[fpc-pascal] How to find address of record member

[Bo Berglund]

On Sun, 8 Jul 2018 17:16:01 +0200, Martin
<fpc at mfriebe.de> wrote:

>On 08/07/2018 17:07, Bo Berglund wrote:
>> Say I have a record type defined like this:
>>
>>    TAppData = packed record  //Total 8 bytes per app
>>      ActiveToken: word; //Matches constant for app if active
>>      Option: word;      //16 option bits
>>      ExpDate: word;     //Integer TDateTime values
>>      Info: word;        //16 info bits
>>    end;
>>
>> The data is stored in a security dongle memory which can be accessed
>> only a word at a time by its word address 0..55.
>>
>> I want to write my code such that the address to the individual member
>> is calculated automatically so if the record type is modified it will
>> still work.
>>
>
>var AppData: TAppData absolute 0;
>begin
>writeln( ptrint( @AppData.Option ));
>
>You can also take a variable without "absolute"
>@AppData.Option - @AppData

I tried this:
var
  offs: word;
begin
  ...
  offs := @FAppData.Info - @FAppData; //Info is 4th member of record
  address := AppBase + (AppNumber -1) * (SizeOf(TAppData) div 2) +
offs;

But it was not accepted by the compiler (on the offs := line)...
Error: Operator not applicable to this operand type

FAppData is declared in the class definition like this:
  TDongle = class
  private
    { Private declarations }
    FAppNumber,
    FKeySerial,
    FKeyFound: boolean;
    FAppData: TAppData;
    ....

I don't know the command 'absolute' and how it can be applied here in
a class definition.


-- 
Bo Berglund
Developer in Sweden