DECLARE EXTERNAL FUNCTION

BY DESCRIPTOR parameter passing
RETURNS PARAMETER n

Available in: DSQL, ESQL

Description: This statement makes an external function (UDF) known to the database.

Syntax:

DECLARE EXTERNAL FUNCTION localname
   [<type_decl> [, <type_decl> ...]]
   RETURNS {<return_type_decl> | PARAMETER 1-based_pos} [FREE_IT]
   ENTRY_POINT 'function_name' MODULE_NAME 'library_name'

<type_decl>         ::=  sqltype [BY DESCRIPTOR] | CSTRING(length)
<return_type_decl>  ::=  sqltype [BY {DESCRIPTOR|VALUE}] | CSTRING(length)

You may choose localname freely; this is the name by which the function will be known to your database. You may also vary the length argument of CSTRING parameters (more about CSTRINGs in the note near the end of the book).

BY DESCRIPTOR parameter passing

Added in: 1.0

Description: Firebird introduces the possibility to pass parameters BY DESCRIPTOR; this mechanism facilitates the processing of NULLs in a meaningful way. Notice that this only works if the person who wrote the function has implemented it. Simply adding “BY DESCRIPTOR” to an existing declaration does not make it work – on the contrary! Always use the declaration block provided by the function designer.

RETURNS PARAMETER `n`

Added in: IB 6

Description: In order to return a BLOB, an extra input parameter must be declared and a “RETURNS PARAMETER n” subclause added – n being the position of said parameter. This subclause dates back to InterBase 6 beta, but somehow didn't make it into the Language Reference (it is documented in the Developer's Guide though).