函数名: ldexp 
 功  能: 计算value*2的幂 
 用  法: #include <math.h> 
         double ldexp(double value, int exp); 
 程序例: 
#include <stdio.h> 
 #include <math.h> 
int main(void) 
 { 
    double value; 
    double x = 2; 
   /* ldexp raises 2 by a power of 3 
       then multiplies the result by 2   */ 
    value = ldexp(x,3); 
    printf("The ldexp value is: %lf\n", 
       value); 
   return 0; 
 }