What im trying to do is take in an input txt file filled with thousands of lines of hexcimals values and convert them to long long ints.

Example:

```
0x7f1a91026b00
0x7f1a91026b03
0x7f1a91027130
0x7f1a91027131
0x7f1a91027134
0x7f1a91027136
0x7f1a91027138
0x7f1a9102713a
0x7f1a9102713c

```

but im reading them as strings, how do i convert these to a type long long int? Ive been told i can use the strtoull() function but im not understanding how it works. This is what i have currently:

```
int main (int argc, char **argv){

    if(argc!=2){
        printf("error\n");
        return 0;
        // if there is no input then print an error
    }

    FILE *file = fopen(argv[1], "r");

    char linestring[BUFSIZ];
    char **ret;
    while (fgets(linestring, sizeof(linestring), file)) // reads the entire file until it hits Null
    {

    strtoul(linestring, ret, 16);
    printf("%s\n", ret);
    printf("%s\n", linestring);

    }

    fclose(file);
}

```

This isn't working obviously but could someone please explain to me what i should do?