gdb: Using Agent Expressions
F.3 Using Agent Expressions
===========================
Agent expressions can be used in several different ways by GDB, and the
debugger can generate different bytecode sequences as appropriate.
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, GDB compiles the source expression into a bytecode
sequence that simply gets values from registers or memory, does
arithmetic, and returns a result.
Another way to use agent expressions is for tracepoint data
collection. GDB generates a different bytecode sequence for collection;
in addition to bytecodes that do the calculation, GDB adds 'trace'
bytecodes to save the pieces of memory that were used.
* The user selects trace points in the program's code at which GDB
should collect data.
* 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.
* GDB transmits the tracepoints and their associated expressions to
the GDB agent, running on the debugging target.
* The agent arranges to be notified when a trace point is hit.
* 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.
* 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.
Info Catalog
gdb: Bytecode Descriptions
gdb: Agent Expressions
gdb: Varying Target Capabilities