Changing a Structures Members array size

This is a discussion on Changing a Structures Members array size within the C++ Programming forums, part of the General Programming Boards category; This is a simple question, so it wont take up too much of anyones time. The Structure 'fd_set' is defined ...

  1. #1
    Xei
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    Question Changing a Structures Members array size

    This is a simple question, so it wont take up too much of anyones time.

    The Structure 'fd_set' is defined as so:
    Code:
    typedef struct fd_set {
      u_int fd_count;
      SOCKET fd_array[FD_SETSIZE];
    } fd_set;
    So the question I ask you is IF my method of increasing the size of 'fd_array' is stable. Here is my method explaining my logic behind it:
    Code:
    //Lets say that we want fd_array to have 3 index's, 0-2.
    fd_set *Set; //Pointer to RealSet eventually.
    BYTE RealSet[(3 * 256)+4+4];
    //256 is the sizeof fd_array, 4 is the size of fd_count, and the other 4 is the size of the pointer(being Set);
    Set = (fd_set *)RealSet;
    //Now we can store(without causing a memory leak) 3 SOCKET members in fd_array, if we store something it actually is stored into RealSet, Set is simply a pointer to RealSet.
    Set->fd_array[0] = Sock1; //Example
    //Or we could use:
    Set[0].fd_array[0] = Sock1;
    I believe that my method works fine, however, I have two questions regarding my method:
    #1. Should RealSet have the size of the Pointer(being 4) also added on to its size?
    #2. When increasing the size of a member in a structure, should I be using the size of the member, or the size of the structure(when multiplying. In this case I used 256 which is the size of the fd_array, and 260 is the size of fd_set, and 4 is the size of fd_count, and 4 is the size of the pointer to fd_set).

    The reason I ask these questions is to make sure that I will not cause memory leaks in the future when I may have slightly more complicated structures. Thankyou.

  2. #2
    Guest Sebastiani's Avatar
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    I don't believe so. First off, when you cast the bytes to an fd_set, you are going to lose the extra bytes due to trimming. Second, all of the functions using fd_sets will constantly use expressions like "sizeof(fd_set)", again which would trim your bytes. Ok. So why not just redefine FD_SETSIZE? But then again, it looks like you're just trying to create an array of fd_set's, is this true? Then just do:
    fd_set Set[3];

    Finally, when you add bytes to the end of a structure, remember that the compiler may or may not place padding between the variables, resulting in a bad deal altogether. Still, if you must try so, there are better options. Consider:

    Code:
    class big_fd_set : public fd_set {
    public:
    BYTE extra_bytes[VALUE];
    fd_set& operator() (){ 
      return *this; 
     }
    };
    Last edited by Sebastiani; 11-07-2002 at 06:55 PM.
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

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