Delphi 7 compared to 2009 (& 2010) Record sizes

Question

I have a weird issue when converting code from Delphi 7 to 2010. It has to do with records. The record defined below, when sized in D7, is 432 bytes, and in D2009 (and 2010) it's 496. I know, that an easy solution is to make it a packed record, then all versions come out to 426 bytes... However, we have data stored where we streamed the record and now we are trying to read those streams with a newer language.

TToTry = Record
 a,b,c,d : Extended;
 e,f,g,h : Extended;
 i : String[15];
 j,k,l,m,n,o,p,q,r,s,t : Array[1..3] of Extended; End;

In investigating this issue, I created another record, and, for whatever reason, the sizes are the same? The record is smaller, but it has the same data types. but it comes out the same size in all versions of the language.

TMyRecord = Record
Ext1  : Extended;
Ext2  : Extended;
Ext3  : Extended;
Ext4  : Extended;
Ext5  : Extended;
Ext6  : Extended;
Int1  : Integer;
Int2  : Integer;
char1 : AnsiChar;
char2 : AnsiChar;
MyString  : String[15];
Arr1  : Array[1..3] of Extended;
Arr2  : Array[1..3] of Extended; end;

Anybody have any insight as to why one record is so different, and the other is the same? Something to do with byte boundary alignments in Delphi for sure. but what changed so drastically from one version to the next?

Answer 1

Well, the first problem is that you stored a non-packed record to disk. Field and array packing is allowed to change between product releases because normally the layout in memory is not visible outside of the process. You've broken that rule.

If the byte padding defaults changed between Delphi 7 and Delphi 2009, find out what the defaults were in D7 and set the defaults to the same in Delphi 2009.

Also check the array packing default. I can't remember if there is a separate setting for this.

Take a look at your record structure in Delphi 2009 in the debug memory view. Some or all of the additional size may be due to padding of the record itself (not the fields within it) so that when the record is used in an array the array elements are on speedy machine boundaries.

If none of this helps, create a temporary packed record type in D2009 and manually insert byte padding fields between the actual data fields until the record size and field alignments match the D7 layout. It's not just size, it's field alignments. Read your old data file in using this temp packed record. Then transfer the data field by field into your "real" record type in D2009 and write out a new file.

And while you're at it, pack that record type in D2009 so this doesn't happen again.