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FPCLASSIFY(3) Library Functions Manual FPCLASSIFY(3)

fpclassify, isfinite, isinf, isnan, isnormal, signbit, finite, finitef, isinff, isnanf
classify a floating-point number

#include <math.h>
int
fpclassify(real-floating x);
int
isfinite(real-floating x);
int
isinf(real-floating x);
int
isnan(real-floating x);
int
isnormal(real-floating x);
int
signbit(real-floating x);

The fpclassify() macro takes an argument of x and returns one of the following manifest constants:
Indicates that x is an infinite number.
Indicates that x is not a number (NaN).
Indicates that x is a normalized number.
Indicates that x is a denormalized number.
Indicates that x is zero (0 or -0).
The isfinite() macro returns a non-zero value if and only if its argument has a finite (zero, subnormal, or normal) value.
The isinf(), isnan(), and isnormal() macros return non-zero if and only if x is an infinity, NaN, or a non-zero normalized number, respectively.
The signbit() macro takes an argument of x and returns non-zero if the value of its sign is negative, otherwise 0.

isgreater(3)

The fpclassify(), isfinite(), isinf(), isnan(), isnormal(), and signbit() macros conform to ISO/IEC 9899:1999 (“ISO C99”).
The symbols isinff(), and isnanf() are provided as compatibility aliases to isinf(), and isnan(), respectively, and their uses are deprecated. Similarly, finite() and finitef() are deprecated versions of isfinite().

The fpclassify(), isfinite(), isinf(), isnan(), isnormal(), and signbit() macros were added in OpenBSD 4.4. 3BSD introduced isinf() and isnan() functions, which accepted double arguments; these have been superseded by the macros described above.
March 10, 2016 OpenBSD-current