C Patterns

C Patterns

This skill focuses on safe C: explicit ownership, explicit errors, predictable control flow, and contracts that survive refactors.

If you’re writing tests, use c-testing.

When to activate

• Writing/refactoring C modules and APIs
• Reviewing for memory safety and integer safety
• Designing error-handling conventions and cleanup paths
• Hardening code for sanitizer/static analysis adoption
• Porting C code across GCC/Clang/MSVC/MinGW

Core rules (high signal)

• Make ownership explicit: allocate/free at the same abstraction level.
• Use a consistent error strategy (return codes, out-params) and document it in headers.
• Avoid in-band error indicators when possible.
• Use a single-exit cleanup path for functions managing multiple resources.
• Treat integer conversions and size calculations as potential bugs.

Recommended workflow

Phase 1 — Define API contract first

Before coding:

• Define ownership of every pointer parameter (borrowed, owned, transferred).
• Define preconditions/postconditions in the header, not only in comments inside .c.
• Define error domain (enum or code space) and whether errno is used.

Phase 2 — Implement with explicit lifetimes

• Use create/destroy, init/deinit, or open/close pairs consistently.
• Keep one cleanup exit path for multi-resource functions.
• Do not mix allocator families (malloc/free vs custom pool vs Win32 heaps).

Phase 3 — Harden integer and bounds logic

• Use size_t for sizes/counts and guard all multiplication/addition used in allocation.
• Validate all signed/unsigned conversions and indexing arithmetic.
• Reject impossible sizes early (n == 0, n > MAX_ALLOWED, overflow guards).

Phase 4 — Concurrency boundaries

• Prefer immutable data or message passing where possible.
• If locks are needed, document lock ownership and lock order in header comments.
• Use atomics for single-value state; mutexes for compound invariants.

Phase 5 — Toolchain-aware portability

• Keep warning set strict on all compilers (GCC/Clang/MSVC).
• Avoid UB-dependent behavior and compiler-specific assumptions.
• Guard compiler-specific attributes/macros with feature checks.

Quick review checklist

• Every pointer has clear ownership semantics in the API.
• Every non-trivial function has deterministic cleanup path.
• Every allocation arithmetic path has overflow guard.
• Every error path returns actionable status.
• Concurrency primitives protect explicitly documented data.
• Code builds clean with strict warnings on at least two toolchains.

Resources

Load on demand:

• references/error-handling.md — error policy, return codes, cleanup patterns
• references/memory-ownership.md — allocation/free rules, zero-length alloc, free-null
• references/integers.md — overflow, size_t usage, bounds checking
• references/concurrency.md — pthreads, C11 atomics, Win32 threads, race detection
• references/tooling.md — warnings, sanitizers, static analysis, binary inspection (MinGW/objdump, MSVC/dumpbin, Linux tools)