#include <math.h>

       double logb(double x);
       float logbf(float x);
       long double logbl(long double x);

       Link with -lm.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
           _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99
       logbf(), logbl():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 ||
           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99

       These functions extract the exponent from the  internal  floating-point
       representation of x and return it as a floating-point value.  The inte-
       ger constant FLT_RADIX, defined in <float.h>, indicates the radix  used
       for  the  system's  floating-point  representation.  If FLT_RADIX is 2,
       logb(x) is equal to floor(log2(x)), except that it is probably faster.

       If x is subnormal, logb() returns the exponent x would have if it  were

       On success, these functions return the exponent of x.

       If x is a NaN, a NaN is returned.

       If  x  is  zero,  then  a  pole  error occurs, and the functions return
       -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

       If x is negative infinity or positive infinity, then positive  infinity
       is returned.

       See  math_error(7) for information on how to determine whether an error
       has occurred when calling these functions.

       The following errors can occur:

       Pole error: x is 0
              A  divide-by-zero  floating-point  exception  (FE_DIVBYZERO)  is

       These functions do not set errno.

       ilogb(3), log(3)

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