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<a name="Using-Agent-Expressions-1"></a>
<h3 class="section">F.3 Using Agent Expressions</h3>

<p>Agent expressions can be used in several different ways by <small>GDB</small>,
and the debugger can generate different bytecode sequences as appropriate.
</p>
<p>One possibility is to do expression evaluation on the target rather
than the host, such as for the conditional of a conditional
tracepoint.  In such a case, <small>GDB</small> compiles the source
expression into a bytecode sequence that simply gets values from
registers or memory, does arithmetic, and returns a result.
</p>
<p>Another way to use agent expressions is for tracepoint data
collection.  <small>GDB</small> generates a different bytecode sequence for
collection; in addition to bytecodes that do the calculation,
<small>GDB</small> adds <code>trace</code> bytecodes to save the pieces of
memory that were used.
</p>
<ul>
<li> The user selects trace points in the program&rsquo;s code at which GDB should
collect data.

</li><li> The user specifies expressions to evaluate at each trace point.  These
expressions may denote objects in memory, in which case those objects&rsquo;
contents are recorded as the program runs, or computed values, in which
case the values themselves are recorded.

</li><li> GDB transmits the tracepoints and their associated expressions to the
GDB agent, running on the debugging target.

</li><li> The agent arranges to be notified when a trace point is hit.

</li><li> When execution on the target reaches a trace point, the agent evaluates
the expressions associated with that trace point, and records the
resulting values and memory ranges.

</li><li> Later, when the user selects a given trace event and inspects the
objects and expression values recorded, GDB talks to the agent to
retrieve recorded data as necessary to meet the user&rsquo;s requests.  If the
user asks to see an object whose contents have not been recorded, GDB
reports an error.

</li></ul>


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