Specify the memory alignment of an opaque type
When the database server passes an opaque data type to a UDR, it aligns the data on a certain byte boundary. By default, the database server uses a four-byte alignment for the internal representation of an opaque type. Four bytes is the standard alignment for 32-bit platforms.
On 64-bit platforms, alignment must be eight bytes.
You can specify a different memory-alignment requirement for your opaque type with the ALIGNMENT modifier of the CREATE OPAQUE TYPE statement. The database server stores the value of the ALIGNMENT modifier in the align column of the sysxtdtypes system catalog table.
| Value for ALIGNMENT modifier | Meaning | Purpose |
|---|---|---|
| 1 | Align structure on one-byte boundary | Structures that begin with one-byte quantities |
| 2 | Align structure on two-byte boundary | Structures that begin with two-byte quantities, such as mi_unsigned_smallint |
| 4 (default) | Align structure on four-byte boundary | Structures that begin with four-byte quantities, such as mi_real or mi_unsigned_integer |
| 8 | Align structure on eight-byte boundary | Structures that contain members of the mi_double_precision data type |
Arrays of a data type must follow the same alignment restrictions as the data type itself. However, structures that begin with single-byte characters (such as mi_boolean or mi_char) can be aligned anywhere.
When you obtain aligned data for an opaque data type from
a varying-length structure, use the mi_get_vardata_align() function.
Make sure that the align argument of mi_get_vardata_align() matches
the value of the align column in the sysxtdtypes system
catalog table for the opaque type. For example, the mi_double_precision data
type is aligned on an eight-byte boundary. If an opaque type contains
an array of mi_double_precision values, use mi_get_vardata_align() with
an align value of 8 to access the data portion
of the mi_double_precision array.
opaque_type_t *buff;
mi_lvarchar *var_struc;
...
buff = (opaque_type_t *)mi_get_vardata_align(var_struc, 8);