15.1.2.18 round(mode)

The round attribute controls the rounding mode of C language fixed-point support (_Fract, _Accum variable types) dialect code (-menable-fixed command-line option) within a function. Specify mode as one of truncation, conventional, or convergent. This attribute overrides the default rounding mode set by -menable-fixed for C language code within the attributed function, but has no effect on functions that may be called by that function.