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<title>GDB’s Obsolete Annotations: Limitations</title>
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<a name="Limitations"></a>
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<p>
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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="previous">Annotations Overview</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>]</p>
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<hr>
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<a name="Limitations-of-the-Annotation-Interface"></a>
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<h2 class="chapter">2 Limitations of the Annotation Interface</h2>
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<p>The level two annotations mechanism is known to have a number of
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technical and architectural limitations. As a consequence, in 2001,
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with the release of <small>GDB</small> 5.1 and the addition of <small>GDB/MI</small>,
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the annotation interface was marked as deprecated.
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</p>
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<p>This chapter discusses the known problems.
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</p>
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<a name="Dependant-on-CLI-output"></a>
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<h3 class="section">2.1 Dependant on <small>CLI</small> output</h3>
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<p>The annotation interface works by interspersing markups with
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<small>GDB</small> normal command-line interpreter output. Unfortunately, this
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makes the annotation client dependant on not just the annotations, but
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also the <small>CLI</small> output. This is because the client is forced to
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assume that specific <small>GDB</small> commands provide specific information.
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Any change to <small>GDB</small>’s <small>CLI</small> output modifies or removes that
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information and, consequently, likely breaks the client.
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</p>
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<p>Since the <small>GDB/MI</small> output is independent of the <small>CLI</small>, it does not
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have this problem.
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</p>
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<a name="Scalability"></a>
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<h3 class="section">2.2 Scalability</h3>
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<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
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value information. These mechanisms are not scalable.
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</p>
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<p>In a graphical environment, where many values can be displayed
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simultaneously, a serious performance problem occurs when the client
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tries to first extract from <small>GDB</small>, and then re-display, all those
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values. The client should instead only request and update the values
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that changed.
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</p>
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<p>The <small>GDB/MI</small> Variable Objects provide just that mechanism.
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</p>
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<a name="Correctness"></a>
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<h3 class="section">2.3 Correctness</h3>
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<p>The annotation interface assumes that a variable’s value can only be
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changed when the target is running. This assumption is not correct. A
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single assignment to a single variable can result in the entire target,
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and all displayed values, needing an update.
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</p>
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<p>The <small>GDB/MI</small> Variable Objects include a mechanism for efficiently
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reporting such changes.
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</p>
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<a name="Reliability"></a>
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<h3 class="section">2.4 Reliability</h3>
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<p>The <small>GDB/MI</small> interface includes a dedicated test directory
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(<samp>gdb/gdb.mi</samp>), and any addition or fix to <small>GDB/MI</small> must include
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testsuite changes.
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</p>
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<a name="Maintainability"></a>
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<h3 class="section">2.5 Maintainability</h3>
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<p>The annotation mechanism was implemented by interspersing <small>CLI</small> print
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statements with various annotations. As a consequence, any <small>CLI</small>
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output change can alter the annotation output.
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</p>
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<p>Since the <small>GDB/MI</small> output is independent of the <small>CLI</small>, and the
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<small>GDB/MI</small> is increasingly implemented independent of the <small>CLI</small>
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code, its long term maintenance is much easier.
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</p>
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<hr>
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<div class="header">
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<p>
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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="previous">Annotations Overview</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>]</p>
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