C99 Compliance Status

shecc implements a subset of C99 suitable for self-hosting and systems programming, prioritizing simplicity, educational value, and minimal dependencies over full standard compliance. This document tracks compliance gaps and non-standard behaviors.

Implemented Features

Core Language

• Basic types: int, short, char, void, _Bool
• Structures and unions with nested definitions
• Enumerations with automatic value assignment
• Function definitions and declarations
• Arrays (single and multi-dimensional)
• Pointers and pointer arithmetic (fully C99-compliant)
• Type definitions (typedef)

Control Flow

• if/else statements
• goto and label statements
• while, do-while, for loops
• switch/case/default statements
• break, continue, return statements

Operators

• Arithmetic: +, -, *, /, %
• Bitwise: &, |, ^, ~, <<, >>
• Logical: &&, ||, !
• Relational: <, >, <=, >=, ==, !=
• Assignment: =, +=, -=, *=, /=, %=, <<=, >>=, &=, |=, ^=
• Increment/decrement: ++, -- (prefix and postfix)
• Conditional: ? :
• Member access: ., ->
• Address/dereference: &, *

Preprocessor (Partial)

• #define for object-like and function-like macros
• #ifdef, #ifndef, #if, #elif, #else, #endif
• #undef for macro removal
• #pragma once, other #pragma options will be ignored
• defined() operator
• __VA_ARGS__ for variadic macros
• __FILE__, __LINE__ built-in macros

Missing Features

Storage Classes & Qualifiers

Feature	Status	Impact
static	Not implemented	No internal linkage or persistent local variables
extern	Not implemented	No external linkage declarations
register	Not implemented	No register hint optimization
auto	Not implemented	Default storage class (implicit)
const	Parsed but ignored	No read-only enforcement
volatile	Not implemented	No volatile semantics
restrict	Not implemented	No pointer aliasing optimization
inline	Not implemented	No function inlining

Type System

Feature	Status	Notes
long	Missing	Only 4-byte integers
long long	Missing	No 64-bit integers
unsigned	Missing	All integers are signed
signed	Missing	Implicit for integers
float	Missing	No floating-point support
double	Missing	No floating-point support
long double	Missing	No floating-point support
Bit-fields	Missing	Cannot pack struct members

Literals & Constants

Feature	Status	Current Behavior
Integer suffixes (u, l, ll)	Not parsed	All literals are int
Wide characters (L'c')	Not supported	Single-byte only
Wide strings (L"...")	Not supported	Single-byte only
Multi-character constants	Not supported	Single character only
Universal characters (\u, \U)	Not supported	ASCII only
Hex escapes (\x...)	Limited	Max 2 hex digits

Preprocessor Gaps

Feature	Status	Description
#include	Partial	Local file inclusion is supported, but lack of capability to include system files
Token pasting (##)	Missing	Cannot concatenate tokens
Stringizing (#)	Missing	Cannot convert to string
__DATE__	Missing	No compile date
__TIME__	Missing	No compile time
__STDC__	Missing	No standard compliance indicator

Advanced Features

Feature	Status	Description
Designated initializers	Missing	No .field = value syntax
Compound literals	Partial	Limited support
Flexible array members	Missing	No [] at struct end
Variable-length arrays	Missing	No runtime-sized arrays
_Complex	Missing	No complex numbers
_Imaginary	Missing	No imaginary numbers
_Static_assert	Missing	No compile-time assertions
_Alignof	Missing	No alignment queries
_Alignas	Missing	No alignment specification
_Generic	Missing	No generic selection

Non-Standard Behaviors

GNU Extensions

• Binary literals: 0b101010
• Escape sequence: \e for ESC character
• void* arithmetic (treated as char*)
• sizeof(void) returns 0 (should be error)
• Computed goto

Implementation-Specific

• Array compound literals in scalar context use first element
• String literals are modifiable (stored in .data, not .rodata)
• No strict aliasing rules
• Left-to-right evaluation order (not always guaranteed in C99)