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<title>Debugging with GDB: Using Agent Expressions</title>
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<a name="Using-Agent-Expressions"></a>
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<p>
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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="previous">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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<a name="Using-Agent-Expressions-1"></a>
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<h3 class="section">F.3 Using Agent Expressions</h3>
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<p>Agent expressions can be used in several different ways by <small>GDB</small>,
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and the debugger can generate different bytecode sequences as appropriate.
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</p>
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<p>One possibility is to do expression evaluation on the target rather
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than the host, such as for the conditional of a conditional
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tracepoint. In such a case, <small>GDB</small> compiles the source
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expression into a bytecode sequence that simply gets values from
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registers or memory, does arithmetic, and returns a result.
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</p>
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<p>Another way to use agent expressions is for tracepoint data
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collection. <small>GDB</small> generates a different bytecode sequence for
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collection; in addition to bytecodes that do the calculation,
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<small>GDB</small> adds <code>trace</code> bytecodes to save the pieces of
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memory that were used.
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</p>
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<ul>
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<li> The user selects trace points in the program’s code at which GDB should
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collect data.
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</li><li> The user specifies expressions to evaluate at each trace point. These
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expressions may denote objects in memory, in which case those objects’
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contents are recorded as the program runs, or computed values, in which
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case the values themselves are recorded.
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</li><li> GDB transmits the tracepoints and their associated expressions to the
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GDB agent, running on the debugging target.
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</li><li> The agent arranges to be notified when a trace point is hit.
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</li><li> When execution on the target reaches a trace point, the agent evaluates
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the expressions associated with that trace point, and records the
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resulting values and memory ranges.
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</li><li> Later, when the user selects a given trace event and inspects the
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objects and expression values recorded, GDB talks to the agent to
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retrieve recorded data as necessary to meet the user’s requests. If the
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user asks to see an object whose contents have not been recorded, GDB
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reports an error.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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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="previous">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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