a struct must have a size?

Your code won't even compile.

You can do something like that. Here is one primitive method, pretty much popular is Windows API:

struct VariableSizeStructure {
    int someData;
    //...
    int Array[1]; //it can only come at the end
};

To use this structure, allocate some heap memory more than the size of the structure. For example, if you need N elements, allocate the size sizeof(int) * (N - 1) + sizeof(VariableSizeStructure) and index Array by indices 0 to N-1.

[EDIT]

I no way recommend this kind if code design in new development. This technique is good to know just because some APIs use it, but the practice itself is rotten. Don't use it in your own code design!

I really appreciate the valuable comment by Stephan below. Apparently, originally I failed to mention the danger of this approach, not to late to do it now. :-)

—SA

The code you posted define an array with 200 elements of type LibCont_dat (it does nothing to the size of the struct).

If you want an expandable array try to use vector or list.

If you want unlimited (or not predefined) number of elements, link list like data structure is useful. In array you must be previously aware of number of members you want.. use link list..
By the way std::vector and std::list are also good options if you are OK with using STL.

hi check these links,

it might be helpful to you..

http://www.dotnetperls.com/struct[^]
http://social.msdn.microsoft.com/forums/en-US/csharplanguage/thread/a5a4597f-ac46-43db-a955-33a41c92fab7[^]
http://www.developerfusion.com/article/84519/mastering-structs-in-c/[^]

hope it helps

have a smile :)