isnan(3m) [sunos man page]

```isnan(3M)						  Mathematical Library Functions						 isnan(3M)

NAME
isnan - test for NaN

SYNOPSIS
C90 Mode
cc [ flag... ] file... -lm [ library... ]
#include <math.h>

int isnan(double x);

C99 Mode
#include <math.h>

int isnan(real--floating x);

DESCRIPTION
In C90 mode, the isnan() function tests whether x is NaN.

In  C99	mode,  the  isnan()  macro determines whether its argument value is NaN. First, an argument represented in a format wider than its
semantic type is converted to its semantic type. The determination is then based on the type of the argument.

RETURN VALUES
Both the isnan() function and macro return non-zero if and only if x is NaN.

ERRORS
No errors are defined.

WARNINGS
In C99 mode, the practice of explicitly supplying a prototype for isnan() after the line

#include <math.h>

is obsolete and will no longer work.

ATTRIBUTES
See attributes(5) for descriptions of the following attributes:

+-----------------------------+-----------------------------+
|      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
+-----------------------------+-----------------------------+
|Interface Stability	     |Standard			   |
+-----------------------------+-----------------------------+
|MT-Level		     |MT-Safe			   |
+-----------------------------+-----------------------------+

fpclassify(3M), isfinite(3M), isinf(3M), isnormal(3M), math.h(3HEAD), signbit(3M), attributes(5), standards(5)

SunOS 5.10							    1 Dec 2003								 isnan(3M)```

Check Out this Related Man Page

```FPCLASSIFY(3)						     Linux Programmer's Manual						     FPCLASSIFY(3)

NAME
fpclassify, isfinite, isnormal, isnan, isinf - floating-point classification macros

SYNOPSIS
#include <math.h>

int fpclassify(x);

int isfinite(x);

int isnormal(x);

int isnan(x);

int isinf(x);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

fpclassify(), isfinite(), isnormal():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
isnan():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
isinf():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
Floating  point	numbers  can  have special values, such as infinite or NaN.  With the macro fpclassify(x) you can find out what type x is.
The macro takes any floating-point expression as argument.  The result is one of the following values:

FP_NAN	     x is "Not a Number".

FP_INFINITE   x is either positive infinity or negative infinity.

FP_ZERO	     x is zero.

FP_SUBNORMAL  x is too small to be represented in normalized format.

FP_NORMAL     if nothing of the above is correct then it must be a normal floating-point number.

The other macros provide a short answer to some standard questions.

isfinite(x)   returns a nonzero value if
(fpclassify(x) != FP_NAN && fpclassify(x) != FP_INFINITE)

isnormal(x)   returns a nonzero value if (fpclassify(x) == FP_NORMAL)

isnan(x)      returns a nonzero value if (fpclassify(x) == FP_NAN)

isinf(x)      returns 1 if x is positive infinity, and -1 if x is negative infinity.

ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).

+-----------------------------+---------------+---------+
|Interface		     | Attribute     | Value   |
+-----------------------------+---------------+---------+
|fpclassify(), isfinite(),    | Thread safety | MT-Safe |
|isnormal(), isnan(), isinf() |		     |	       |
+-----------------------------+---------------+---------+
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C99.

For isinf(), the standards merely say that the return value is nonzero if and only if the argument has an infinite value.

NOTES
In glibc 2.01 and earlier, isinf() returns a nonzero value (actually: 1) if x is positive infinity or negative infinity.  (This is all that
C99 requires.)