100 lines
5.3 KiB
HTML
100 lines
5.3 KiB
HTML
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Agent Expressions - Debugging with GDB</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="Debugging with GDB">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="prev" href="Remote-Protocol.html#Remote-Protocol" title="Remote Protocol">
|
||
|
|
<link rel="next" href="Target-Descriptions.html#Target-Descriptions" title="Target Descriptions">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
Copyright (C) 1988-2019 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with the
|
||
|
|
Invariant Sections being ``Free Software'' and ``Free Software Needs
|
||
|
|
Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
|
||
|
|
and with the Back-Cover Texts as in (a) below.
|
||
|
|
|
||
|
|
(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
|
||
|
|
this GNU Manual. Buying copies from GNU Press supports the FSF in
|
||
|
|
developing GNU and promoting software freedom.''
|
||
|
|
-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Agent-Expressions"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Target-Descriptions.html#Target-Descriptions">Target Descriptions</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="Remote-Protocol.html#Remote-Protocol">Remote Protocol</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h2 class="appendix">Appendix F The GDB Agent Expression Mechanism</h2>
|
||
|
|
|
||
|
|
<p>In some applications, it is not feasible for the debugger to interrupt
|
||
|
|
the program's execution long enough for the developer to learn anything
|
||
|
|
helpful about its behavior. If the program's correctness depends on its
|
||
|
|
real-time behavior, delays introduced by a debugger might cause the
|
||
|
|
program to fail, even when the code itself is correct. It is useful to
|
||
|
|
be able to observe the program's behavior without interrupting it.
|
||
|
|
|
||
|
|
<p>Using GDB's <code>trace</code> and <code>collect</code> commands, the user can
|
||
|
|
specify locations in the program, and arbitrary expressions to evaluate
|
||
|
|
when those locations are reached. Later, using the <code>tfind</code>
|
||
|
|
command, she can examine the values those expressions had when the
|
||
|
|
program hit the trace points. The expressions may also denote objects
|
||
|
|
in memory — structures or arrays, for example — whose values GDB
|
||
|
|
should record; while visiting a particular tracepoint, the user may
|
||
|
|
inspect those objects as if they were in memory at that moment.
|
||
|
|
However, because GDB records these values without interacting with the
|
||
|
|
user, it can do so quickly and unobtrusively, hopefully not disturbing
|
||
|
|
the program's behavior.
|
||
|
|
|
||
|
|
<p>When GDB is debugging a remote target, the GDB <dfn>agent</dfn> code running
|
||
|
|
on the target computes the values of the expressions itself. To avoid
|
||
|
|
having a full symbolic expression evaluator on the agent, GDB translates
|
||
|
|
expressions in the source language into a simpler bytecode language, and
|
||
|
|
then sends the bytecode to the agent; the agent then executes the
|
||
|
|
bytecode, and records the values for GDB to retrieve later.
|
||
|
|
|
||
|
|
<p>The bytecode language is simple; there are forty-odd opcodes, the bulk
|
||
|
|
of which are the usual vocabulary of C operands (addition, subtraction,
|
||
|
|
shifts, and so on) and various sizes of literals and memory reference
|
||
|
|
operations. The bytecode interpreter operates strictly on machine-level
|
||
|
|
values — various sizes of integers and floating point numbers — and
|
||
|
|
requires no information about types or symbols; thus, the interpreter's
|
||
|
|
internal data structures are simple, and each bytecode requires only a
|
||
|
|
few native machine instructions to implement it. The interpreter is
|
||
|
|
small, and strict limits on the memory and time required to evaluate an
|
||
|
|
expression are easy to determine, making it suitable for use by the
|
||
|
|
debugging agent in real-time applications.
|
||
|
|
|
||
|
|
<ul class="menu">
|
||
|
|
<li><a accesskey="1" href="General-Bytecode-Design.html#General-Bytecode-Design">General Bytecode Design</a>: Overview of the interpreter.
|
||
|
|
<li><a accesskey="2" href="Bytecode-Descriptions.html#Bytecode-Descriptions">Bytecode Descriptions</a>: What each one does.
|
||
|
|
<li><a accesskey="3" href="Using-Agent-Expressions.html#Using-Agent-Expressions">Using Agent Expressions</a>: How agent expressions fit into the big picture.
|
||
|
|
<li><a accesskey="4" href="Varying-Target-Capabilities.html#Varying-Target-Capabilities">Varying Target Capabilities</a>: How to discover what the target can do.
|
||
|
|
<li><a accesskey="5" href="Rationale.html#Rationale">Rationale</a>: Why we did it this way.
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<!-- @node Rationale -->
|
||
|
|
<!-- @section Rationale -->
|
||
|
|
</body></html>
|
||
|
|
|