ilogb, ilogbf, ilogbl

Article
06/05/2017

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The latest version of this topic can be found at ilogb, ilogbf, ilogbl2.

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  
);

Parameters

[in] 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, ilogb always takes and returns a double.

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>

For additional compatibility information, see Compatibility.

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