memcpy question

This is a discussion on memcpy question within the C Programming forums, part of the General Programming Boards category; I'm seeing a weird problem with memcpy. I have the following code: Code: int main() { char data[] = { ...

  1. #1
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    memcpy question

    I'm seeing a weird problem with memcpy. I have the following code:

    Code:
    int main()
    {
            char data[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
    
    
            struct mystruct {
    
                    char a;
                    short b;
                    char c;
                    long d;
                    char e;
                    short f;
                    char g;
            } ms;
    
            memcpy(&ms, data, 16);
    Ok, I expected a = 1, but for some bizarre reason, c = 5! It skipped over 4 completely. Not only that, but b = 3 and 4, it skipped over 2! This totally messes up all of my code.

    Whats the best way to fix this issue without manually specifying offsets (i.e. c = data[3]) and make sure all the other values equal what they are supposed to?
    Last edited by absoult-tech1; 10-02-2010 at 11:41 PM.

  2. #2
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    Google struct padding.
    Even if there's no padding, it won't work.(as what you expect).
    Just assign to individual member.
    Last edited by Bayint Naung; 10-02-2010 at 11:43 PM.

  3. #3
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    Quote Originally Posted by Bayint Naung View Post
    Google struct padding.
    Just assign to individual member.

    I know its padding the structure. My question is, how do I solve that problem without manually specifying offsets? It becomes an issue with portability and endianness between machines, and much further complicates the code.
    Last edited by absoult-tech1; 10-02-2010 at 11:45 PM.

  4. #4
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    Structs may include padding (spare space between members) for performance reasons. memcpy() will happily write data to those padding bytes.

    Rather than trying to use memcpy() to initialise the struct - such things are sensitive to how the compiler lays out the structure in memory - initialise the struct directly.

    Code:
    struct mystruct ms = {1, 1027, 4, <other values you expect>};
    Right 98% of the time, and don't care about the other 3%.

  5. #5
    and the hat of wrongness Salem's Avatar
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    Code:
    ms.a = data[0];
    ms.b = (data[1]<<8) + data[2];
    etc etc
    If you want portability across all endian implementations and struct packing.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  6. #6
    Algorithm Dissector iMalc's Avatar
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    Reorder the variables in your struct so that they are ordered from largest to smallest. (long -> short -> short -> char -> char -> char -> char)
    That minimises the padding.
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