That code works fine for me. I would suspect that you are not linking against what you think you are linking against.
How about you add the following line:
```
printf("sqlite3 libversion: %s\n", sqlite3_libversion());
```
after the line
```
printf("bytes_written=%llu %s\n",bytes_written,sql);
```
and see if that sheds any light ...
```
bytes_written=0 insert INTO mqtt6 values ('abcd',1623066863471168257,1623066863471168257,35) RETURNING (SELECT sum(size_t) from mqtt6);
sqlite3 libversion: 3.36.0
```