17.03.2015 12:45, liviusliv...@poczta.onet.pl wrote:
> i assign it to whole record not for member by member
So, you don't know what values should be in what member ?
> but how offsets can hange any in this representation?
It show us exactly how compiler works
> I should do for one integer "word" alignment
> but for smallint dword
> and for next integer different alignment then for first integer?
> I should gues this or are some rules? Somewhere in FBSource - that after
> smallint another integer+WordBool should be aligned whith DWord?
Rules as simple - for simple types alignment is a sizeof this type.
> think about this simplified record declaration - how will you declare this?
>
> GenRowsInMessage = record
> iValue: Integer;
> iValueNull: WordBool;
> sValue: SmallInt;
> sValueNull: WordBool;
> iValue2: Integer;
> iValue2Null: WordBool;
> end
Alignments (offsets) of members within record should be:
member data type sizeof datatype offset
iValue Integer 4 0
iValueNull WordBool 2 4
sValue SmallInt 2 6
sValueNull WordBool 2 8
iValue2 Integer 4 12
iValue2Null WordBool 2 16
but only if record is *not* packed and declared with {$A+}.
Note, iValue2 have offset 12, not 10 !
BTW, Delphi docs is not fully consistent, see:
1.
http://docwiki.embarcadero.com/RADStudio/XE7/en/Internal_Data_Formats#Record_Types
When a record type is declared in the {$A+} state (the default), and when the
declaration
does not include a packed modifier, the type is an unpacked record type, and
the fields of
the record are aligned for efficient access by the CPU, and according to the
platform. *The
alignment is controlled by the type of each field. Every data type has an
inherent alignment,
which is automatically computed by the compiler.* The alignment can be 1, 2, 4,
or 8, and
represents the byte boundary on which a value of the type must be stored in
order to provide
the most efficient access.
This is how C++ compiler works and what we are expected. But
2. http://docwiki.embarcadero.com/RADStudio/XE7/en/Align_fields_%28Delphi%29
The $A directive controls the maximum alignment used by the compiler when
introducing padding
to align a field.
...
In the {$A8} or {$A+} state, fields in record types that are declared without
the packed
modifier and fields in class structures are aligned on quadword boundaries.
I.e. it is not consistent with link (1). I hope, that {$A+} and {$A8} is not
the same and
think you should try {$A+} not leaving "default" value
Regards,
Vlad
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