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A list of missing standard C features:

C89

• trigraphs (deliberately left out)

C99

• Proper long double support in platforms with extended floats (currently equivalent to double)
• <tgmath.h> header
• digraphs
• Universal character names (\uXXXX, \UXXXXXXXX)
• IEEE 754 float support Annex F IEC 60559 (FLT_EVAL_METHOD, FENV_ACCESS pragma) (not even GCC or Clang care about this)

Stack-allocated VLAs are a stub and actually compile to a fixed size array at the moment (but noone should use them anyway)

C11

• atomics

C23

• Decimal floating-point types (_Decimal32, _Decimal64, and _Decimal128)
• Bit-precise integers (_BitInt)
• char8_t
• Attributes ([[...]] syntax)
• Labels followed by declarations and }
• auto for type inference, typeof_unqual, constexpr
• unreachable macro in <stddef.h>
• checked int arithmetic (<stdckdint.h>)
• tagged type compatibility N3037
• storage class specifier for compound literals
• preprocessor #embed directive, __has_include, __has_c_attribute
• Pragmas for float rounding direction (STDC FENV_ROUND, STDC FENV_DEC_ROUND)
• IEEE 754 decimal floats, interchange and extended types. various <float.h> macros,...