Next: Floating Point Errors, Previous: Floating Point Numbers, Up: Arithmetic

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
xis “Not a Number” (see Infinity and NaN)`FP_INFINITE`

- The value of
xis either plus or minus infinity (see Infinity and NaN)`FP_ZERO`

- The value of
xis zero. In floating-point formats like IEEE 754, where zero can be signed, this value is also returned ifxis 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 ofxin 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

xis finite: not plus or minus infinity, and not NaN. It is equivalent to(fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE)

`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

xis finite and normalized. It is equivalent to(fpclassify (x) == FP_NORMAL)

— Macro: int **isnan** (*float-type*` x`)

This macro returns a nonzero value if

xis 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: intisinff(float x)

— Function: intisinfl(long double x)

This function returns

`-1`

ifxrepresents negative infinity,`1`

ifxrepresents positive infinity, and`0`

otherwise.

— Function: int **isnan** (`double x`)

— Function: intisnanf(float x)

— Function: intisnanl(long double x)

This function returns a nonzero value if

xis 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: intfinitef(float x)

— Function: intfinitel(long double x)

This function returns a nonzero value if

xis finite or a “not a number” value, and zero otherwise.

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