Padding in Structure

This is a discussion on Padding in Structure within the C Programming forums, part of the General Programming Boards category; Size of below structure is 32 bytes, how it is 32 bytes? can you explain me? 2+2+2+2+4+8+8=28 bytes Code: #pragma ...

  1. #1
    C is Sea. I know a drop! ganesh bala's Avatar
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    Thumbs up Padding in Structure

    Size of below structure is 32 bytes, how it is 32 bytes?
    can you explain me?

    2+2+2+2+4+8+8=28 bytes


    Code:
    #pragma pack(push, 8)
    struct MY_STRUCT
    {
    char a[2];
    short b;
    short c;
    short d;
    int e;
    long long x;
    long long y;
    };
    what will pragma pack directive do ? will this changes alignment?

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    A long long would normally start on a 8-byte boundary, so it couldn't start at byte 12. pragma pack, if the compiler understands it in the first place, would suggest removing all "extra" padding bytes in the structure.

  3. #3
    Captain Crash brewbuck's Avatar
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    a, b, c, d, can pack, for a total of 8 bytes. This is an okay alignment for int e, so it packs there, for a total of 12 bytes. This is NOT an okay alignment for long long f, so 4 bytes are inserted to align it at 16 bytes. 16 bytes plus the two long longs makes 32, which is sufficiently aligned for the largest alignment in the struct (the alignment of long long which is 8), so no further pad bytes are used, leading to a grand total of 32 bytes.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  4. #4
    C is Sea. I know a drop! ganesh bala's Avatar
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    Thanks tabstop n brewbuck :-)

  5. #5
    C is Sea. I know a drop! ganesh bala's Avatar
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    This is another one... I am getting 32 as size of struct....

    but , i found only 28..

    4+(padding)4+8+4+1+(padding)3+4=28... where i was wrong??

    Code:
    struct MY_STRUCT
    {
    
    	int ctrl1;
    	double flag;
    	int ctrl2;
    	char flag2;
    	int ctrl3;
    };

  6. #6
    and the hat of wrongness Salem's Avatar
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    http://www.cplusplus.com/reference/c.../offsetof.html
    Use this to explore your struct alignment questions.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  7. #7
    CSharpener vart's Avatar
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    order the members of the struct from longest types to shortest if you want to acheive more compact form without using pack
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  8. #8
    C is Sea. I know a drop! ganesh bala's Avatar
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    Thanks salem n vart..

    i checked with offsetof...

    its showing last one=> int offset is at 24...

    this means size of struct should be 28...

    But i m getting 32...

    Code:
    Anyway thanks for offsetof ... New function i learnt today

  9. #9
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    The largest element is the double, which is 8 bytes, so the compiler adds 4 bytes to make the size of the whole struct a multiple of 8.

    If the size is 28, and you stack 2 of them in an array, the double element in the second one won't be aligned to a 8-byte boundary (28+8).

  10. #10
    C is Sea. I know a drop! ganesh bala's Avatar
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    Thanks Cyberfish 4 ur help...

    i think There are lot of things we have to consider in alignment ...

  11. #11
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    If you are using that pragma directive with an 8 byte alignment then 32 is the right answer as in
    Code:
    struct MY_STRUCT
    {
    
    	int ctrl1;   /* 8 bytes */
    	double flag; /* 8 bytes */
    	int ctrl2;   /* this one and the next can fit into a 8 byte word */
    	char flag2;  
    	int ctrl3;   /* 8 bytes */
    };
    Last edited by itCbitC; 01-29-2009 at 09:31 AM.

  12. #12
    C is Sea. I know a drop! ganesh bala's Avatar
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    Talking

    Thanks itcbitc

    Now i understood what Pragma directive will do

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