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Q29825: Structure Packing: /Zp4 or #pragma pack(4), and /Zp2

Article: Q29825
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | docerr | mspl13_c
Last Modified: 12-OCT-1988

The /Zp4 compiler option or #pragma pack(4) results in a more
efficient use of space than indicated on Pages 100 to 102 in both the
"Microsoft C 5.00 Optimizing Compiler User's Guide" and the "Microsoft
C 5.10 Optimizing Compiler User's Guide." When the C compiler aligns
structure members, it uses the /Zp4 or #pragma pack(4) (or /Zp2 or
#pragma pack(2) values) as the maximum number of bytes for alignment,
but will word (even-byte) align individual struct members whose size
is one or two bytes. Contiguous char or array of char members that are
immediately preceded by char members will be byte aligned.

When specifying 4-byte (or 2-byte) boundaries for aligning structure
members, the actual alignment is dependent on the size of the structure
member. To avoid wasting space, the compiler will word align structure
members that are one or two bytes in size. Structure members whose size
is four or eight bytes or greater will be aligned on 4-byte boundaries
with the /Zp4 switch or #pragma pack(4) as documented.

Char or array of char struct members will be byte aligned if they are
preceded by a char struct member, otherwise they will be word aligned.
(Search on "Zp" and "char" to find articles with more information on
the alignment of char struct members.)

The sizeof operator correctly indicates the actual size of the
structure with any alignment of struct members.

Note that this struct member alignment is different than that which
occurred with previous versions of Microsoft C compilers. If using
object files containing structures produced by C 4.00 or earlier
compiler versions, it may be necessary to use dummy bytes to word
align contiguous char struct members.

To gain complete control over the alignment of struct members, you may
use the /Zp or /Zp1 compiler options, or #pragma pack(1), and pad your
struct with an appropriate number of dummy char or array of char
members.

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