struct sizeof result not expected

Question

Hey,

I have a a struct defined thusly:

typedef struct _CONFIGURATION_DATA {
    BYTE configurationIndicator;
    ULONG32 baudRate;
    BYTE stopBits;
    BYTE parity;
    BYTE wordLength;
    BYTE flowControl;
    BYTE padding;
} CONFIGURATION_DATA;

Now, by my reckoning, that struct is 10 bytes long. However, sizeof reports that it is 16 bytes long? Anyone know why?

I am compiling using the build tools in the Windows DDK.

Answer 1

Change the order of the elements. Start with the ULONG, followed by the BYTEs. This will improve the struct's alignment in memory.

Answer 2

Alignment.

use

#pragma pack(1)

...struct goes here...

#pragma pack()

I would also recommend reordering things, and if necessary padding then with RESERVED bytes, so that multi-byte integral types will be better aligned. This will make processing faster for tbe CPU, and your code smaller.

Answer 3

This is due to padding, because on your platform, an ULONG32 apparently must be aligned on 4-byte boundaries. Since the start and end of the struct apparently also must be aligned, the first and last BYTE will be padded with 3 bytes each.

Answer 4

The extra size you are measuring is the padding introduced by the compiler.

Presumably, you are working on a 32 bits system, so you will have 3 bytes of padding between configurationIndicator and baudRate, and 3 more bytes of padding at the end of the struct.