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Why "packed record" ?

Posted on 2003-10-31
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Last Modified: 2010-04-05
Hi!

When should one use packed records ?  besides "compressing" the record, is there any advantage or disadvantae when using them ?

Also, what effect does packed record have when interfacing to a C dll ?
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Question by:delphi112497
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7 Comments
 
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Accepted Solution

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DragonSlayer earned 100 total points
ID: 9662478
In the 32-bit world, data in a record is padded to fit a 4-byte boundary, e.g. a 3-byte variable will take up 4 bytes (including 1 padding) in order to speed up access.

In the olden days of 16-bit, records are padded to fit a 2-byte boundary.

let's assume the following structure in a 16-bit world:

type
  T16BitStructure = record
    a: Integer;
    b: array [0 .. 1] of Char;
    c: Integer;
  end;

back then, Integers are 16-bit data types. So the above structure will take up 6 bytes (I gave example in Pascal, but it applies to C too).

now back to the 32-bit world, for compatibility, you will change Integer to the 16-bit SmallInt:

type
  T32BitStructure = record
    a: SmallInt;
    b: array [0 .. 1] of Char;
    c: SmallInt;
  end;

however, the above will not work because each variable is actually padded to be 4 bytes. Hence, your new structure will not be able to read the 16-bit data structure correctly.

Add the word packed, and voila! It is solved :)

type
  T32BitStructure = packed record
    a: SmallInt;
    b: array [0 .. 1] of Char;
    c: SmallInt;
  end;
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Expert Comment

by:DeerBear
ID: 9662497
Hi,

I would say you must use packed records at any time.
It allows for best speed and it's necessary when creating
API-style routines, in order to be compatible with other
programming languages.

HTH,

Andrew
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LVL 11

Assisted Solution

by:robert_marquardt
robert_marquardt earned 100 total points
ID: 9662588
The alignment to 4 or 8 byte boundaries improves speed especially when using SSE or SSE2 instructions.
packed record (= alignment 1) usually gives you a small loss in speed which is countered by less memory requirement.
Usually the C structs of the Windows API are all packed structures so you need packed record to match the layout.
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Expert Comment

by:swift99
ID: 9662909
DragonSlayer and robert_marquardt's explanations are both sound.
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Expert Comment

by:Motaz
ID: 9666795
Thats a wonderful information, in the past I was thought that the items of un-packed array can be stored in different locations (Non contiguous allocation).
But now I have a question of Dragon, is that applied to typed files of that records too or only to memory.

Motaz
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Expert Comment

by:Motaz
ID: 9666802
sorry, I mean I have a question for Dragon

Motaz
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Expert Comment

by:DragonSlayer
ID: 9668124
it applies to files too.
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