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__attribute__s

GNU-like attribute syntax. Implemented:

• aligned(alignment), packed
• noreturn (treated as C11 _Noreturn)
• mode

Other attributes for correctness hints/diagnostics are recognized (format, nonnull, etc) but discarded.

__builtin functions

• variadic arguments: __builtin_va_start/copy/end/arg, and __builtin_va_list type
• __builtin_trap, __builtin_unreachable
• __builtin_bswap16/32/64
• __builtin_constant_p
• __builtin_expect
• __builtin_alloca
• __builtin_huge_val*/inf*/nan*/nans*
• __builtin_isinf, __builtin_isnan
• __builtin_ffs*/clz*/ctz*/clrsb*/popcount*/parity*

Other GNU C language extensions

• Statement expressions ({ ... })
• __typeof__, __alignof__
• __asm__ for symbol renaming
• zero-sized arrays, empty structs/unions
• returning void expression in void function
• elvis operator a ?: b
• forward-declared enums
• __FUNCTION__, __PRETTY_FUNCTION__ as synonyms for __func__
• applying _Alignof to an expression
• some constant-foldable expressions in contexts where integer constant expressions are required
• '\e' escape sequence
• Various alternate keywords: __const, __inline, etc

Preprocessor extensions

• #include_next
• #pragma once, #warning
• #pragma GCC diagnostic/#pragma clang diagnostic (stub)
• __has_include/__has_include_next
• __has_attribute/__has_builtin
• __COUNTER__, __VA_OPT__, ##__VA_ARGS__ comma swallowing
• named variadic macro arguments #define mac(args...)
• alternate #line directives: #line-number filename
• embedding preprocessor directives within macro arguments