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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).