denote objects
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in memory — structures or arrays, for example — whose values GDB
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should record; while visiting a particular tracepoint, the user may
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inspect those objects as if they were in memory at that moment.
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However, because GDB records these values without interacting with the
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user, it can do so quickly and unobtrusively, hopefully not disturbing
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the program’s behavior.
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</p>
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<p>When GDB is debugging a remote target, the GDB <em>agent</em> code running
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on the target computes the values of the expressions itself. To avoid
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having a full symbolic expression evaluator on the agent, GDB translates
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expressions in the source language into a simpler bytecode language, and
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then sends the bytecode to the agent; the agent then executes the
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bytecode, and records the values for GDB to retrieve later.
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</p>
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<p>The bytecode language is simple; there are forty-odd opcodes, the bulk
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of which are the usual vocabulary of C operands (addition, subtraction,
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shifts, and so on) and various sizes of literals and memory reference
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operations. The bytecode interpreter operates strictly on machine-level
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values — various sizes of integers and floating point numbers — and
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requires no information about types or symbols; thus, the interpreter’s
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internal data structures are simple, and each bytecode requires only a
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few native machine instructions to implement it. The interpreter is
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small, and strict limits on the memory and time required to evaluate an
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expression are easy to determine, making it suitable for use by the
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debugging agent in real-time applications.
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</p>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="General-Bytecode-Design.html#General-Bytecode-Design" accesskey="1">General Bytecode Design</a>:</td><td> </td><td align="left" valign="top">Overview of the interpreter.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="2">Bytecode Descriptions</a>:</td><td> </td><td align="left" valign="top">What each one does.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Using-Agent-Expressions.html#Using-Agent-Expressions" accesskey="3">Using Agent Expressions</a>:</td><td> </td><td align="left" valign="top">How agent expressions fit into the big picture.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="4">Varying Target Capabilities</a>:</td><td> </td><td align="left" valign="top">How to discover what the target can do.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Rationale.html#Rationale" accesskey="5">Rationale</a>:</td><td> </td><td align="left" valign="top">Why we did it this way.
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</td></tr>
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</table>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Target-Descriptions.html#Target-Descriptions" accesskey="n" rel="next">Target Descriptions</a>, Previous: <a href="Remote-Protocol.html#Remote-Protocol" accesskey="p" rel="prev">Remote Protocol</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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