How to write UDF's in Delphi?

It's quite simple, the only thing you need to remember is that you must always use ib_util_malloc() to allocate memory if your UDF returns string result. The UDF must be declared as FREE_IT, so that Firebird releases the memory after it reads the string.

To use ib_util_malloc(), you need to import it from ib_util.dll into your program - and make sure you use it instead of regular memory alocating functions. Here's a simple example of Delphi UDF:


function ib_util_malloc(l: integer): pointer; cdecl; external 'ib_util.dll';

function ChangeMyString(const p: PChar): PChar; cdecl;
var
s: string;
begin
s := DoSomething(string(p));
Result := ib_util_malloc(Length(s) + 1);
StrPCopy(Result, s);
end;


Declaration in Firebird:

DECLARE EXTERNAL FUNCTION ChangeMyString
CString(255)
RETURNS CString(255) FREE_IT
ENTRY_POINT 'ChangeMyString' MODULE_NAME '......'


For newer versions of Delphi, the code is a little bit different. For example, for Delphi XE3 with 64bit Firebird, you can use something like this:


function ib_util_malloc(l: integer): pointer; cdecl; external 'ib_util.dll';

function ChangeMyString(const p: PAnsiChar): PAnsiChar; cdecl;
var
s: string;
begin
s := DoSomething(string(p));
Result := ib_util_malloc(Length(s) + 1);
StrPCopy(Result, AnsiString(s));
end;