4

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.

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  • forced alignment of some sort? Commented Dec 16, 2009 at 10:50
  • Remember: The size of a structure may not be equal to the sum of the sizes of its members; implementations are allowed to insert padding between members and after the last members. <b>ALWAYS</b> access members by name, not by offset as the offset may change between compiler versions and vendors. Commented Dec 16, 2009 at 20:12

4 Answers 4

Reset to default
12

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.

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7 Comments

Only do this if you've got a good reason to -- many architectures disallow misaligned accesses (so the compiler will have to use byte loads and saves for safety) and most introduce a performance penalty.
True. But I'd expect the compiler to generate proper instructions to access misaligned integral types when architecture does not support it. This is a common pattern to access binary data in files, communications and other IO. On the contrary, i'd say avoid writing a structure if you do not specify the alignment explicitly, because it will change with compiler version/settings.
Actually, the reason i need it packed like this is because it gets written to a socket, and the communications spec dicatates a 10-byte message.
@Pavel -- you might 'expect' the compiler to do something but there is no guarantee that it will! For example the MS VC++ cross-compiler will not automatically generate instructions for misaligned types, you have to explicitly tell it to do so using pointers declared with __unaligned.
@Pavel: I certainly wouldn't "expect" that. You're just spoiled because you're used to x86 hardware. Writing programs that crash at the slightest provocation is a "common pattern" too.
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4

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

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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.

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2

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.

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