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,
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
- 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.