ilogb-ilogbf-ilogbl2.md

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ilogb, ilogbf, ilogbl2	API reference for ilogb, ilogbf, and ilogbl2; which retrieve an integer that represents the unbiased base-2 exponent of the specified value.	9/1/2020	ilogb ilogbf ilogbl	msvcrt.dll msvcr80.dll msvcr90.dll msvcr100.dll msvcr100_clr0400.dll msvcr110.dll msvcr110_clr0400.dll msvcr120.dll msvcr120_clr0400.dll ucrtbase.dll api-ms-win-crt-math-l1-1-0.dll	DLLExport	apiref	ilogb ilogbf ilogbl math/ilogb math/ilogbf math/ilogbl	ilogb function ilogbf function ilogbl function	9ef19d57-1caa-41d5-8233-2faad3562fcb

ilogb, ilogbf, ilogbl

Retrieves an integer that represents the unbiased base-2 exponent of the specified value.

Syntax

int ilogb(
   double x
);

int ilogb(
   float x
); //C++ only

int ilogb(
   long double x
); //C++ only

int ilogbf(
   float x
);

int ilogbl(
   long double x
);

#define ilogbl(X) // Requires C11 or higher

Parameters

x
The specified value.

Return value

If successful, these functions return the base-2 exponent of x as a signed int value.

Otherwise, the functions return one of the following values, defined in <math.h>:

Input	Result
±0	FP_ILOGB0
± INF, ± NAN, IND	FP_ILOGBNAN

Errors are reported as specified in _matherr.

Remarks

Because C++ allows overloading, you can call overloads of ilogb that take and return float and long double types. In a C program, unless you're using the <tgmath.h> macro to call this function, ilogb always takes and returns a double.

If you use the <tgmath.h> ilogb() macro, the type of the argument determines which version of the function is selected. See Type-generic math for details.

Calling this function is similar to calling the equivalent logb function, then casting the return value to int.

Requirements

Routine	C header	C++ header
ilogb, ilogbf, ilogbl	<math.h>	<cmath>
ilogb macro	<tgmath.h>

For more compatibility information, see Compatibility.

See also

Alphabetical function reference
frexp
logb, logbf, logbl, _logb, _logbf