为什么这个小写变大写的程序运行不了啊

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main()
{
FILE *fp;
char str[100];
int i=0;
if((fp=fopen("a1","w"))==NULL)
{
printf("can't open file!\n");
exit(0);
}
printf("input a string:\n");
gets(str);
while(str[i]!='!')
{
if(str[i]>='a'&&str[i]<='z')
str[i]=str[i]-32;
fputc(str[i],fp);
i++;
}
fclose(fp);
fp=fopen("a1","r");
fgets(str,strlen(str)+1,fp);
printf("%s\n",str);
fclose(fp);
return 0;
}

fp=fopen("a1","w")，这个a1应该是个文件，比如a1.txt