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| <h4 class="subsection">13.1.9 Starting and Stopping Trace Experiments</h4> |
| |
| |
| <a name="index-tstart-726"></a> |
| <a name="index-start-a-new-trace-experiment-727"></a> |
| <a name="index-collected-data-discarded-728"></a> |
| <dl><dt><code>tstart</code><dd>This command takes no arguments. It starts the trace experiment, and |
| begins collecting data. This has the side effect of discarding all |
| the data collected in the trace buffer during the previous trace |
| experiment. |
| |
| <p><a name="index-tstop-729"></a><a name="index-stop-a-running-trace-experiment-730"></a><br><dt><code>tstop</code><dd>This command takes no arguments. It ends the trace experiment, and |
| stops collecting data. |
| |
| <p><strong>Note</strong>: a trace experiment and data collection may stop |
| automatically if any tracepoint's passcount is reached |
| (see <a href="Tracepoint-Passcounts.html#Tracepoint-Passcounts">Tracepoint Passcounts</a>), or if the trace buffer becomes full. |
| |
| <p><a name="index-tstatus-731"></a><a name="index-status-of-trace-data-collection-732"></a><a name="index-trace-experiment_002c-status-of-733"></a><br><dt><code>tstatus</code><dd>This command displays the status of the current trace data |
| collection. |
| </dl> |
| |
| <p>Here is an example of the commands we described so far: |
| |
| <pre class="smallexample"> (gdb) <b>trace gdb_c_test</b> |
| (gdb) <b>actions</b> |
| Enter actions for tracepoint #1, one per line. |
| > collect $regs,$locals,$args |
| > while-stepping 11 |
| > collect $regs |
| > end |
| > end |
| (gdb) <b>tstart</b> |
| [time passes ...] |
| (gdb) <b>tstop</b> |
| </pre> |
| <p><a name="index-disconnected-tracing-734"></a>You can choose to continue running the trace experiment even if |
| <span class="sc">gdb</span> disconnects from the target, voluntarily or |
| involuntarily. For commands such as <code>detach</code>, the debugger will |
| ask what you want to do with the trace. But for unexpected |
| terminations (<span class="sc">gdb</span> crash, network outage), it would be |
| unfortunate to lose hard-won trace data, so the variable |
| <code>disconnected-tracing</code> lets you decide whether the trace should |
| continue running without <span class="sc">gdb</span>. |
| |
| <dl> |
| <dt><code>set disconnected-tracing on</code><dt><code>set disconnected-tracing off</code><dd><a name="index-set-disconnected_002dtracing-735"></a>Choose whether a tracing run should continue to run if <span class="sc">gdb</span> |
| has disconnected from the target. Note that <code>detach</code> or |
| <code>quit</code> will ask you directly what to do about a running trace no |
| matter what this variable's setting, so the variable is mainly useful |
| for handling unexpected situations, such as loss of the network. |
| |
| <br><dt><code>show disconnected-tracing</code><dd><a name="index-show-disconnected_002dtracing-736"></a>Show the current choice for disconnected tracing. |
| |
| </dl> |
| |
| <p>When you reconnect to the target, the trace experiment may or may not |
| still be running; it might have filled the trace buffer in the |
| meantime, or stopped for one of the other reasons. If it is running, |
| it will continue after reconnection. |
| |
| <p>Upon reconnection, the target will upload information about the |
| tracepoints in effect. <span class="sc">gdb</span> will then compare that |
| information to the set of tracepoints currently defined, and attempt |
| to match them up, allowing for the possibility that the numbers may |
| have changed due to creation and deletion in the meantime. If one of |
| the target's tracepoints does not match any in <span class="sc">gdb</span>, the |
| debugger will create a new tracepoint, so that you have a number with |
| which to specify that tracepoint. This matching-up process is |
| necessarily heuristic, and it may result in useless tracepoints being |
| created; you may simply delete them if they are of no use. |
| |
| <p><a name="index-circular-trace-buffer-737"></a>If your target agent supports a <dfn>circular trace buffer</dfn>, then you |
| can run a trace experiment indefinitely without filling the trace |
| buffer; when space runs out, the agent deletes already-collected trace |
| frames, oldest first, until there is enough room to continue |
| collecting. This is especially useful if your tracepoints are being |
| hit too often, and your trace gets terminated prematurely because the |
| buffer is full. To ask for a circular trace buffer, simply set |
| ‘<samp><span class="samp">circular_trace_buffer</span></samp>’ to on. You can set this at any time, |
| including during tracing; if the agent can do it, it will change |
| buffer handling on the fly, otherwise it will not take effect until |
| the next run. |
| |
| <dl> |
| <dt><code>set circular-trace-buffer on</code><dt><code>set circular-trace-buffer off</code><dd><a name="index-set-circular_002dtrace_002dbuffer-738"></a>Choose whether a tracing run should use a linear or circular buffer |
| for trace data. A linear buffer will not lose any trace data, but may |
| fill up prematurely, while a circular buffer will discard old trace |
| data, but it will have always room for the latest tracepoint hits. |
| |
| <br><dt><code>show circular-trace-buffer</code><dd><a name="index-show-circular_002dtrace_002dbuffer-739"></a>Show the current choice for the trace buffer. Note that this may not |
| match the agent's current buffer handling, nor is it guaranteed to |
| match the setting that might have been in effect during a past run, |
| for instance if you are looking at frames from a trace file. |
| |
| </dl> |
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