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100 lines
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<title>Agent Expressions - Debugging with GDB</title>
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<a name="Agent-Expressions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Target-Descriptions.html#Target-Descriptions">Target Descriptions</a>,
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Previous: <a rel="previous" accesskey="p" href="Remote-Protocol.html#Remote-Protocol">Remote Protocol</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<h2 class="appendix">Appendix F The GDB Agent Expression Mechanism</h2>
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<p>In some applications, it is not feasible for the debugger to interrupt
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the program's execution long enough for the developer to learn anything
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helpful about its behavior. If the program's correctness depends on its
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real-time behavior, delays introduced by a debugger might cause the
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program to fail, even when the code itself is correct. It is useful to
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be able to observe the program's behavior without interrupting it.
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<p>Using GDB's <code>trace</code> and <code>collect</code> commands, the user can
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specify locations in the program, and arbitrary expressions to evaluate
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when those locations are reached. Later, using the <code>tfind</code>
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command, she can examine the values those expressions had when the
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program hit the trace points. The expressions may also denote objects
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in memory — structures or arrays, for example — whose values GDB
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should record; while visiting a particular tracepoint, the user may
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inspect those objects as if they were in memory at that moment.
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However, because GDB records these values without interacting with the
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user, it can do so quickly and unobtrusively, hopefully not disturbing
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the program's behavior.
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<p>When GDB is debugging a remote target, the GDB <dfn>agent</dfn> code running
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on the target computes the values of the expressions itself. To avoid
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having a full symbolic expression evaluator on the agent, GDB translates
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expressions in the source language into a simpler bytecode language, and
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then sends the bytecode to the agent; the agent then executes the
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bytecode, and records the values for GDB to retrieve later.
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<p>The bytecode language is simple; there are forty-odd opcodes, the bulk
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of which are the usual vocabulary of C operands (addition, subtraction,
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shifts, and so on) and various sizes of literals and memory reference
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operations. The bytecode interpreter operates strictly on machine-level
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values — various sizes of integers and floating point numbers — and
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requires no information about types or symbols; thus, the interpreter's
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internal data structures are simple, and each bytecode requires only a
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few native machine instructions to implement it. The interpreter is
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small, and strict limits on the memory and time required to evaluate an
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expression are easy to determine, making it suitable for use by the
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debugging agent in real-time applications.
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<ul class="menu">
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<li><a accesskey="1" href="General-Bytecode-Design.html#General-Bytecode-Design">General Bytecode Design</a>: Overview of the interpreter.
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<li><a accesskey="2" href="Bytecode-Descriptions.html#Bytecode-Descriptions">Bytecode Descriptions</a>: What each one does.
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<li><a accesskey="3" href="Using-Agent-Expressions.html#Using-Agent-Expressions">Using Agent Expressions</a>: How agent expressions fit into the big picture.
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<li><a accesskey="4" href="Varying-Target-Capabilities.html#Varying-Target-Capabilities">Varying Target Capabilities</a>: How to discover what the target can do.
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<li><a accesskey="5" href="Rationale.html#Rationale">Rationale</a>: Why we did it this way.
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</ul>
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<!-- @node Rationale -->
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<!-- @section Rationale -->
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