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| <h4 class="subsection">13.1.4 Tracepoint Conditions</h4> |
| |
| <p><a name="index-conditional-tracepoints-704"></a><a name="index-tracepoint-conditions-705"></a> |
| The simplest sort of tracepoint collects data every time your program |
| reaches a specified place. You can also specify a <dfn>condition</dfn> for |
| a tracepoint. A condition is just a Boolean expression in your |
| programming language (see <a href="Expressions.html#Expressions">Expressions</a>). A |
| tracepoint with a condition evaluates the expression each time your |
| program reaches it, and data collection happens only if the condition |
| is true. |
| |
| <p>Tracepoint conditions can be specified when a tracepoint is set, by |
| using ‘<samp><span class="samp">if</span></samp>’ in the arguments to the <code>trace</code> command. |
| See <a href="Create-and-Delete-Tracepoints.html#Create-and-Delete-Tracepoints">Setting Tracepoints</a>. They can |
| also be set or changed at any time with the <code>condition</code> command, |
| just as with breakpoints. |
| |
| <p>Unlike breakpoint conditions, <span class="sc">gdb</span> does not actually evaluate |
| the conditional expression itself. Instead, <span class="sc">gdb</span> encodes the |
| expression into an agent expression (see <a href="Agent-Expressions.html#Agent-Expressions">Agent Expressions</a> |
| suitable for execution on the target, independently of <span class="sc">gdb</span>. |
| Global variables become raw memory locations, locals become stack |
| accesses, and so forth. |
| |
| <p>For instance, suppose you have a function that is usually called |
| frequently, but should not be called after an error has occurred. You |
| could use the following tracepoint command to collect data about calls |
| of that function that happen while the error code is propagating |
| through the program; an unconditional tracepoint could end up |
| collecting thousands of useless trace frames that you would have to |
| search through. |
| |
| <pre class="smallexample"> (gdb) <kbd>trace normal_operation if errcode > 0</kbd> |
| </pre> |
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