From where these two extra bytes came?

This is a discussion on From where these two extra bytes came? within the C Programming forums, part of the General Programming Boards category; Please see following snap code. Sizes of individual members of structure are correct but when I print size of structure ...

  1. #1
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    From where these two extra bytes came?

    Please see following snap code. Sizes of individual members of structure are correct but when I print size of structure it gives two bytes extra.

    #define BYTE unsigned char
    #define INT16 unsigned short

    typedef struct Element
    {
    BYTE ID;
    BYTE Type;
    BYTE Ptr;
    INT16 Cnt;
    INT16 Idx;
    BYTE Status;
    };

    struct Element Element;


    main()
    {
    printf("Size of BYTE %d\n",sizeof(Element.ID));
    printf("Size of INT16 %d\n",sizeof(Element.Type));
    printf("Size of WORD %d\n",sizeof(Element.Ptr));
    printf("Size of DWORD %d\n",sizeof(Element.Cnt));
    printf("Size of DWORD %d\n",sizeof(Element.Idx));
    printf("Size of DWORD %d\n",sizeof(Element.Status));

    printf("Size of Element %d\n",sizeof(struct Element));

    }


    I am working in VC++ 6.0 and on NT platform(0x86).

    Size of struct element is 10, which is 2 bytes extra, if you add each member sizes. from where these two extra bytes came?

    Is Borland 4.0 also exbhits same way?

  2. #2
    moi
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    after the three bytes, the compiler needs to add one byte of waste before the int16s so that they will be aligned on 2 byte boundaries. similarly, after the end of the struct there is another 1 byte waste so that the next struct will begin on 2 byte boundary (beginning of struct is always aligned on the requirements of the strictest of its members).

    the offsetof() macro can tell you where in the struct each memeber starts.
    hello, internet!

  3. #3
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    So you mean to say, If I rearrage the strcuture following way then it will give size as 8 only.

    typedef struct Element
    {
    BYTE ID;
    BYTE Type;
    BYTE Ptr;
    BYTE Status;
    INT16 Cnt;
    INT16 Idx;
    };

  4. #4
    moi
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    yep
    hello, internet!

  5. #5
    Registered User jawwadalam's Avatar
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    Originally posted by moi
    after the three bytes, the compiler needs to add one byte of waste before the int16s so that they will be aligned on 2 byte boundaries. similarly, after the end of the struct there is another 1 byte waste so that the next struct will begin on 2 byte boundary (beginning of struct is always aligned on the requirements of the strictest of its members ).

    the offsetof() macro can tell you where in the struct each memeber starts.
    moi.. I have quoted your post with highlights which I really could understand.. which are..

    1. You have written next struct will begin on 2 byte boundary . I wanted to ask.. Is struct is a object of a struct or of any data structure....

    2. Secondly I couldnt understand ..what is strictest of structure members..

    explian... .?
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