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