// 加筆部分はこれでいいよ
unsigned char write_block_data[17];
char str[sizeof(write_block_data + 1];
fprintf(stdout, "...");
fgets(str, sizeof(str), stdin);
char * p = strchr(str, '\n');
if (p) * p = '\0';
sprintf(write_block_data, "%16.16s", str);
// 最後の一行コピペすんのめんどくせ