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| |
| <h4 class="subsection">13.1.6 Tracepoint Action Lists</h4> |
| |
| |
| <a name="index-actions-889"></a> |
| <a name="index-tracepoint-actions-890"></a> |
| <dl><dt><code>actions </code><span class="roman">[</span><var>num</var><span class="roman">]</span><dd>This command will prompt for a list of actions to be taken when the |
| tracepoint is hit. If the tracepoint number <var>num</var> is not |
| specified, this command sets the actions for the one that was most |
| recently defined (so that you can define a tracepoint and then say |
| <code>actions</code> without bothering about its number). You specify the |
| actions themselves on the following lines, one action at a time, and |
| terminate the actions list with a line containing just <code>end</code>. So |
| far, the only defined actions are <code>collect</code>, <code>teval</code>, and |
| <code>while-stepping</code>. |
| |
| <p><code>actions</code> is actually equivalent to <code>commands</code> (see <a href="Break-Commands.html#Break-Commands">Breakpoint Command Lists</a>), except that only the defined |
| actions are allowed; any other <span class="sc">gdb</span> command is rejected. |
| |
| <p><a name="index-remove-actions-from-a-tracepoint-891"></a>To remove all actions from a tracepoint, type ‘<samp><span class="samp">actions </span><var>num</var></samp>’ |
| and follow it immediately with ‘<samp><span class="samp">end</span></samp>’. |
| |
| <pre class="smallexample"> (gdb) <b>collect </b><var>data</var> // collect some data |
| |
| (gdb) <b>while-stepping 5</b> // single-step 5 times, collect data |
| |
| (gdb) <b>end</b> // signals the end of actions. |
| </pre> |
| <p>In the following example, the action list begins with <code>collect</code> |
| commands indicating the things to be collected when the tracepoint is |
| hit. Then, in order to single-step and collect additional data |
| following the tracepoint, a <code>while-stepping</code> command is used, |
| followed by the list of things to be collected after each step in a |
| sequence of single steps. The <code>while-stepping</code> command is |
| terminated by its own separate <code>end</code> command. Lastly, the action |
| list is terminated by an <code>end</code> command. |
| |
| <pre class="smallexample"> (gdb) <b>trace foo</b> |
| (gdb) <b>actions</b> |
| Enter actions for tracepoint 1, one per line: |
| > collect bar,baz |
| > collect $regs |
| > while-stepping 12 |
| > collect $pc, arr[i] |
| > end |
| end |
| </pre> |
| <p><a name="index-collect-_0040r_007b_0028tracepoints_0029_007d-892"></a><br><dt><code>collect</code><span class="roman">[</span><code>/</code><var>mods</var><span class="roman">]</span> <var>expr1</var><code>, </code><var>expr2</var><code>, ...</code><dd>Collect values of the given expressions when the tracepoint is hit. |
| This command accepts a comma-separated list of any valid expressions. |
| In addition to global, static, or local variables, the following |
| special arguments are supported: |
| |
| <dl> |
| <dt><code>$regs</code><dd>Collect all registers. |
| |
| <br><dt><code>$args</code><dd>Collect all function arguments. |
| |
| <br><dt><code>$locals</code><dd>Collect all local variables. |
| |
| <br><dt><code>$_ret</code><dd>Collect the return address. This is helpful if you want to see more |
| of a backtrace. |
| |
| <p><em>Note:</em> The return address location can not always be reliably |
| determined up front, and the wrong address / registers may end up |
| collected instead. On some architectures the reliability is higher |
| for tracepoints at function entry, while on others it's the opposite. |
| When this happens, backtracing will stop because the return address is |
| found unavailable (unless another collect rule happened to match it). |
| |
| <br><dt><code>$_probe_argc</code><dd>Collects the number of arguments from the static probe at which the |
| tracepoint is located. |
| See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>. |
| |
| <br><dt><code>$_probe_arg</code><var>n</var><dd><var>n</var> is an integer between 0 and 11. Collects the <var>n</var>th argument |
| from the static probe at which the tracepoint is located. |
| See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>. |
| |
| <br><dt><code>$_sdata</code><dd><a name="index-g_t_0024_005fsdata_0040r_007b_002c-collect_007d-893"></a>Collect static tracepoint marker specific data. Only available for |
| static tracepoints. See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>. On the UST static tracepoints library backend, an |
| instrumentation point resembles a <code>printf</code> function call. The |
| tracing library is able to collect user specified data formatted to a |
| character string using the format provided by the programmer that |
| instrumented the program. Other backends have similar mechanisms. |
| Here's an example of a UST marker call: |
| |
| <pre class="smallexample"> const char master_name[] = "$your_name"; |
| trace_mark(channel1, marker1, "hello %s", master_name) |
| </pre> |
| <p>In this case, collecting <code>$_sdata</code> collects the string |
| ‘<samp><span class="samp">hello $yourname</span></samp>’. When analyzing the trace buffer, you can |
| inspect ‘<samp><span class="samp">$_sdata</span></samp>’ like any other variable available to |
| <span class="sc">gdb</span>. |
| </dl> |
| |
| <p>You can give several consecutive <code>collect</code> commands, each one |
| with a single argument, or one <code>collect</code> command with several |
| arguments separated by commas; the effect is the same. |
| |
| <p>The optional <var>mods</var> changes the usual handling of the arguments. |
| <code>s</code> requests that pointers to chars be handled as strings, in |
| particular collecting the contents of the memory being pointed at, up |
| to the first zero. The upper bound is by default the value of the |
| <code>print elements</code> variable; if <code>s</code> is followed by a decimal |
| number, that is the upper bound instead. So for instance |
| ‘<samp><span class="samp">collect/s25 mystr</span></samp>’ collects as many as 25 characters at |
| ‘<samp><span class="samp">mystr</span></samp>’. |
| |
| <p>The command <code>info scope</code> (see <a href="Symbols.html#Symbols">info scope</a>) is |
| particularly useful for figuring out what data to collect. |
| |
| <p><a name="index-teval-_0040r_007b_0028tracepoints_0029_007d-894"></a><br><dt><code>teval </code><var>expr1</var><code>, </code><var>expr2</var><code>, ...</code><dd>Evaluate the given expressions when the tracepoint is hit. This |
| command accepts a comma-separated list of expressions. The results |
| are discarded, so this is mainly useful for assigning values to trace |
| state variables (see <a href="Trace-State-Variables.html#Trace-State-Variables">Trace State Variables</a>) without adding those |
| values to the trace buffer, as would be the case if the <code>collect</code> |
| action were used. |
| |
| <p><a name="index-while_002dstepping-_0040r_007b_0028tracepoints_0029_007d-895"></a><br><dt><code>while-stepping </code><var>n</var><dd>Perform <var>n</var> single-step instruction traces after the tracepoint, |
| collecting new data after each step. The <code>while-stepping</code> |
| command is followed by the list of what to collect while stepping |
| (followed by its own <code>end</code> command): |
| |
| <pre class="smallexample"> > while-stepping 12 |
| > collect $regs, myglobal |
| > end |
| > |
| </pre> |
| <p class="noindent">Note that <code>$pc</code> is not automatically collected by |
| <code>while-stepping</code>; you need to explicitly collect that register if |
| you need it. You may abbreviate <code>while-stepping</code> as <code>ws</code> or |
| <code>stepping</code>. |
| |
| <br><dt><code>set default-collect </code><var>expr1</var><code>, </code><var>expr2</var><code>, ...</code><dd><a name="index-set-default_002dcollect-896"></a><a name="index-default-collection-action-897"></a>This variable is a list of expressions to collect at each tracepoint |
| hit. It is effectively an additional <code>collect</code> action prepended |
| to every tracepoint action list. The expressions are parsed |
| individually for each tracepoint, so for instance a variable named |
| <code>xyz</code> may be interpreted as a global for one tracepoint, and a |
| local for another, as appropriate to the tracepoint's location. |
| |
| <br><dt><code>show default-collect</code><dd><a name="index-show-default_002dcollect-898"></a>Show the list of expressions that are collected by default at each |
| tracepoint hit. |
| |
| </dl> |
| |
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