move filename to a fat16 image

fat16.c

int write_dir(FILE *fp, char *fname, struct fat_dir *dir){
    strncpy((char *) dir->name, (char *) fname, sizeof(fname));

    if (fwrite(dir, 1, sizeof(struct fat_dir), fp) <= 0)
        return -1;
    return 0;
}

int write_data(FILE *fp, char *fname, struct fat_dir *dir, struct fat_bpb *bpb){

    FILE *localf = fopen(fname, "r");
    int c;

    while ((c = fgetc(localf)) != EOF){
        if (fputc(c, fp) != c)
            return -1;
    }
    return 0;
}

void mv(FILE *fp, char *filename, struct fat_bpb *bpb){
    if (access(filename, F_OK) < 0)
        return;
    struct fat_dir *dirs = ls(fp, bpb); // find empty place to store file.
    int dirs_len = sizeof (struct fat_dir) * bpb->possible_rentries;

    int i;
    uint32_t data_addrs = bpb_froot_addr(bpb);
    struct fat_dir *curdir;

    for (i=0; i < dirs_len; i++){
        data_addrs += bpb->bytes_p_sect;
        if (dirs[i].name[0] == '\0'){
            curdir = &dirs[i]; /* found the first free directory entry */
            curdir->starting_cluster = i + 1;
            break;
        }
        else if (dirs[i].name[0] == DIR_FREE_ENTRY){
            curdir = &dirs[i];
            wipe(fp, curdir, bpb);
            return;
        }
    }

    int dir_addr = bpb_froot_addr(bpb) + i * 32;
    fseek(fp, dir_addr, SEEK_SET);

    off_t filesize = fsize(filename);
    if (filesize > 0){
        curdir->file_size = filesize;
    }
    else {
        return;
    }

    if (write_dir(fp, filename, curdir) < 0)
        return;
    fseek(fp, data_addrs, SEEK_SET);
    if (write_data(fp, filename, curdir, bpb) < 0)
        return;

    bpb->snumber_sect += 1;
    free(dirs);
}