6.16 Fixed-Point Types

As an extension, GNU C supports fixed-point types as defined in the N1169 draft of ISO/IEC DTR 18037. Support for fixed-point types in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. Not all targets support fixed-point types.

The fixed-point types are short _Fract, _Fract, long _Fract, long long _Fract, unsigned short _Fract, unsigned _Fract, unsigned long _Fract, unsigned long long _Fract, _Sat short _Fract, _Sat _Fract, _Sat long _Fract, _Sat long long _Fract, _Sat unsigned short _Fract, _Sat unsigned _Fract, _Sat unsigned long _Fract, _Sat unsigned long long _Fract, short _Accum, _Accum, long _Accum, long long _Accum, unsigned short _Accum, unsigned _Accum, unsigned long _Accum, unsigned long long _Accum, _Sat short _Accum, _Sat _Accum, _Sat long _Accum, _Sat long long _Accum, _Sat unsigned short _Accum, _Sat unsigned _Accum, _Sat unsigned long _Accum, _Sat unsigned long long _Accum.

Fixed-point data values contain fractional and optional integral parts. The format of fixed-point data varies and depends on the target machine.

Support for fixed-point types includes:

  • prefix and postfix increment and decrement operators (++, --)
  • unary arithmetic operators (+, -, !)
  • binary arithmetic operators (+, -, *, /)
  • binary shift operators (<<, >>)
  • relational operators (<, <=, >=, >)
  • equality operators (==, !=)
  • assignment operators (+=, -=, *=, /=, <<=, >>=)
  • conversions to and from integer, floating-point, or fixed-point types

Use a suffix in a fixed-point literal constant:

  • hr’ or ‘HR’ for short _Fract and _Sat short _Fract
  • r’ or ‘R’ for _Fract and _Sat _Fract
  • lr’ or ‘LR’ for long _Fract and _Sat long _Fract
  • llr’ or ‘LLR’ for long long _Fract and _Sat long long _Fract
  • uhr’ or ‘UHR’ for unsigned short _Fract and _Sat unsigned short _Fract
  • ur’ or ‘UR’ for unsigned _Fract and _Sat unsigned _Fract
  • ulr’ or ‘ULR’ for unsigned long _Fract and _Sat unsigned long _Fract
  • ullr’ or ‘ULLR’ for unsigned long long _Fract and _Sat unsigned long long _Fract
  • hk’ or ‘HK’ for short _Accum and _Sat short _Accum
  • k’ or ‘K’ for _Accum and _Sat _Accum
  • lk’ or ‘LK’ for long _Accum and _Sat long _Accum
  • llk’ or ‘LLK’ for long long _Accum and _Sat long long _Accum
  • uhk’ or ‘UHK’ for unsigned short _Accum and _Sat unsigned short _Accum
  • uk’ or ‘UK’ for unsigned _Accum and _Sat unsigned _Accum
  • ulk’ or ‘ULK’ for unsigned long _Accum and _Sat unsigned long _Accum
  • ullk’ or ‘ULLK’ for unsigned long long _Accum and _Sat unsigned long long _Accum

GCC support of fixed-point types as specified by the draft technical report is incomplete:

  • Pragmas to control overflow and rounding behaviors are not implemented.

Fixed-point types are supported by the DWARF debug information format.