Thread: Casting of structs in union on char array

  1. #1
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    Casting of structs in union on char array

    Hi I have the following struct as below

    Code:
    typedef struct{
       union{
          typedef struct{
               int b;
          }c;
          typedef struct{
               int e;
          }f;
       }u;
    }a;
    I have a pointer declared

    Code:
    a* pter=(a*) malloc(sizeof(a));
    and a char buffer which I want to cast the struct c onto

    I am not sure how to go about doing this

    Code:
    pter=(a*)(c*)buffer;
    Or how do I access the struct c from the union?

  2. #2
    ATH0 quzah's Avatar
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    You don't need to typecast the return from malloc. If you do, you either forgot a header or you are using C++ not C. On to your question: The same way you access any nested structure's members:
    Code:
    a *ptr = malloc( sizeof *ptr );
    ptr->u.c.b = 0xDEADBEEF;
    One has to wonder why you wrapped your union in a struct that contains nothing else.


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
    Lurking whiteflags's Avatar
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    Aren't the two structs in the union basically the same?

  4. #4
    ATH0 quzah's Avatar
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    Ug. He's adding typedefs inside his union. So he didn't actually create instances of f and c, he just made types without actual variables. I discovered this when I was going to make a joke about "No, they're not the same, one is named f and one is named c!" My compiler weeps. It didn't error or warn, it just started crying.


    Quzah.
    Hope is the first step on the road to disappointment.

  5. #5
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    Quote Originally Posted by Edelweiss View Post
    Hi I have the following struct as below
    Or how do I access the struct c from the union?
    That would be a.c.b ...

    What exactly are you trying to accomplish... not in code, in words...
    If you simply want to overlay a char array on an int you can do it like this...
    Code:
    union t_ctoi
      { unsigned char chs[sizeof(int)];
         int  num; } 
    ctoi;
    This will let you do stuff like this...
    Code:
    ctoi.num = 4242424242;
    printf("%c" ctoi.chs[2];

  6. #6
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    Ops...sorry for giving a lousy example.

    Wanted to cast a struct from a union onto a char buffer and the structs in that union are supposed to have different variables.

    But I figured it out already. I initialized struct a and and access the struct which i wanted and cast it on the char buffer.

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