C Pocket Reference

Classification of Floating-Point Numbers

ANSI C99 defines five categories of floating-point numbers, listed in Table 1-28. A symbolic constant for each category is defined in the header file math.h .

Table 1-28. Floating-point categories

Macro name	Category
FP_ZERO	Floating-point numbers with the value 0
FP_NORMAL	Floating-point numbers in normalized representation
FP_SUBNORMAL[]	Floating-point numbers in subnormal representation
FP_INFINITE	Floating-point numbers that represent an infinite value
FP_NAN	Not a Number (NAN): bit patterns that do not represent a valid floating-point number
[1] Tiny numbers may be represented in subnormal notation.

The macros in Table 1-29 can be used to classify a real floating-point number x with respect to the categories in Table 1-28 without causing an error condition.

Table 1-29. Macros for floating-point number classification

Macro	Result
fpclassify(x)	Returns one of the constants described in Table 1-28 to indicate the category to which x belongs.
isfinite(x)	Returns “true” (i. e., a value other than 0) if the value of x is finite (0, normal, subnormal, not infinite, or NAN), otherwise 0.
isinf(x)	Returns “true” if x is an infinity, otherwise 0.
isnormal(x)	Returns “true” if the value of x is a normalized floating-point number not equal to 0. Returns 0 in all other cases.
isnan(x)	Returns “true” if x is “not a number” (NaN), otherwise 0.
signbit(x)	Returns “true” if x is negative (i. e., if the sign ...