Fixed-length opaque data type
If the C structure that holds your opaque type is always the same size, regardless of the data it holds, you can declare the opaque type as a fixed-length opaque type.
You tell the database server that an opaque type is fixed length when you register the opaque type. In the CREATE OPAQUE TYPE statement, you must include the INTERNALLENGTH modifier to specify the fixed size of the C structure. The database server stores the value of the INTERNALLENGTH modifier in the length column of the sysxtdtypes system catalog table.
The circle_t C structure (which Figure 1 defines)
is a fixed-length structure because all of its member fields have
a constant size. Therefore, the following CREATE OPAQUE TYPE statement
registers a fixed-length opaque type named circle for the circle_t structure:
CREATE OPAQUE TYPE circle (INTERNALLENGTH = 24);The size of a fixed-length opaque data type must match
the value that the C-language sizeof directive returns for
the C structure. On most compilers, the sizeof directive performs
cast promotion to the nearest four-byte size to ensure that the pointer
match on arrays of structures works correctly. However, you do not
need to round up for the size of a fixed-length opaque data type.
Instead you can specify alignment for the opaque data type with the
ALIGNMENT modifier. For more information, see Specify the memory alignment of an opaque type.
Important: The routine manager does perform cast promotion on
argument values smaller than the size of the MI_DATUM data
type when it pushes routine arguments onto the stack. On some platforms,
small values can create problems with pointer matching. For more information,
see Push arguments onto the thread stack.
The size of the fixed-length opaque type determines the passing mechanism for the opaque type. For more information, see Determine the passing mechanism for an opaque type.
You can obtain information about support functions for
the circle fixed-length opaque type in Write opaque-type support functions. The following table
lists the circle support functions that this section declares.