Write idiomatic, efficient C code with best-practice patterns

C: Idiomatic Efficiency Reference is a skill of before/after C coding guidelines across seven areas: memory management, pointers and arrays, error handling, strings, structs and enums, preprocessor and headers, and a summary anti-pattern table.

Use this when writing or reviewing C code and want idiomatic, safer patterns applied consistently. It's language-specific guidance, not an architectural framework, and the skill notes over-compression can hurt readability, so judgement still applies.

Memory management: always check malloc's return value before use, use sizeof *ptr instead of sizeof(Type) so it stays correct when the type changes, avoid casting malloc's result since it hides a missing #include, null a pointer immediately after free, and use a single cleanup label with goto for multi-allocation functions instead of leaking on early-return paths. Pointers and arrays: pass array length alongside a pointer or bundle both into a struct rather than tracking size manually, prefer arr[i] indexing over raw pointer arithmetic, and avoid VLAs in production code, a stack overflow risk, in favor of heap allocation with an error check. Error handling: use named error codes via errno/perror instead of magic numbers, and use early-return or goto cleanup instead of deeply nested if blocks - goto cleanup is described as idiomatic C for resource teardown, not something to avoid on principle. Strings: replace strcpy with strncpy plus explicit null-termination or snprintf, never compare string content with ==, since that compares pointers, and avoid repeated strcat in a loop, an O(n squared) pattern, in favor of tracking a write position with snprintf. Structs and enums: typedef structs to avoid repeating the struct keyword, always zero-initialize a struct rather than leaving it uninitialized, and use named enums instead of magic integer constants for state values.

Preprocessor and headers: prefer a static inline function over a macro for logic, since macros lack type safety and can double-evaluate arguments like x++, and always use an #ifndef/#define include guard, or the widely supported but non-standard #pragma once. A closing anti-pattern table maps specific bad patterns - sprintf, gets, casting malloc's result, VLAs for large arrays, void* callbacks with no context param, global mutable state, assert used for runtime error handling, missing const on read-only pointer params, and mixing signed/unsigned comparisons - to their preferred replacements.

C developers writing or reviewing code who want a concrete before/after reference for memory safety, string handling, and idiomatic patterns instead of relying on habit or memory.