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124 lines
5.9 KiB
HTML
124 lines
5.9 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>Using Agent Expressions (Debugging with GDB)</title>
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<meta name="description" content="Using Agent Expressions (Debugging with GDB)">
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<meta name="keywords" content="Using Agent Expressions (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="Agent-Expressions.html#Agent-Expressions" rel="up" title="Agent Expressions">
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<link href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" rel="next" title="Varying Target Capabilities">
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<link href="Bytecode-Descriptions.html#Bytecode-Descriptions" rel="prev" title="Bytecode Descriptions">
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</style>
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</head>
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<body lang="en">
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<a name="Using-Agent-Expressions"></a>
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<div class="header">
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<p>
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Next: <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="n" rel="next">Varying Target Capabilities</a>, Previous: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="p" rel="prev">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</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="Using-Agent-Expressions-1"></a>
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<h3 class="section">F.3 Using Agent Expressions</h3>
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<p>Agent expressions can be used in several different ways by <small>GDB</small>,
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and the debugger can generate different bytecode sequences as appropriate.
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</p>
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<p>One possibility is to do expression evaluation on the target rather
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than the host, such as for the conditional of a conditional
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tracepoint. In such a case, <small>GDB</small> compiles the source
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expression into a bytecode sequence that simply gets values from
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registers or memory, does arithmetic, and returns a result.
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</p>
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<p>Another way to use agent expressions is for tracepoint data
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collection. <small>GDB</small> generates a different bytecode sequence for
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collection; in addition to bytecodes that do the calculation,
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<small>GDB</small> adds <code>trace</code> bytecodes to save the pieces of
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memory that were used.
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</p>
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<ul>
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<li> The user selects trace points in the program’s code at which GDB should
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collect data.
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</li><li> The user specifies expressions to evaluate at each trace point. These
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expressions may denote objects in memory, in which case those objects’
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contents are recorded as the program runs, or computed values, in which
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case the values themselves are recorded.
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</li><li> GDB transmits the tracepoints and their associated expressions to the
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GDB agent, running on the debugging target.
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</li><li> The agent arranges to be notified when a trace point is hit.
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</li><li> When execution on the target reaches a trace point, the agent evaluates
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the expressions associated with that trace point, and records the
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resulting values and memory ranges.
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</li><li> Later, when the user selects a given trace event and inspects the
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objects and expression values recorded, GDB talks to the agent to
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retrieve recorded data as necessary to meet the user’s requests. If the
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user asks to see an object whose contents have not been recorded, GDB
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reports an error.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="n" rel="next">Varying Target Capabilities</a>, Previous: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="p" rel="prev">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</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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