huangmeng
/
Embedded_AI

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1994-2019 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 no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License". -->
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
<head>
<title>GDB&rsquo;s Obsolete Annotations: Limitations</title>

<meta name="description" content="GDB&rsquo;s Obsolete Annotations: Limitations">
<meta name="keywords" content="GDB&rsquo;s Obsolete Annotations: Limitations">
<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="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="index.html#Top" rel="up" title="Top">
<link href="Migrating-to-GDB_002fMI.html#Migrating-to-GDB_002fMI" rel="next" title="Migrating to GDB/MI">
<link href="Annotations-Overview.html#Annotations-Overview" rel="prev" title="Annotations Overview">
<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="Limitations"></a>
<div class="header">
<p>
Next: <a href="Migrating-to-GDB_002fMI.html#Migrating-to-GDB_002fMI" accesskey="n" rel="next">Migrating to GDB/MI</a>, Previous: <a href="Annotations-Overview.html#Annotations-Overview" accesskey="p" rel="prev">Annotations Overview</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
</div>
<hr>
<a name="Limitations-of-the-Annotation-Interface"></a>
<h2 class="chapter">2 Limitations of the Annotation Interface</h2>

<p>The level two annotations mechanism is known to have a number of
technical and architectural limitations.  As a consequence, in 2001,
with the release of <small>GDB</small> 5.1 and the addition of <small>GDB/MI</small>,
the annotation interface was marked as deprecated.
</p>
<p>This chapter discusses the known problems.
</p>
<a name="Dependant-on-CLI-output"></a>
<h3 class="section">2.1 Dependant on <small>CLI</small> output</h3>

<p>The annotation interface works by interspersing markups with
<small>GDB</small> normal command-line interpreter output.  Unfortunately, this
makes the annotation client dependant on not just the annotations, but
also the <small>CLI</small> output.  This is because the client is forced to
assume that specific <small>GDB</small> commands provide specific information.
Any change to <small>GDB</small>&rsquo;s <small>CLI</small> output modifies or removes that
information and, consequently, likely breaks the client.
</p>
<p>Since the <small>GDB/MI</small> output is independent of the <small>CLI</small>, it does not
have this problem.
</p>
<a name="Scalability"></a>
<h3 class="section">2.2 Scalability</h3>

<p>The annotation interface relies on value annotations (see <a href="Value-Annotations.html#Value-Annotations">Value Annotations</a>) and the display mechanism as a way of obtaining up-to-date
value information.  These mechanisms are not scalable.
</p>
<p>In a graphical environment, where many values can be displayed
simultaneously, a serious performance problem occurs when the client
tries to first extract from <small>GDB</small>, and then re-display, all those
values.  The client should instead only request and update the values
that changed.
</p>
<p>The <small>GDB/MI</small> Variable Objects provide just that mechanism.
</p>
<a name="Correctness"></a>
<h3 class="section">2.3 Correctness</h3>

<p>The annotation interface assumes that a variable&rsquo;s value can only be
changed when the target is running.  This assumption is not correct.  A
single assignment to a single variable can result in the entire target,
and all displayed values, needing an update.
</p>
<p>The <small>GDB/MI</small> Variable Objects include a mechanism for efficiently
reporting such changes.
</p>
<a name="Reliability"></a>
<h3 class="section">2.4 Reliability</h3>

<p>The <small>GDB/MI</small> interface includes a dedicated test directory
(<samp>gdb/gdb.mi</samp>), and any addition or fix to <small>GDB/MI</small> must include
testsuite changes.
</p>
<a name="Maintainability"></a>
<h3 class="section">2.5 Maintainability</h3>

<p>The annotation mechanism was implemented by interspersing <small>CLI</small> print
statements with various annotations.  As a consequence, any <small>CLI</small>
output change can alter the annotation output.
</p>
<p>Since the <small>GDB/MI</small> output is independent of the <small>CLI</small>, and the
<small>GDB/MI</small> is increasingly implemented independent of the <small>CLI</small>
code, its long term maintenance is much easier.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Migrating-to-GDB_002fMI.html#Migrating-to-GDB_002fMI" accesskey="n" rel="next">Migrating to GDB/MI</a>, Previous: <a href="Annotations-Overview.html#Annotations-Overview" accesskey="p" rel="prev">Annotations Overview</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
</div>


</body>
</html>