Q67735: How Structures Are Packed with /Zp

Article: Q67735
Product(s): See article
Version(s): 
Operating System(s): 6.00 6.00a | 6.00 6.00a
Keyword(s): MS-DOS | OS/2 | mspl13_c
Last Modified: 30-JAN-1991

ENDUSER | docerr s_quickc

The C compiler contains a command-line option as well as a pragma to
pack structures, /Zp and pack, respectively. Packing a structure means
to align an element of a structure on a 1-, 2-, or 4-byte boundary.
Packing can be used for indexing purposes, as well as to decrease
processor access times. If /Zp or the pragma is not used, the default
structure packing value is 2.

The amount of padding used before a particular structure field is
determined by the field size and packing value. The packing value may
change the offset of particular members of the structure.

All offsets of structure members are relative to 0 (zero). Each member
size is compared to the packing value (also called the alignment
value). The element is then aligned on a boundary of the smallest of
the field size and packing value.

Finally, the structure itself may be padded to allow for arrays of
structures to be aligned properly. The rule is simple. All structures
are padded to a multiple of the pack size except one case. If a
structure is packed on 4 byte boundaries but doesn't contain any
elements larger than 2 bytes, it is padded to a multiple of 2.

Below is an example of a structure that was packed with 1-byte packing
[/Zp1 or #pragma pack (1)]. The structure is shown first, followed by
a summary of what happens to the structure in memory, and finally the
generated assembly listing is shown.

Structure
---------

struct
{
   char a;
   int b;
   char c;
} dummy;

Packed Structure (/Zp1)
-----------------------

struct
{
   char a;
   int b;
   char c;
}

Assembly Code Generated in Small Model
--------------------------------------

_BSS SEGMENT WORD PUBLIC 'BSS'
_BSS ENDS
.
.
.
_BSS SEGMENT
COMM NEAR _dummy:BYTE:4
_BSS ENDS

Notice that the size in the assembly listing shows 4 bytes. Because
the structure is exactly 4-bytes in size, there is no need for padding
at the end.

The following is an example with 2-byte packing [/Zp2 or #pragma
pack(2)]:

Structure
---------

struct
{
   char a;
   int b;
   char c;
} dummy;

Packed Structure (/Zp2)
-----------------------

struct
{
   char a;
   (Filler character here)
   int b;
   char c;
   (Filler character here)
}

Assembly Code Generated in Small Model
--------------------------------------

_BSS SEGMENT WORD PUBLIC 'BSS'
_BSS ENDS
.
.
.
_BSS SEGMENT
COMM NEAR _dummy:BYTE:6
_BSS ENDS

In this case, notice that the int is padded to start on a 2-byte
boundary and the actual structure was padded to be a multiple of 2.
Therefore, the length is 6 bytes.

The following is an example of a structure packed with /Zp4:

Structure
---------

struct
{
   char a;
   int  b;
   long c;
   char d;
} dummy;

Packed Structure (/Zp4)
-----------------------

struct
{
   char a;
   (1 padding character here)
   int  b;
   long c;
   char d;
   (3 filler characters here)
}

Assembly Code Generated in Small Model
--------------------------------------

_BSS SEGMENT WORD PUBLIC 'BSS'
_BSS ENDS
.
.
.
_BSS SEGMENT
COMM NEAR _dummy:BYTE:12
_BSS ENDS

This is a little more complex. The first padding occurs with the
integer. Because the field size for an integer is 2 and the alignment
value is 4, the integer will be aligned on a 2-byte boundary (field
size is smaller). The long integer needs to be on a 4-byte alignment.
However, because it is already on a 4-byte boundary, no padding
characters are needed. Finally, because we have a long in this
structure, the entire structure is padded to be a multiple of 4.

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