[fpc-pascal] DLL call works on FPC 2.6.4 but not with D7

[Paul Breneman]

On 07/01/2014 11:53 AM, Jonas Maebe wrote:
> On 01/07/14 17:27, Paul Breneman wrote:
>> Sorry, but after I sent the email above I realized that I was editing a
>> file in the wrong folder.  Free Pascal doesn't work with "var" on the
>> line below:
>>    var str : Pchar;
>>
>> This is what works on FPC 2.6.4:
>> function ps5000aGetUnitInfo
>>           (handle : smallInt;
>>            Str : Pchar;
>>            stringLength : smallInt;
>>            var requiredSize : smallInt;
>>            info : integer
>>            ) : integer;
>> {$IFDEF WIN32} stdcall; external 'ps5000a.dll';{$ENDIF}
>
> It will work in both FPC and Delphi if you pass @MyReturnStr[1],
> provided that the length of MyReturnStr is sufficiently large. When
> dealing with addresses to strings/arrays, always explicitly specify the
> first element you want to use rather than the entire string/array. That
> will always work regardless of the string or array type
> (static/dynamic/open/... array, ansi/short/wide/..., and even pchar).


Thanks Jonas for the help!  This now works for me in FPC (MyReturnStr is 
PChar):
       Status := ps5000aGetUnitInfo( ps5000a_handle, @MyReturnStr[ 0], 
StringLen, requiredSize, RInfo);

But in D7 using "@MyReturnStr[ 0]" gives an Access violation exception 
in the dll.  The address the DLL is trying to write is about the same as 
in FPC so I still don't understand why FPC works but D7 doesn't.