Use Conditional Breakpoints and Watchpoints in GDB

Topic Overview

Advanced GDB Breakpoints

GDB offers breakpoint mechanisms beyond simple line breaks. Conditional breakpoints, watchpoints, and catchpoints let you pause execution exactly when specific conditions are met, which saves hours of manual stepping through loops.

Breakpoint Types

Conditional breakpoints stop only when a predicate is true:

• break foo if x==5: Stop in foo only when x equals 5
• condition 1 x==10: Add a condition to breakpoint #1

Hardware watchpoints use CPU debug registers:

• watch variable: Stop when a variable is written
• rwatch variable: Stop when a variable is read

Catchpoints intercept events:

• catch syscall open: Stop on specific system calls
• catch throw: Stop when C++ exceptions are thrown

For This Exercise

You will write a program with a loop and document which GDB commands would catch specific state changes. These features are essential for debugging complex programs where the bug only triggers after thousands of iterations.

When to Use Each Type

Conditional breakpoints shine in loops that run thousands of iterations: instead of pressing continue repeatedly, you stop only when the counter hits the failing value. Watchpoints catch memory corruption the moment a variable changes, even if you do not know which line writes to it. Catchpoints on catch syscall reveal when a program opens unexpected files or spawns child processes during security reviews.

Write a C program that simulates GDB's breakpoint bookkeeping: read a breakpoint table and an execution trace from stdin, decide for each trace event whether each breakpoint would fire, and print the hit count per breakpoint.

Requirements:

• Parse breakpoint definitions: plain (b), conditional (c, condition var==value with optional @thread), and watch (w, optional condition newval==value) breakpoints
• Parse a trace of exec / write / read events: "exec ADDR TID X" (X is the loop variable's value at that stop), "write ADDR TID ACC OLD NEW", "read ADDR TID ACC"
• A plain breakpoint fires on exec at its address; a conditional one additionally checks the thread id and the variable value; a watchpoint fires only on a write to its watched address (optionally when the new value matches)
• Print "Breakpoint hit counts:" followed by "name: hits" for each breakpoint in input order