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<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 <span class="sc">gdb</span> 5.1 and the addition of <span class="sc">gdb/mi</span>,
the annotation interface was marked as deprecated.

   <p>This chapter discusses the known problems.

<h3 class="section">2.1 Dependant on <span class="sc">cli</span> output</h3>

<p>The annotation interface works by interspersing markups with
<span class="sc">gdb</span> normal command-line interpreter output.  Unfortunately, this
makes the annotation client dependant on not just the annotations, but
also the <span class="sc">cli</span> output.  This is because the client is forced to
assume that specific <span class="sc">gdb</span> commands provide specific information. 
Any change to <span class="sc">gdb</span>'s <span class="sc">cli</span> output modifies or removes that
information and, consequently, likely breaks the client.

   <p>Since the <span class="sc">gdb/mi</span> output is independent of the <span class="sc">cli</span>, it does not
have this problem.

<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>In a graphical environment, where many values can be displayed
simultaneously, a serious performance problem occurs when the client
tries to first extract from <span class="sc">gdb</span>, and then re-display, all those
values.  The client should instead only request and update the values
that changed.

   <p>The <span class="sc">gdb/mi</span> Variable Objects provide just that mechanism.

<h3 class="section">2.3 Correctness</h3>

<p>The annotation interface assumes that a variable'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>The <span class="sc">gdb/mi</span> Variable Objects include a mechanism for efficiently
reporting such changes.

<h3 class="section">2.4 Reliability</h3>

<p>The <span class="sc">gdb/mi</span> interface includes a dedicated test directory
(<samp><span class="file">gdb/gdb.mi</span></samp>), and any addition or fix to <span class="sc">gdb/mi</span> must include
testsuite changes.

<h3 class="section">2.5 Maintainability</h3>

<p>The annotation mechanism was implemented by interspersing <span class="sc">cli</span> print
statements with various annotations.  As a consequence, any <span class="sc">cli</span>
output change can alter the annotation output.

   <p>Since the <span class="sc">gdb/mi</span> output is independent of the <span class="sc">cli</span>, and the
<span class="sc">gdb/mi</span> is increasingly implemented independent of the <span class="sc">cli</span>
code, its long term maintenance is much easier.

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