I read the input file , which has 911 lines , and want to copy these 911 lines into the output file , after making a few changes to each line..

I am getting segmentation fault when i run this.. I dont know why.. can anyone please help..

```
#include<stdio.h>
void main()
{
    int i;
    FILE *fin,*fop;char* str;
    fin=fopen("atk561011.txt","r");
    if(fin=NULL) printf("ip err");
    fop=fopen("svmip.txt","w");
    if(fop=NULL) printf("op err");
    for(i=1;i<=911;i++)
    {
        fgets(str,150,fin);
        if((i>300&&i<=360)||(i>600&&i<=660)) 
            str[7]='1';
        else 
            str[7]='0';
        fputs(str+7,fop);
        putc('\n',fop);
    }
    fclose(fin);
    fclose(fop);
}

```