casting pointers

This is a discussion on casting pointers within the C Programming forums, part of the General Programming Boards category; hi, i have: Code: struct test{ int a; int b; char c[20]; }; struct test *ptest; is it now safe ...

  1. #1
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    casting pointers

    hi,

    i have:

    Code:
    struct test{
           int a;
           int b;
           char c[20];
    };
    
    struct test *ptest;
    is it now safe to do something like this:

    Code:
    BYTE *b = (BYTE *) ptest;
    int tmp;
    for (tmp = 0; tmp < sizeof(struct test); tmp++){
       BYTE c = *(b+tmp)
       do_something(c)
    }

    why would i want to do this? i have a function that needs to treat with arbitrary data, something like this:

    Code:
    send_data(BYTE *data, int length);
    and i want to pass that struct into that function.


    felix

  2. #2
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    > i have a function that needs to treat with arbitrary data, something like this:
    Most people make such functions accept a void* parameter, then the casting problem goes away.
    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
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    unfortunatly i cannot change that functions signature because it comes from a library...

  4. #4
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    "Safe" depends on your definition. Yes, you're allowed to access the individual bytes in a struct using the mechanism you're using. In fact, if you look up fread() and fwrite() you'll find it allows you to output data in this way (if you use appropriate casts). What is less safe is any interpretation of the meaning of those bytes: the layout of data in structures is often operating system and compiler dependent.

  5. #5
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    Thanks Grumpy.

    lets say now that i copyy those bytes each one after another into a freshly allocated BYTE Buffer and cast the thing back into a struct test pointer, e.g. like this:

    Code:
    BYTE * b = malloc(sizeof(struct test) * sizeof(BYTE));
    .... copy every single byte
    struct test *reborn = (struct test *) b;
    is this valid?

  6. #6
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    Yes it is valid, within some constraints.

    The thing that is not necessarily valid is;
    Code:
        struct test *a = malloc(sizeof(struct test));
        /*   initialsie members of a */
        fwrite(a, sizeof (struct test), 1, some_file_stream);
    and (with code built using another compiler)
    Code:
        struct test *a = malloc(sizeof(struct test));
        fread(a, sizeof (struct test), 1, a_stream_for_the_same_file);
    
         /* access members of a */
    These two bits of code will interoperate correctly if built using the same compiler (and same compiler settings) on the same operating system. They will not necessarily interoperate if the code is built using different compiler (or different compiler settings) or on a different operating system. The reasons are that;

    1) sizeof (struct test) is implementation dependent;
    2) layout of individual members in a struct is implementation dependent.

  7. #7
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    good, thanks.

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