double frexp(double);

Mantissa m. in *n is the exponent.

compile in linux with: gcc frexp.c -o frexp -lm -Wall

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <math.h> /* including math library */
 
int main ( void ) 
{
  double x;
  int n;
 
  x = 42.0;
  printf ("%lf * 2^%d = %f\n", frexp (x , &n), n, x);
  return 0;
}

 output:
 user@host:~$ ./frexp 
 0.656250 * 2^0 = 42.000000