1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2023 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." -->
<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ -->
<head>
<title>Debugging with GDB: Using Agent Expressions</title>
<meta name="description" content="Debugging with GDB: Using Agent Expressions">
<meta name="keywords" content="Debugging with GDB: Using Agent Expressions">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Agent-Expressions.html#Agent-Expressions" rel="up" title="Agent Expressions">
<link href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" rel="next" title="Varying Target Capabilities">
<link href="Bytecode-Descriptions.html#Bytecode-Descriptions" rel="previous" title="Bytecode Descriptions">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.indentedblock {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
div.smalllisp {margin-left: 3.2em}
kbd {font-style:oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nocodebreak {white-space:nowrap}
span.nolinebreak {white-space:nowrap}
span.roman {font-family:serif; font-weight:normal}
span.sansserif {font-family:sans-serif; font-weight:normal}
ul.no-bullet {list-style: none}
-->
</style>
</head>
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Using-Agent-Expressions"></a>
<div class="header">
<p>
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="previous">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>
</div>
<hr>
<a name="Using-Agent-Expressions-1"></a>
<h3 class="section">F.3 Using Agent Expressions</h3>
<p>Agent expressions can be used in several different ways by <small>GDB</small>,
and the debugger can generate different bytecode sequences as appropriate.
</p>
<p>One possibility is to do expression evaluation on the target rather
than the host, such as for the conditional of a conditional
tracepoint. In such a case, <small>GDB</small> compiles the source
expression into a bytecode sequence that simply gets values from
registers or memory, does arithmetic, and returns a result.
</p>
<p>Another way to use agent expressions is for tracepoint data
collection. <small>GDB</small> generates a different bytecode sequence for
collection; in addition to bytecodes that do the calculation,
<small>GDB</small> adds <code>trace</code> bytecodes to save the pieces of
memory that were used.
</p>
<ul>
<li> The user selects trace points in the program’s code at which GDB should
collect data.
</li><li> The user specifies expressions to evaluate at each trace point. These
expressions may denote objects in memory, in which case those objects’
contents are recorded as the program runs, or computed values, in which
case the values themselves are recorded.
</li><li> GDB transmits the tracepoints and their associated expressions to the
GDB agent, running on the debugging target.
</li><li> The agent arranges to be notified when a trace point is hit.
</li><li> When execution on the target reaches a trace point, the agent evaluates
the expressions associated with that trace point, and records the
resulting values and memory ranges.
</li><li> Later, when the user selects a given trace event and inspects the
objects and expression values recorded, GDB talks to the agent to
retrieve recorded data as necessary to meet the user’s requests. If the
user asks to see an object whose contents have not been recorded, GDB
reports an error.
</li></ul>
<hr>
<div class="header">
<p>
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="previous">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>
</div>
</body>
</html>
|
