a query on structure/union

This is a discussion on a query on structure/union within the C++ Programming forums, part of the General Programming Boards category; hi all, this may be a basic problem, but confusing me as a beginner. here follows a small piece of ...

  1. #1
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    Unhappy a query on structure/union

    hi all,

    this may be a basic problem, but confusing me as a beginner.
    here follows a small piece of code :-

    struct ask
    {
    int a1;
    char ch[10];
    };

    int main()
    {
    ask z;
    cout<<"size of ask="<<sizeof(z)<<"\n";
    }

    it should result :-
    size of ask=14 (4 for int+10 for the array)
    but giving : 16
    I checked with sizeof(z.a1) which gives correctly 4.

    please clarify !

  2. #2
    geek SilentStrike's Avatar
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    The structure is padded. This means that it is filled with junk data so as to be aligned on word boundries (32 bits -> 4 bytes, rounded up to the nearest number such that it mod 4 is 0). On some processors (x86) this helps performance, other processors (MIPS, or so I hear) simply refuse to run things not word-aligned.
    Prove you can code in C++ or C# at TopCoder, referrer rrenaud
    Read my livejournal

  3. #3
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    thanx silent strike for response !
    can u plz clarify a bit ?
    I'm giving u some piece of code with outputs I got :

    struct a
    {
    int x;
    char y;
    }v1;

    now,
    sizeof(v1.x) ---> gives 4
    sizeof(v1.y) ---> gives 1
    sizeof(v1) -----> gives 8 ( ?)

    why extra 3 bytes?

    again,
    union a
    {
    int x;
    char y[10];
    }v1;

    now,
    sizeof(v1.x) ---> gives 4
    sizeof(v1.y) ---> gives 10
    sizeof(v1) -----> gives 12( but, should be 10bytes , no ?)

  4. #4
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    thanx SilentStrike again,
    got it.
    only initialization of structure members will do !

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