函数名: gcvt 
 功  能: 把浮点数转换成字符串 
 用  法: #include <stdlib.h> 
         char *gcvt(double value, int ndigit, char *buf); 
 程序例: 
#include <stdlib.h> 
 #include <stdio.h> 
int main(void) 
 { 
    char str[25]; 
    double num; 
    int sig = 5; /* significant digits */ 
   /* a regular number */ 
    num = 9.876; 
    gcvt(num, sig, str); 
    printf("string = %s\n", str); 
   /* a negative number */ 
    num = -123.4567; 
    gcvt(num, sig, str); 
    printf("string = %s\n", str); 
   /* scientific notation */ 
    num = 0.678e5; 
    gcvt(num, sig, str); 
    printf("string = %s\n", str); 
   return(0); 
 }