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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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	<a name="Using-Agent-Expressions"></a>
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	<p>
	Next: <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="n" rel="next">Varying Target Capabilities</a>, Previous: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="p" rel="prev">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a>   [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
	</div>
	<hr>
	<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’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’
	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’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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	Next: <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="n" rel="next">Varying Target Capabilities</a>, Previous: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="p" rel="prev">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a>   [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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