cmetal

A self-contained learning environment for advanced C, taught by fixing broken code.

C is taught everywhere, but almost always up to the point where a program compiles. The hard part of the language starts after that: undefined behavior, aliasing, the lifetime of memory, const discipline, error handling that survives a real call chain. Books explain these; almost nothing lets you practice them with immediate feedback. That is the gap cmetal fills — the way rustlings did for Rust, but for the parts of C that actually hurt.

Each exercise is a .c file with a real bug or a TODO. You open it, fix it, and save. cmetal recompiles, runs the binary — and the tests and sanitizers where the exercise calls for them — and tells you, in seconds, whether you got it right.

  Exercise: 02_memory/memory2

  ✗ AddressSanitizer: heap-use-after-free
      free(buf);
      return buf[0];   // <- read after free

  Press [h] for a hint.

Fix it, save, and the same screen turns green — and the official solution unlocks in my_solutions/ so you can compare it with yours.

The loop

1. Run cmetal init, then cmetal from the workspace it creates (a git clone of the repository works the same way). Your working copies live in my_exercises/; the originals in exercises/ stay pristine.
2. Open the exercise's .c file in your editor. Fix the bug.
3. Save. cmetal recompiles and re-verifies automatically — watch mode is the core experience.
4. Stuck? Press h for progressive hints.
5. Green? The solution unlocks. On to the next.

What it is for

• Deliberate practice on hard C. 62 exercises across 20 topics, from pointer decay to hash tables, garbage collection and bytecode, each built around a bug you will actually meet in production code. See the curriculum.
• Real toolchains, not a sandbox. gcc and clang, AddressSanitizer and UBSan — the tools you use at work. Choosing a compiler.
• A curriculum you can extend. Adding an exercise is writing a broken program, its fix, and a few hints; one script gatekeeps quality. See Contributing an exercise.

What it is not

The goal is not to teach every corner of C, but the C that appears in production systems. cmetal is not a C course from zero, not a C++ tutor, and not an IDE or build system. It assumes you know C syntax and want to get good at the parts that bite. It is a sharp tool for one job: deliberate practice on advanced C. Where that line moves next is tracked in the roadmap.

The CLI is written in Rust for a fast experience on Linux and macOS; the exercises are pure C11. (On Windows, use WSL: native toolchains lack the sanitizers the exercises rely on.)