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 later

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>:

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

For more compatibility information, see Compatibility.

See also

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