int get_checksum(char *filename) {
    // New prototype, if no such function already exists in standard C-libraries
    int result;           // Or char/float/whatever


    // ...


    return result;
}
int main(void) {

    char filename[] = { "foo.dat" };
    char file_url[] = { "http://example.com/foo.dat" }
    int old_checksum;     // Or char/float/whatever
    int new_checksum;     // Or char/float/whatever


    // ...


    // Now assume that old_checksum has a value from before:

    dl_file(filename, file_url);    // Some prototype for downloading the file
    if ((new_checksum = get_checksum(filename)) == -1) {
        // Badness
    }
    else {
        if (new_checksum != old_checksum) {
            old_checksum = new_checksum;
            // Parse the file
        }
        else {
            // Do nothing
        }
    }


    // ...


}

struct stat {
    dev_t     st_dev;     /* ID of device containing file */
    ino_t     st_ino;     /* inode number */
    mode_t    st_mode;    /* protection */
    nlink_t   st_nlink;   /* number of hard links */
    uid_t     st_uid;     /* user ID of owner */
    gid_t     st_gid;     /* group ID of owner */
    dev_t     st_rdev;    /* device ID (if special file) */
    off_t     st_size;    /* total size, in bytes */
    blksize_t st_blksize; /* blocksize for file system I/O */
    blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
    time_t    st_atime;   /* time of last access */
    time_t    st_mtime;   /* time of last modification */
    time_t    st_ctime;   /* time of last status change */
};

unsigned int hash(vector<char> file){
    unsigned int result;
    int *arr = (int*)file.data();

    for(int i = 0;i < file.size() / sizeof(unsigned int);i++)
        result ^= arr[i];

    return result;
}