logb, logbf, logbl, _logb, _logbf

Extracts the exponent value of a floating-point argument.

double logb(
   double x 
float logb(
   float x 
); // C++ only
long double logb(
   long double x 
); // C++ only 
float logbf(
   float x 
long double logbl(
   long double x 
double _logb(
   double x 
float _logbf(
   float x 


A floating-point value.

logb returns the unbiased exponent value of x as a signed integer represented as a floating-point value.

The logb functions extract the exponential value of the floating-point argument x, as though x were represented with infinite range. If the argument x is denormalized, it is treated as if it were normalized.

Because C++ allows overloading, you can call overloads of logb that take and return float or long double values. In a C program, logb always takes and returns a double.


SEH exception

Matherr exception




± 0




Required header



logb, logbf, logbl, _logbf


For more compatibility information, see Compatibility.


All versions of the C run-time libraries.

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.