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1.37 log2, log2f—base 2 logarithm

Synopsis 

     #include <math.h>
     double log2(double x);
     float log2f(float x);

Description
The log2 functions compute the base-2 logarithm of x. A domain error occurs if the argument is less than zero. A range error occurs if the argument is zero.

The Newlib implementations are not full, intrinisic calculations, but rather are derivatives based on log. (Accuracy might be slightly off from a direct calculation.) In addition to functions, they are also implemented as macros defined in math.h: 

      #define log2(x) (log (x) / _M_LN2)
      #define log2f(x) (logf (x) / (float) _M_LN2)

To use the functions instead, just undefine the macros first.

You can use the (non-ANSI) function matherr to specify error handling for these functions, indirectly through the respective log function.
Returns
The log2 functions return log base-2(x) on success. When x is zero, the returned value is -HUGE_VAL and errno is set to ERANGE. When x is negative, the returned value is NaN (not a number) and errno is set to EDOM. You can control the error behavior via matherr.
Portability
C99, POSIX, System V Interface Definition (Issue 6).