Experiments modernising old C code

Mon, 18 May 2026 00:00:00 +0000

I wanted to see what modern compilers make of old C code: how much louder they get, what they flag, and how much work it is to clean up under a stricter, security-conscious flag set. The experiment also doubles as a test of AI coding agents on legacy modernisation. Two approaches:

Fix in place. Turn on strict warnings, have the agent resolve them. The cleanup tends to surface latent security bugs, undefined behaviour, and portability problems as a side effect.
Rewrite from scratch. Keep the original as reference, have the agent reimplement in modern C17, validate with tests.

Rewriting straight into Rust or Go would be its own experiment. Here the focus is C-to-C: how well do current models produce clean, modern C while preserving the behaviour of a 20-year-old codebase.

Gcc on Martin's Blog

Experiments modernising old C code