Specifying The Subtypes

Specifying the Subtypes

The subtypes can be specified as the subtype number or as the subtype mnemonic name.Custom subtypes must be represented by negative numbers (from -1 to -32,768), or their user-defined name from the RDB$TYPES table.An attempt to declare more than one BLOB filter with the same combination of the input and output types will fail with an error.

INPUT_TYPE: clause defining the BLOB subtype of the object to be converted
OUTPUT_TYPE: clause defining the BLOB subtype of the object to be created.

Note

Mnemonic names can be defined for custom BLOB subtypes and inserted manually into the system table RDB$TYPES system table:

INSERT INTO RDB$TYPES (RDB$FIELD_NAME, RDB$TYPE, RDB$TYPE_NAME)
VALUES ('RDB$FIELD_SUB_TYPE', -33, 'MIDI');

After the transaction is committed, the mnemonic names can be used in declarations when you create new filters.

The value of the column RDB$FIELD_NAME must always be 'RDB$FIELD_SUB_TYPE'.If a mnemonic names was defined in upper case, they can be used case-insensitively and without quotation marks when a filter is declared, following the rules for other object names.

Warning

In general, the system tables are not writable by users.However, inserting custom types into RDB$TYPES is still possible if the user is an administrator, or has the system privilege CREATE_USER_TYPES.

Parameters

ENTRY_POINT: clause defining the name of the entry point (the name of the imported function) in the module.
MODULE_NAME: The clause defining the name of the module where the exported function is located.By default, modules must be located in the UDF folder of the root directory on the server.The UDFAccess parameter in firebird.conf allows editing of access restrictions to filter libraries.

Any user connected to the database can declare a BLOB filter.