Exploring The GNU GDB Agent Expressions: A Complete Overview

GDB provides a lightweight and efficient debugging mechanism known as the GDB Agent for real-time applications. The GDB Agent has a low performance impact and does not interrupt program execution, making it ideal for debugging remote targets in real-time scenarios. This mechanism is particularly useful for debugging systems where performance is critical and has a minimal impact on the target system’s resources. With GDB Agent, developers can easily debug real-time applications and diagnose issues without affecting the program’s performance.

1- The GDB Agent

When the program’s correctness depends on its real-time behavior, It is useful to be able to observe the program’s behavior without interrupting it.

[1]

GDB allows the user to define tracepoints in their program, along with arbitrary expressions to evaluate when those tracepoints are reached. The user can later examine the values of these expressions at the tracepoints. When visiting a tracepoint, the user can inspect the recorded objects as if they were in memory at that moment. GDB collects this information unobtrusively and quickly, without disrupting the program’s behavior.

• The GDB agent expressions can refer to objects in memory, such as structures or arrays, whose values should be recorded.

2- Design of The GDB Agent

2.1 GDB Agent Expression Design Overview

The GDB agent 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.

[GDB Manual]

2.2 Minimal Overhead Design

The design of the GDB agent is kept simple and requires only few native machine instructions when evaluating the tracepoint expressions. This way, the performance overhead of the GDB agent is minimized when debugging a remote target, making it suitable for use as the debugging agent in real-time applications.

2.3 Simple Agent Expression Evaluator

When GDB is debugging a remote target, the GDB agent 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.

The bytecode interpreter operates strictly on machine-level values 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.

[1] GDB Manual

3- The GDB Agent Commands

• trace
  • The user can specify the locations in the program to be traced.
• collect
  • The user can specify expressions to evaluate when traced locations are reached.
• tfind
  • The user can examine the values the expressions had when the program hit the trace points.

References

[1] GDB Manual : Appendix F – The GDB Agent Expression Mechanism