NaNs and infinities
The floating point system supports two special representations:

Infinity Positive infinity in a format compares greater than all
other representable numbers in the same format.
Arithmetic operations on infinities are quite intuitive.
For example,
adding any representable number to infinity is a valid operation the result
of which is positive infinity.
Subtracting positive infinity from
itself is invalid.
If some arithmetic operation overflows, and
the overflow trap is disabled, in some rounding modes the
result is infinity.

NotaNumber (NaN)
These floating point representations are not numbers.
They can be used to carry diagnostic information.
There are two kinds of NaNs: signaling NaNs and quiet NaNs.
Signaling NaNs raise the invalid operation exception whenever they are
used as operands in floating point operations.
Quiet NaNs propagate
through most operations without raising any exception.
The result of these operations is the same quiet NaN.
NaNs are sometimes produced
by the arithmetic operations themselves.
For example, 0.0 divided by 0.0, when
the invalid operation trap is disabled, produces a quiet NaN.
The header file ieeefp.h defines the
interface for the floating point exception and environment control.
This header defines three interfaces:

Rounding Control

Exception Control

Exception Handling
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Rounding control
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Specialcase values
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SCO OpenServer Release 6.0.0  02 June 2005