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: