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Record alignment, Delphi 4 vs Delphi 5
Posted on 2000-02-14
I wrote to a file in Delphi 4 using the following record structure:
TRouteFileHeader = record // 40 in Delphi 5, 32 in Delphi 4
HRouteId: string[8]; // 9
HRouteVersion: string[5]; // 6
HOperationCount: Integer; // 4
HLoadDate: TDateTime; // 8
HFileVersion: string[2]; // 3
end;
I recompiled in Delphi 5 and tried to read the same file. The first three fields were okay, but the last two were not. When I did a sizeof on the record, it was 40 bytes. In Delphi 4 it was only 32 bytes (which seems about right).
I think this may have something to do with aligning records on 32 bit boundries. I tried to change that setting, and it did make a difference. I couldn't get the Delphi 5 program to correctly read the file though.
My question is, is there a setting in Delphi 5 that will allow it to correctly read this file as written in Delphi 4?
TRouteFileHeader = record // 40 in Delphi 5, 32 in Delphi 4
HRouteId: string[8]; // 9
HRouteVersion: string[5]; // 6
HOperationCount: Integer; // 4
HLoadDate: TDateTime; // 8
HFileVersion: string[2]; // 3
end;
I recompiled in Delphi 5 and tried to read the same file. The first three fields were okay, but the last two were not. When I did a sizeof on the record, it was 40 bytes. In Delphi 4 it was only 32 bytes (which seems about right).
I think this may have something to do with aligning records on 32 bit boundries. I tried to change that setting, and it did make a difference. I couldn't get the Delphi 5 program to correctly read the file though.
My question is, is there a setting in Delphi 5 that will allow it to correctly read this file as written in Delphi 4?
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6 Comments
The answer is no, there is not setting (sorry). You need to declare your record like this if the D4 record size was 32:
TRouteFileHeader = packed record
HRouteId: string[8];
Dummy1 : Byte;
HRouteVersion: string[5];
Dummy2 : Byte;
HOperationCount: Integer;
HLoadDate: TDateTime;
HFileVersion: string[2];
end;
In general, any record that you write to a file should be declared as 'packed'
Cheers,
Raymond.
TRouteFileHeader = packed record
HRouteId: string[8];
Dummy1 : Byte;
HRouteVersion: string[5];
Dummy2 : Byte;
HOperationCount: Integer;
HLoadDate: TDateTime;
HFileVersion: string[2];
end;
In general, any record that you write to a file should be declared as 'packed'
Cheers,
Raymond.
Your welcome.
As a general rule experts here do not mind if you verify an answer works for you before grading.
Let me know how it goes.
Cheers,
Raymond.
As a general rule experts here do not mind if you verify an answer works for you before grading.
Let me know how it goes.
Cheers,
Raymond.
Raymond,
I tried it today and didn't work. I think using a packed record made things worse. I didn't get a chance to document exactly what I got, but it didn't look too good. I still wonder what they changed in Delphi 5 to cause this problem.
Regards,
Bill
I tried it today and didn't work. I think using a packed record made things worse. I didn't get a chance to document exactly what I got, but it didn't look too good. I still wonder what they changed in Delphi 5 to cause this problem.
Regards,
Bill
I found the definitive guide in section 18-7 of the D3 Object Pascal Guide. According to that, the record should look like this (sorry for the incorrect initial answer):
TRouteFileHeader = packed record
HRouteId: string[8];
HRouteVersion: string[5];
Dummy1 : Byte;
HOperationCount: Integer;
HLoadDate: TDateTime;
HFileVersion: string[2];
Dummy2 : Byte;
end;
I don't think there were changes between D3 and D4, though there have been for D4 to D5.
Cheers,
Raymond.
TRouteFileHeader = packed record
HRouteId: string[8];
HRouteVersion: string[5];
Dummy1 : Byte;
HOperationCount: Integer;
HLoadDate: TDateTime;
HFileVersion: string[2];
Dummy2 : Byte;
end;
I don't think there were changes between D3 and D4, though there have been for D4 to D5.
Cheers,
Raymond.
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