28.5 Floating Arithmetic Exception Flags

Sticky exception flags record the occurrence of particular conditions: once set, they remain set until the program explicitly clears them.

The conditions include invalid operand, division-by_zero, inexact result (i.e., one that required rounding), underflow, and overflow. Some extended floating-point designs offer several additional exception flags. The functions feclearexcept, feraiseexcept, fetestexcept, fegetexceptflags, and fesetexceptflags provide a standardized interface to those flags. See Status bit operations in The GNU C Library Reference Manual.

One important use of those flags is to do a computation that is normally expected to be exact in floating-point arithmetic, but occasionally might not be, in which case, corrective action is needed. You can clear the inexact result flag with a call to feclearexcept (FE_INEXACT), do the computation, and then test the flag with fetestexcept (FE_INEXACT); the result of that call is 0 if the flag is not set (there was no rounding), and 1 when there was rounding (which, we presume, implies the program has to correct for that).