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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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