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| <h3 class="section">E.3 Using Agent Expressions</h3> |
| |
| <p>Agent expressions can be used in several different ways by <span class="sc">gdb</span>, |
| and the debugger can generate different bytecode sequences as appropriate. |
| |
| <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, <span class="sc">gdb</span> compiles the source |
| expression into a bytecode sequence that simply gets values from |
| registers or memory, does arithmetic, and returns a result. |
| |
| <p>Another way to use agent expressions is for tracepoint data |
| collection. <span class="sc">gdb</span> generates a different bytecode sequence for |
| collection; in addition to bytecodes that do the calculation, |
| <span class="sc">gdb</span> adds <code>trace</code> bytecodes to save the pieces of |
| memory that were used. |
| |
| <ul> |
| <li>The user selects trace points in the program's code at which GDB should |
| collect data. |
| |
| <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>GDB transmits the tracepoints and their associated expressions to the |
| GDB agent, running on the debugging target. |
| |
| <li>The agent arranges to be notified when a trace point is hit. |
| |
| <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>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. |
| |
| </ul> |
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