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

[Bo Berglund]

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.

So far I have this for the setter:
//Info is 4th member of record
address := AppBase + (AppNumber -1) * (SizeOf(TAppData) div 2) + 3; 
WriteMemory(address, Value);

Appbase is defined in a global constants unit as where the array of
app records in dongle memory starts.

While programming I have found that mistakes are easy if fixed offset
constants are used, like 3 in the example.

Is there some way to get the address within the record of a member
value?


-- 
Bo Berglund
Developer in Sweden