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<title>Limitations - GDB's Obsolete Annotations</title>
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<a name="Limitations"></a>
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Next: <a rel="next" accesskey="n" href="Migrating-to-GDB_002fMI.html#Migrating-to-GDB_002fMI">Migrating to GDB/MI</a>,
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Previous: <a rel="previous" accesskey="p" href="Annotations-Overview.html#Annotations-Overview">Annotations Overview</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</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 <span class="sc">gdb</span> 5.1 and the addition of <span class="sc">gdb/mi</span>,
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the annotation interface was marked as deprecated.
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<p>This chapter discusses the known problems.
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<h3 class="section">2.1 Dependant on <span class="sc">cli</span> output</h3>
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<p>The annotation interface works by interspersing markups with
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<span class="sc">gdb</span> 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 <span class="sc">cli</span> output. This is because the client is forced to
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assume that specific <span class="sc">gdb</span> commands provide specific information.
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Any change to <span class="sc">gdb</span>'s <span class="sc">cli</span> output modifies or removes that
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information and, consequently, likely breaks the client.
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<p>Since the <span class="sc">gdb/mi</span> output is independent of the <span class="sc">cli</span>, it does not
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have this problem.
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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>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 <span class="sc">gdb</span>, 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>The <span class="sc">gdb/mi</span> Variable Objects provide just that mechanism.
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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>The <span class="sc">gdb/mi</span> Variable Objects include a mechanism for efficiently
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reporting such changes.
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<h3 class="section">2.4 Reliability</h3>
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<p>The <span class="sc">gdb/mi</span> interface includes a dedicated test directory
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(<samp><span class="file">gdb/gdb.mi</span></samp>), and any addition or fix to <span class="sc">gdb/mi</span> must include
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testsuite changes.
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<h3 class="section">2.5 Maintainability</h3>
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<p>The annotation mechanism was implemented by interspersing <span class="sc">cli</span> print
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statements with various annotations. As a consequence, any <span class="sc">cli</span>
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output change can alter the annotation output.
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<p>Since the <span class="sc">gdb/mi</span> output is independent of the <span class="sc">cli</span>, and the
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<span class="sc">gdb/mi</span> is increasingly implemented independent of the <span class="sc">cli</span>
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code, its long term maintenance is much easier.
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