题目：写一个函数，求一个字符串的长度，在main函数中输入字符串，并输出其长度。
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1.程序分析：
2.程序源代码：
main()
{
int len;
char *str20;
printf("please input a string:\n");
scanf("%s",str);
len=length(str);
printf("the string has %d characters.",len);
}
length(p)
char *p;
{
int n;
n=0;
while(*p!='\0')
{
n++;
p++;
}
return n;
}