The last parameter better to be declared as pointer to STRING:

DECLARE INTEGER myfunction IN myfile.dll;
	INTEGER Interfacenum, INTEGER DeviceNum,;
	STRING @buffer

LOCAL DataBuf
DataBuf = REPLICATE(Chr(0), 16)
Result = myfunction (0, 0, @DataBuf)

After function returns, for sure, the buffer will be stuffed with proper values. Treat characters as bytes.