The fenv.h file defines macros for each of the four IEEE rounding direction modes: FE_TONEAREST, FE_UPWARD (toward positive infinity), FE_DOWNWARD (toward negative infinity), and FE_TOWARDZERO. C99 defines two functions to control rounding direction modes: fesetround sets the current rounding direction to the direction specified by its argument (which must be one of the four macros above), and fegetround returns the value of the macro corresponding to the current rounding direction.
On x86-based systems, the fenv.h file defines macros for each of three rounding precision modes: FE_FLTPREC (single precision), FE_DBLPREC (double precision), and FE_LDBLPREC (extended double precision). Although they are not part of C99, libm on x86 provides two functions to control the rounding precision mode: fesetprec sets the current rounding precision to the precision specified by its argument which must be one of the three macros above, and fegetprec returns the value of the macro corresponding to the current rounding precision.