Here is a sketch of a full non-stop debugging cycle, showing how agent
expressions fit into the process.
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. Note that,
on some systems, the target operating system is completely responsible
for collecting the data; refer to Section E.5 Tracing on Symmetrix.
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.