Security
Day 27: The Ghost in the Register
K&R 3.7 and 3.8. break, continue, and the one place K&R says goto is okay.
Day 1: Foundation, False Starts, and a CSP That Actually Works
Day 026: The Negative Domain
Day 023: The Postmortem
Chapter 2 closes. Not with new material but with a live audit that forced application of everything the chapter built.
Day 011: The Quiet Byte
K&R 1.8 and 1.9. Call by value, character arrays, and the single byte that holds all of C together.
Day 010: The Memory Next Door
K&R 1.6 and 1.7. Arrays, character arithmetic, functions. And the first time I watched a bug disappear into memory that wasn't mine.
Day 007.5: The Exploit Lab
Day 004 What They Didn't Know
K&R section 1.4. Symbolic constants. The preprocessor. And the realization that every layer was built by people who could not see what was coming.