20.4 Floating-Point Number Classification Functions

ISO C99 defines macros that let you determine what sort of
floating-point number a variable holds.

— Macro: int fpclassify (float-type x)

This is a generic macro which works on all floating-point types and
which returns a value of type int. The possible values are:

FP_NAN

The floating-point number x is “Not a Number” (see Infinity and NaN)

FP_INFINITE

The value of x is either plus or minus infinity (see Infinity and NaN)

FP_ZERO

The value of x is zero. In floating-point formats like IEEE 754, where zero can be signed, this value is also returned if
x is negative zero.

FP_SUBNORMAL

Numbers whose absolute value is too small to be represented in the
normal format are represented in an alternate, denormalized format
(see Floating Point Concepts). This format is less precise but can
represent values closer to zero. fpclassify returns this value
for values of x in this alternate format.

FP_NORMAL

This value is returned for all other values of x. It indicates
that there is nothing special about the number.

fpclassify is most useful if more than one property of a number
must be tested. There are more specific macros which only test one
property at a time. Generally these macros execute faster than
fpclassify, since there is special hardware support for them.
You should therefore use the specific macros whenever possible.

— Macro: int isfinite (float-type x)

This macro returns a nonzero value if x is finite: not plus or
minus infinity, and not NaN. It is equivalent to

isfinite is implemented as a macro which accepts any
floating-point type.

— Macro: int isnormal (float-type x)

This macro returns a nonzero value if x is finite and normalized.
It is equivalent to

(fpclassify (x) == FP_NORMAL)

— Macro: int isnan (float-type x)

This macro returns a nonzero value if x is NaN. It is equivalent
to

(fpclassify (x) == FP_NAN)

Another set of floating-point classification functions was provided by
BSD. The GNU C library also supports these functions; however, we
recommend that you use the ISO C99 macros in new code. Those are standard
and will be available more widely. Also, since they are macros, you do
not have to worry about the type of their argument.

— Function: int isinf (double x)

— Function: int isinff (float x)

— Function: int isinfl (long double x)

This function returns -1 if x represents negative infinity,
1 if x represents positive infinity, and 0 otherwise.

— Function: int isnan (double x)

— Function: int isnanf (float x)

— Function: int isnanl (long double x)

This function returns a nonzero value if x is a “not a number”
value, and zero otherwise.

Note: The isnan macro defined by ISO C99 overrides
the BSD function. This is normally not a problem, because the two
routines behave identically. However, if you really need to get the BSD
function for some reason, you can write

(isnan) (x)

— Function: int finite (double x)

— Function: int finitef (float x)

— Function: int finitel (long double x)

This function returns a nonzero value if x is finite or a “not a
number” value, and zero otherwise.

Portability Note: The functions listed in this section are BSD
extensions.

Published under the terms of the GNU General Public License