Thanks Rob, that makes sense. I'd wondered if something like that was happening, but wasn't sure. I'd love to just use WideStrings, but I need the DLLs to be callable from other programming languages. What I've done now is modified the DLL to be able to return the size of those PWideChar fields. So the calling application now does (in pseudo-code)
var Intf: TInterfaceDescriptor; IDSize, NameSize, AdditionalSize: Integer; GetDescriptorSizes(IntfID, IDSize, NameSize, AdditionalSize); Intf.ID := StrAllocW(IDSize); Intf.Name := StrAllocW(NameSize); Intf.Additional := StrAllocW(AdditionalSize); GetDescriptor(IntfID, Intf); //Does stuff with the descriptor (like display values in TTntEdit controls) StrDisposeW(Intf.ID); StrDisposeW(Intf.Name); StrDisposeW(Intf.Additional); The code for GetDescriptor in the DLL is pretty much the same as before, except now it uses StrCopyW() to copy the value into the record it has been passed. This all works nicely and the troublesome ID value from before is now correctly returned to the calling application. With the slight exception that when it trys to free the memory for the ID it raises an "invalid pointer operation" error. All other ID values work correctly, it is just this one that is being obstinant. For posterity's sake the troublesome ID in question is 4 characters long and its value is "Biox". I can't help feeling that I'm missing something really (really) simple here. Regards, Darren -- Darren Ferguson Senior Software Engineer Clinical Software Division Medtel Australia Adelaide, South Australia -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rob Kennedy Sent: Friday, 21 April 2006 5:40 AM To: Borland's Delphi Discussion List Subject: RE: PWideChar and DLL problem Since IntfID is a function returning WideString, that line is actually compiled as though it were like this: var TempIntfID: WideString; TempIntfID := AClass.IntfID; Descriptor.ID := PWideChar(TempIntfID); When this GetDescriptor function returns, the local variables are released. That includes the TempIntfID variables. Therefore, Descriptor.ID is left pointing at memory that has already been released. The most straightforward way to solve that is to use WideStrings instead of PWideChars. Then the compiler manages the strings' lifetimes for you. Otherwise, you need to allocate memory for the Descriptor.ID field in advance. You can use the JclWideStrings unit to do this: Descriptor.ID := StrNewW(PWideChar(AClass.IntfID)); That allocates a new PWideChar buffer and fills it with the contents of the IntfID string. Use StrDisposeW when you're finished with Descriptor.ID. -- Rob _______________________________________________ Delphi mailing list -> [email protected] http://www.elists.org/mailman/listinfo/delphi _______________________________________________ Delphi mailing list -> [email protected] http://www.elists.org/mailman/listinfo/delphi
