Memory alignment
When the database server passes the data type to a UDR, it aligns opaque-type data on a specified byte boundary. Alignment requirements depend on the C definition of the opaque data type and on the system (hardware and compiler) on which the opaque data type is compiled.
You can specify the memory-alignment requirement for your opaque
data type with the ALIGNMENT modifier of the CREATE OPAQUE TYPE statement.
The following table summarizes valid alignment values.
| ALIGNMENT value | Meaning | Purpose |
|---|---|---|
| 1 | Align structure on single-byte boundary. | Structures that begin with 1-byte quantities |
| 2 | Align structure on 2-byte boundary. | Structures that begin with 2-byte quantities such as mi_unsigned_smallint |
| 4 | Align structure on 4-byte boundary. | Structures that begin with 4-byte quantities such as float or mi_unsigned_integer |
| 8 | Align structure on 8-byte boundary. | Structures that contain members of the C double data type |
Structures that begin with single-byte characters, char, can be aligned anywhere. Arrays of a data type should follow the same alignment restrictions as the data type itself.
For example, the following CREATE OPAQUE TYPE statement specifies
a fixed-length opaque data type, called LongLong, of 18 bytes
that must be aligned on a 1-byte boundary:
CREATE OPAQUE TYPE LongLong (INTERNALLENGTH=18, ALIGNMENT=1);If you do not include the ALIGNMENT modifier in the CREATE OPAQUE TYPE statement, the default alignment is a 4-byte boundary.