94 lines
4.2 KiB
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94 lines
4.2 KiB
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<title>Using Agent Expressions - Debugging with GDB</title>
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<a name="Using-Agent-Expressions"></a>
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Next: <a rel="next" accesskey="n" href="Varying-Target-Capabilities.html#Varying-Target-Capabilities">Varying Target Capabilities</a>,
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Previous: <a rel="previous" accesskey="p" href="Bytecode-Descriptions.html#Bytecode-Descriptions">Bytecode Descriptions</a>,
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Up: <a rel="up" accesskey="u" href="Agent-Expressions.html#Agent-Expressions">Agent Expressions</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 <span class="sc">gdb</span>,
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and the debugger can generate different bytecode sequences as appropriate.
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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, <span class="sc">gdb</span> 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>Another way to use agent expressions is for tracepoint data
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collection. <span class="sc">gdb</span> generates a different bytecode sequence for
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collection; in addition to bytecodes that do the calculation,
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<span class="sc">gdb</span> adds <code>trace</code> bytecodes to save the pieces of
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memory that were used.
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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>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>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>The agent arranges to be notified when a trace point is hit.
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<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>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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</ul>
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</body></html>
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