Size of structures is messed up

This is a discussion on Size of structures is messed up within the C++ Programming forums, part of the General Programming Boards category; Code: struct little{ unsigned char ident; unsigned short int data; }; Code: char debug[256]; sprintf(debug,"%i+%i=%i",sizeof(unsigned char),sizeof(unsigned short int),sizeof(little)); MessageBox(0,debug,"Debug",MB_OK); The ...

  1. #1
    Reverse Engineer maxorator's Avatar
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    Size of structures is messed up

    Code:
    struct little{
        unsigned char ident;
        unsigned short int data;
    };
    Code:
        char debug[256];
        sprintf(debug,"%i+%i=%i",sizeof(unsigned char),sizeof(unsigned short int),sizeof(little));
        MessageBox(0,debug,"Debug",MB_OK);
    The messagebox says 1+2=4. How is this possible?
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  2. #2
    and the hat of wrongness Salem's Avatar
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    Structures have internal and trailing padding to preserve alignment contraints.
    This has been discussed many times before.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    Reverse Engineer maxorator's Avatar
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    How can I get around this "padding"? I want to save it into a binary file as data, but it adds some bytes with random values.
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  5. #5
    Reverse Engineer maxorator's Avatar
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    I just used good old char array. No padding problems.
    So that instead of:
    Code:
    boke[count].ident=i+1;
    boke[count].data=number;
    I have:
    Code:
    boke[count*3]=i+1;
    boke[count*3+1]=number/256;
    boke[count*3+2]=number%256;
    Low-level stuff always helps out.
    Last edited by maxorator; 01-02-2007 at 05:31 AM.
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    CSharpener vart's Avatar
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    try to reorder from longest types to shortest...
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  7. #7
    and the hat of wrongness Salem's Avatar
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    > but it adds some bytes with random values.
    So long as you use the same structure for writing and reading, the content of the padding bytes doesn't matter.

    If you're paranoid about it, memset the whole structure to 0 before assigning individual fields.

    Or use a non-portable "#pragma" of some sort to force the compiler to squeeze out the empty space (at the expense of more complicated code)
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
    Reverse Engineer maxorator's Avatar
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    I suppose this should work too:
    Code:
    WORD number=61001;
    char* boke=new char[5*MAXPARTS];
    WORD* arr;
    boke[count*3]=i+1;
    arr=(WORD*)((int)boke+count*3+1);
    *arr=number;
    //do something with it
    delete[] boke;
    Same with integer:
    Code:
    int number=548841;
    char* boke=new char[5*MAXPARTS];
    int* arr;
    boke[count*5]=i+1;
    arr=(WORD*)((int)boke+count*5+1);
    *arr=number;
    //do something with it
    delete[] boke;
    Last edited by maxorator; 01-02-2007 at 06:01 AM.
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  9. #9
    and the hat of wrongness Salem's Avatar
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    > int* arr;
    > boke[count*5]=i+1;
    > arr=boke+count*5+1;
    > *arr=number;
    Nope - that will get you a bus error (or unaligned access error) on some machines.
    Did you get any warnings when you compiled it?

    If you must pack ints into a char array, then you need to use memcpy() to be sure of it working.
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  10. #10
    Reverse Engineer maxorator's Avatar
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    No warnings - work excellenty (I changed the code a bit so it won't give me "couldn't convert" errors)
    I've done exactly the same thing in assembly (where it is normal) before.
    I think this is a very logical thing and I see no problem why it shouldn't work (integers don't always have to start at even memory addresses so I see no reason to get unaligned access error).
    Last edited by maxorator; 01-02-2007 at 06:07 AM.
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  11. #11
    and the hat of wrongness Salem's Avatar
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    Good solid "works for me" logic - keep it up
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  12. #12
    Reverse Engineer maxorator's Avatar
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    It seems as logical as 2+2=4 to me that it works.
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  13. #13
    and the hat of wrongness Salem's Avatar
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    So the gcc option of -Wcast-align is basically a waste of time in your opinion then, based on your limited experience?
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  14. #14
    Reverse Engineer maxorator's Avatar
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    Quote Originally Posted by Salem
    So the gcc option of -Wcast-align is basically a waste of time in your opinion then, based on your limited experience?
    I don't think that aligns memory... but I might be wrong.
    Last edited by maxorator; 01-03-2007 at 05:10 AM.
    "The Internet treats censorship as damage and routes around it." - John Gilmore

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