Debugger Discipline

The hypothesis loop

1. Reproduce reliably (same input, same failure).
2. Localize with the largest step that keeps the failure: binary search over the pipeline.
3. Hypothesize one mechanism ("the index is stale after the erase").
4. Experiment: breakpoint/watchpoint that distinguishes hypothesis A from B.
5. Confirm, fix, and add the regression test that would have caught it.

printf-debugging is a for-loop over guesses; the debugger is a binary search. Both are tools, but only one scales to heisenbugs.

The moves that matter

• Breakpoints — stop at a line/function. Conditional breakpoints (i == 999) skip 998 useless stops.
• Watchpoints — break when a variable changes: the killer feature for "who corrupts this field?" (watch -l s->count).
• Stepping — step into, next over, finish out. Stepping through library code is noise; step your code, skip the standard library.
• Backtrace + frame inspection — after a crash, bt lists the stack; frame 0 is where it died, frames up are why.

Post-mortem state of mind

A crash dump is a recording of the past. The skill is reading it: which frame is library-internal (skim), which is yours (study), which values are plausible vs corrupted (distrust pointers that look like small integers).