ilogb, ilogbf, ilogbl

Artikel
08/03/2021
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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, return the base-2 exponent of x as a signed int value.

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

Input	Result
±0	FP_ILOGB0
±inf, ±nan, indefinite	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 additional compatibility information, see Compatibility.