关于C的有趣问题

//以下程序是实现小写转大写程序
#include<stdio.h>
void to_upper(char *str)
{
    for(;*str !='\0';str++)
    {
        if(unsigned(*str-'a')<='z'-'a')
            *str-='a'-'A';//小写转成大写
    }
}
void main()
{
    char *str="Are you ready?";
    to_upper(str);
    printf("%s\n",str);
}
以上程序在编译和连接都通过了，但却得不出结果！！
char *str="Are you ready?";对应的汇编
004010A8   mov         dword ptr [ebp-4],offset string "Are you ready?" (00422020)
改正为：
#include<stdio.h>
void to_upper(char *str)
{
    for(;*str !='\0';str++)
    {
        if(unsigned(*str-'a')<='z'-'a')
            *str-='a'-'A';//小写转成大写
    }
}
void main()
{
    char str[]="Are you ready?";
    to_upper(str);
    printf("%s\n",str);
}
输出：ARE YOU READY？
char str[]="Are you ready?"对应的汇编：
004010A8   mov         eax,[string "Are you ready?" (00422020)]
004010AD   mov         dword ptr [ebp-10h],eax
004010B0   mov         ecx,dword ptr [string "Are you ready?"+4 (00422024)]
004010B6   mov         dword ptr [ebp-0Ch],ecx
004010B9   mov         edx,dword ptr [string "Are you ready?"+8 (00422028)]
004010BF   mov         dword ptr [ebp-8],edx
004010C2