Firebird crashes after some time of using my UDF?

You are probably allocating memory and not freeing it. Firebird knows nothing about memory allocations in Delphi or whatever tool you might be using. If your UDF returns string result, you must always use ib_util_malloc() to allocate memory for it. 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. Delphi example:

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

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

Declaration in Firebird:

ENTRY_POINT 'ChangeMyString' MODULE_NAME '......'

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