5.10. EXTERNAL FUNCTION

External functions, also known as User-Defined Functions (UDFs) are programs written in an external programming language and stored in dynamically loaded libraries. Once declared in a database, they become available in dynamic and procedural statements as though they were implemented in the SQL language.

External functions extend the possibilities for processing data with SQL considerably. To make a function available to a database, it is declared using the statement DECLARE EXTERNAL FUNCTION.

The library containing a function is loaded when any function included in it is called.

ⓘ
Note

External functions declared as DECLARE EXTERNAL FUNCTION are a legacy from previous versions of Firebird. Their capabilities are inferior to the capabilities of the new type of external functions, UDR (User-Defined Routine). Such functions are declared as CREATE FUNCTION …​ EXTERNAL …​. See CREATE FUNCTION for details.

ⓘ
Note

External functions may be contained in more than one library — or module, as it is referred to in the syntax.

⚠
Caution

UDFs are fundamentally insecure. We recommend avoiding their use whenever possible, and disabling UDFs in your database configuration (UdfAccess = None in firebird.conf). If you do need to call native code from your database, use a UDR external engine instead.

See alsoFUNCTION

5.10.1. DECLARE EXTERNAL FUNCTION

Used forDeclaring a user-defined function (UDF) to the database

Available inDSQL, ESQL

Syntax 

   |DECLARE EXTERNAL FUNCTION funcname
   |  [{ <arg_desc_list> | ( <arg_desc_list> ) }]
   |  RETURNS { <return_value> | ( <return_value> ) }
   |  ENTRY_POINT 'entry_point' MODULE_NAME 'library_name'
   | 
   |<arg_desc_list> ::=
   |  <arg_type_decl> [, <arg_type_decl> ...]
   | 
   |<arg_type_decl> ::=
   |  <udf_data_type> [BY {DESCRIPTOR | SCALAR_ARRAY} | NULL]
   | 
   |<udf_data_type> ::=
   |    <scalar_datatype>
   |  | BLOB
   |  | CSTRING(length) [ CHARACTER SET charset ]
   | 
   |<scalar_datatype> ::=
   |  !! See Scalar Data Types Syntax !!
   | 
   |<return_value> ::=
   |  { <udf_data_type> | PARAMETER param_num }
   |  [{ BY VALUE | BY DESCRIPTOR [FREE_IT] | FREE_IT }]

Table 5.27. DECLARE EXTERNAL FUNCTION Statement Parameters
ParameterDescription

funcname

Function name in the database. It may consist of up to 31 characters. It should be unique among all internal and external function names in the database and need not be the same name as the name exported from the UDF library via ENTRY_POINT.

entry_point

The exported name of the function

library_name

The name of the module (MODULE_NAME) from which the function is exported. This will be the name of the file, without the .dll or .so file extension.

length

The maximum length of a null-terminated string, specified in bytes

charset

Character set of the CSTRING

param_num

The number of the input parameter, numbered from 1 in the list of input parameters in the declaration, describing the data type that will be returned by the function

The DECLARE EXTERNAL FUNCTION statement makes a user-defined function available in the database. UDF declarations must be made in each database that is going to use them. There is no need to declare UDFs that will never be used.

The name of the external function must be unique among all function names. It may be different from the exported name of the function, as specified in the ENTRY_POINT argument.

5.10.1.1. DECLARE EXTERNAL FUNCTION Input Parameters

The input parameters of the function follow the name of the function and are separated with commas. Each parameter has an SQL data type specified for it. Arrays cannot be used as function parameters. In addition to the SQL types, the CSTRING type is available for specifying a null-terminated string with a maximum length of LENGTH bytes. There are several mechanisms for passing a parameter from the Firebird engine to an external function, each of these mechanisms will be discussed below.

By default, input parameters are passed by reference. There is no separate clause to explicitly indicate that parameters are passed by reference.

When passing a NULL value by reference, it is converted to the equivalent of zero, for example, a number 0 or an empty string (''). If the keyword NULL is specified after a parameter, then with passing a NULL values, the null pointer will be passed to the external function.

ⓘ
Note

Declaring a function with the NULL keyword does not guarantee that the function will correctly handle a NULL input parameter. Any function must be written or rewritten to correctly handle NULL values. Always use the function declaration as provided by its developer.

If BY DESCRIPTOR is specified, then the input parameter is passed by descriptor. In this case, the UDF parameter will receive a pointer to an internal structure known as a descriptor. The descriptor contains information about the datatype, subtype, precision, character set and collation, scale, a pointer to the data itself and some flags, including the NULL indicator. This declaration only works if the external function is written using a handle.

🛑
Warning

When passing a function parameter by descriptor, the passed value is not cast to the declared data type.

The BY SCALAR_ARRAY clause is used when passing arrays as input parameters. Unlike other types, you cannot return an array from a UDF.

5.10.1.1.1. Clauses and Keywords
RETURNS clause

(Required) specifies the output parameter returned by the function. A function is scalar, it returns one value (output parameter). The output parameter can be of any SQL type (except an array or an array element) or a null-terminated string (CSTRING). The output parameter can be passed by reference (the default), by descriptor or by value. If the BY DESCRIPTOR clause is specified, the output parameter is passed by descriptor. If the BY VALUE clause is specified, the output parameter is passed by value.

PARAMETER keyword

specifies that the function returns the value from the parameter under number param_num. It is necessary if you need to return a value of data type BLOB.

FREE_IT keyword

means that the memory allocated for storing the return value will be freed after the function is executed. It is used only if the memory was allocated dynamically in the UDF. In such a UDF, the memory must be allocated with the help of the ib_util_malloc function from the ib_util module, a requirement for compatibility with the functions used in Firebird code and in the code of the shipped UDF modules, for allocating and freeing memory.

ENTRY_POINT clause

specifies the name of the entry point (the name of the imported function), as exported from the module.

MODULE_NAME clause

defines the name of the module where the exported function is located. The link to the module should not be the full path and extension of the file, if that can be avoided. If the module is located in the default location (in the ../UDF subdirectory of the Firebird server root) or in a location explicitly configured in firebird.conf, it makes it easier to move the database between different platforms. The UDFAccess parameter in the firebird.conf file allows access restrictions to external functions modules to be configured.

Any user connected to the database can declare an external function (UDF).

5.10.1.2. Who Can Create an External Function

The DECLARE EXTERNAL FUNCTION statement can be executed by:

The user who created the function becomes its owner.

5.10.1.3. Examples using DECLARE EXTERNAL FUNCTION

  1. Declaring the addDay external function located in the fbudf module. The input and output parameters are passed by reference.

      |DECLARE EXTERNAL FUNCTION addDay
  |  TIMESTAMP, INT
  |  RETURNS TIMESTAMP
  |  ENTRY_POINT 'addDay' MODULE_NAME 'fbudf';

  2. Declaring the invl external function located in the fbudf module. The input and output parameters are passed by descriptor.

      |DECLARE EXTERNAL FUNCTION invl
  |  INT BY DESCRIPTOR, INT BY DESCRIPTOR
  |  RETURNS INT BY DESCRIPTOR
  |  ENTRY_POINT 'idNvl' MODULE_NAME 'fbudf';

  3. Declaring the isLeapYear external function located in the fbudf module. The input parameter is passed by reference, while the output parameter is passed by value.

      |DECLARE EXTERNAL FUNCTION isLeapYear
  |  TIMESTAMP
  |  RETURNS INT BY VALUE
  |  ENTRY_POINT 'isLeapYear' MODULE_NAME 'fbudf';

  4. Declaring the i64Truncate external function located in the fbudf module. The input and output parameters are passed by descriptor. The second parameter of the function is used as the return value.

      |DECLARE EXTERNAL FUNCTION i64Truncate
  |  NUMERIC(18) BY DESCRIPTOR, NUMERIC(18) BY DESCRIPTOR
  |  RETURNS PARAMETER 2
  |  ENTRY_POINT 'fbtruncate' MODULE_NAME 'fbudf';

See alsoSection 5.10.2, “ALTER EXTERNAL FUNCTION”, Section 5.10.3, “DROP EXTERNAL FUNCTION”, CREATE FUNCTION

5.10.2. ALTER EXTERNAL FUNCTION

Used forChanging the entry point and/or the module name for a user-defined function (UDF)

Available inDSQL

Syntax 

  |ALTER EXTERNAL FUNCTION funcname
  |  [ENTRY_POINT 'new_entry_point']
  |  [MODULE_NAME 'new_library_name']

Table 5.28. ALTER EXTERNAL FUNCTION Statement Parameters
ParameterDescription

funcname

Function name in the database

new_entry_point

The new exported name of the function

new_library_name

The new name of the module (MODULE_NAME from which the function is exported). This will be the name of the file, without the .dll or .so file extension.

The ALTER EXTERNAL FUNCTION statement changes the entry point and/or the module name for a user-defined function (UDF). Existing dependencies remain intact after the statement containing the change(s) is executed.

The ENTRY_POINT clause

is for specifying the new entry point (the name of the function as exported from the module).

The MODULE_NAME clause

is for specifying the new name of the module where the exported function is located.

Any user connected to the database can change the entry point and the module name.

5.10.2.1. Who Can Alter an External Function

The ALTER EXTERNAL FUNCTION statement can be executed by:

  • Administrators

  • Owner of the external function

  • Users with the ALTER ANY FUNCTION privilege

5.10.2.2. Examples using ALTER EXTERNAL FUNCTION

Changing the entry point for an external function 

  |ALTER EXTERNAL FUNCTION invl ENTRY_POINT 'intNvl';

Changing the module name for an external function 

  |ALTER EXTERNAL FUNCTION invl MODULE_NAME 'fbudf2';

See alsoSection 5.10.1, “DECLARE EXTERNAL FUNCTION”, Section 5.10.3, “DROP EXTERNAL FUNCTION”

5.10.3. DROP EXTERNAL FUNCTION

Used forRemoving a user-defined function (UDF) from a database

Available inDSQL, ESQL

Syntax 

  |DROP EXTERNAL FUNCTION funcname

Table 5.29. DROP EXTERNAL FUNCTION Statement Parameter
ParameterDescription

funcname

Function name in the database

The DROP EXTERNAL FUNCTION statement deletes the declaration of a user-defined function from the database. If there are any dependencies on the external function, the statement will fail and the appropriate error will be raised.

Any user connected to the database can delete the declaration of an internal function.

5.10.3.1. Who Can Drop an External Function

The DROP EXTERNAL FUNCTION statement can be executed by:

  • Administrators

  • Owner of the external function

  • Users with the DROP ANY FUNCTION privilege

5.10.3.2. Example using DROP EXTERNAL FUNCTION

Deleting the declaration of the addDay function. 

  |DROP EXTERNAL FUNCTION addDay;

See alsoSection 5.10.1, “DECLARE EXTERNAL FUNCTION”