194 lines
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194 lines
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<html lang="en">
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<head>
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<title>Tracepoint Actions - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Set-Tracepoints.html#Set-Tracepoints" title="Set Tracepoints">
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<link rel="prev" href="Trace-State-Variables.html#Trace-State-Variables" title="Trace State Variables">
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<link rel="next" href="Listing-Tracepoints.html#Listing-Tracepoints" title="Listing Tracepoints">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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</head>
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<body>
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<div class="node">
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<a name="Tracepoint-Actions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Listing-Tracepoints.html#Listing-Tracepoints">Listing Tracepoints</a>,
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Previous: <a rel="previous" accesskey="p" href="Trace-State-Variables.html#Trace-State-Variables">Trace State Variables</a>,
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Up: <a rel="up" accesskey="u" href="Set-Tracepoints.html#Set-Tracepoints">Set Tracepoints</a>
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<hr>
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</div>
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<h4 class="subsection">13.1.6 Tracepoint Action Lists</h4>
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<a name="index-actions-889"></a>
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<a name="index-tracepoint-actions-890"></a>
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<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
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tracepoint is hit. If the tracepoint number <var>num</var> is not
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specified, this command sets the actions for the one that was most
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recently defined (so that you can define a tracepoint and then say
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<code>actions</code> without bothering about its number). You specify the
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actions themselves on the following lines, one action at a time, and
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terminate the actions list with a line containing just <code>end</code>. So
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far, the only defined actions are <code>collect</code>, <code>teval</code>, and
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<code>while-stepping</code>.
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<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
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actions are allowed; any other <span class="sc">gdb</span> command is rejected.
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<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>’
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and follow it immediately with ‘<samp><span class="samp">end</span></samp>’.
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<pre class="smallexample"> (gdb) <b>collect </b><var>data</var> // collect some data
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(gdb) <b>while-stepping 5</b> // single-step 5 times, collect data
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(gdb) <b>end</b> // signals the end of actions.
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</pre>
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<p>In the following example, the action list begins with <code>collect</code>
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commands indicating the things to be collected when the tracepoint is
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hit. Then, in order to single-step and collect additional data
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following the tracepoint, a <code>while-stepping</code> command is used,
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followed by the list of things to be collected after each step in a
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sequence of single steps. The <code>while-stepping</code> command is
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terminated by its own separate <code>end</code> command. Lastly, the action
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list is terminated by an <code>end</code> command.
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<pre class="smallexample"> (gdb) <b>trace foo</b>
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(gdb) <b>actions</b>
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Enter actions for tracepoint 1, one per line:
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> collect bar,baz
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> collect $regs
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> while-stepping 12
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> collect $pc, arr[i]
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> end
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end
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</pre>
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<p><a name="index-collect-_0040r_007b_0028tracepoints_0029_007d-892"></a><br><dt><code>collect</code><span class="roman">[</span><code>/</code><var>mods</var><span class="roman">]</span> <var>expr1</var><code>, </code><var>expr2</var><code>, ...</code><dd>Collect values of the given expressions when the tracepoint is hit.
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This command accepts a comma-separated list of any valid expressions.
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In addition to global, static, or local variables, the following
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special arguments are supported:
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<dl>
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<dt><code>$regs</code><dd>Collect all registers.
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<br><dt><code>$args</code><dd>Collect all function arguments.
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<br><dt><code>$locals</code><dd>Collect all local variables.
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<br><dt><code>$_ret</code><dd>Collect the return address. This is helpful if you want to see more
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of a backtrace.
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<p><em>Note:</em> The return address location can not always be reliably
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determined up front, and the wrong address / registers may end up
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collected instead. On some architectures the reliability is higher
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for tracepoints at function entry, while on others it's the opposite.
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When this happens, backtracing will stop because the return address is
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found unavailable (unless another collect rule happened to match it).
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<br><dt><code>$_probe_argc</code><dd>Collects the number of arguments from the static probe at which the
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tracepoint is located.
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See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>.
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<br><dt><code>$_probe_arg</code><var>n</var><dd><var>n</var> is an integer between 0 and 11. Collects the <var>n</var>th argument
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from the static probe at which the tracepoint is located.
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See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>.
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<br><dt><code>$_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
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static tracepoints. See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>. On the UST static tracepoints library backend, an
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instrumentation point resembles a <code>printf</code> function call. The
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tracing library is able to collect user specified data formatted to a
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character string using the format provided by the programmer that
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instrumented the program. Other backends have similar mechanisms.
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Here's an example of a UST marker call:
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<pre class="smallexample"> const char master_name[] = "$your_name";
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trace_mark(channel1, marker1, "hello %s", master_name)
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</pre>
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<p>In this case, collecting <code>$_sdata</code> collects the string
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‘<samp><span class="samp">hello $yourname</span></samp>’. When analyzing the trace buffer, you can
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inspect ‘<samp><span class="samp">$_sdata</span></samp>’ like any other variable available to
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<span class="sc">gdb</span>.
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</dl>
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<p>You can give several consecutive <code>collect</code> commands, each one
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with a single argument, or one <code>collect</code> command with several
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arguments separated by commas; the effect is the same.
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<p>The optional <var>mods</var> changes the usual handling of the arguments.
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<code>s</code> requests that pointers to chars be handled as strings, in
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particular collecting the contents of the memory being pointed at, up
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to the first zero. The upper bound is by default the value of the
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<code>print elements</code> variable; if <code>s</code> is followed by a decimal
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number, that is the upper bound instead. So for instance
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‘<samp><span class="samp">collect/s25 mystr</span></samp>’ collects as many as 25 characters at
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‘<samp><span class="samp">mystr</span></samp>’.
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<p>The command <code>info scope</code> (see <a href="Symbols.html#Symbols">info scope</a>) is
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particularly useful for figuring out what data to collect.
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<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
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command accepts a comma-separated list of expressions. The results
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are discarded, so this is mainly useful for assigning values to trace
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state variables (see <a href="Trace-State-Variables.html#Trace-State-Variables">Trace State Variables</a>) without adding those
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values to the trace buffer, as would be the case if the <code>collect</code>
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action were used.
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<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,
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collecting new data after each step. The <code>while-stepping</code>
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command is followed by the list of what to collect while stepping
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(followed by its own <code>end</code> command):
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<pre class="smallexample"> > while-stepping 12
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> collect $regs, myglobal
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> end
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>
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</pre>
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<p class="noindent">Note that <code>$pc</code> is not automatically collected by
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<code>while-stepping</code>; you need to explicitly collect that register if
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you need it. You may abbreviate <code>while-stepping</code> as <code>ws</code> or
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<code>stepping</code>.
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<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
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hit. It is effectively an additional <code>collect</code> action prepended
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to every tracepoint action list. The expressions are parsed
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individually for each tracepoint, so for instance a variable named
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<code>xyz</code> may be interpreted as a global for one tracepoint, and a
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local for another, as appropriate to the tracepoint's location.
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<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
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tracepoint hit.
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</dl>
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</body></html>
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