c-programming

name: c-programming description: > Expert-level C programming assistance based on modern C standards (C99/C11/C17/C23) and the pedagogical approach of "C Programming: A Modern Approach" by K.N. King. Use when the user is writing C code, debugging C programs, asking about C syntax or semantics, discussing C data types, pointers, arrays, strings, structs, unions, enums, the preprocessor, the C standard library, memory management, file I/O, bitwise operations, or program organization. Also triggers on mentions of gcc, clang, Makefile with C files, segfault debugging, undefined behavior, pointer arithmetic, malloc/free, printf/scanf formatting, header files, linkage, storage duration, translation units, or any C standard library header (stdio.h, stdlib.h, string.h, math.h, etc.). Covers the full language from fundamentals through advanced topics like function pointers, abstract data types, and low-level programming.

C Programming Expert

Language Standards

Default to C99 as baseline (widely supported, most practical). Note C23 features when relevant.

Compile with: gcc -std=c99 -Wall -Wextra -pedantic (or -std=c11, -std=c17, -std=c23).

Compilation Model

source.c  →  preprocessor  →  translation unit  →  compiler  →  object file  →  linker  →  executable
(.c)         (cpp/gcc -E)     (expanded .c)        (gcc -c)     (.o)           (gcc)      (a.out)

Each .c file is compiled independently. Headers (.h) are textually included by the preprocessor. The linker resolves cross-file references using external linkage.

Type System Quick Reference

Integer types (minimum guaranteed sizes)

All integer types exist in signed (default) and unsigned variants. Use <stdint.h> for exact-width types: int8_t, int16_t, int32_t, int64_t, uint*_t.

Floating-point types

Other types

Operator Precedence (high to low)

Common pitfall: & / ^ / | bind looser than == / !=. Always parenthesize: if ((x & MASK) == EXPECTED).

Undefined Behavior — Critical Rules

C has undefined behavior (UB) that the compiler may exploit for optimization. Key UB to avoid:

1. Signed integer overflow — use unsigned or check before operations
2. Dereferencing NULL or dangling pointers — always validate
3. Out-of-bounds array access — no runtime checks in C
4. Using uninitialized variables — always initialize
5. Double free or use-after-free — set pointers to NULL after free()
6. Violating strict aliasing — don't cast between incompatible pointer types (use memcpy or unions)
7. Modifying a variable twice between sequence points — e.g., i = i++ is UB
8. Shifting by negative or >= bit-width — 1 << 32 is UB for 32-bit int
9. Dividing by zero — check divisor
10. Misaligned pointer access — use memcpy for unaligned reads

Reference Documents

Load these as needed based on the specific topic: