[ale] Copy a Structure - A C Question

[Terry Lee Tucker]

Wow! What a great answer. Thank you for taking the time to expound on 
this issue. I tried bcopy after making the post and it worked just as 
you stated. I just tried the assignement operator as you suggested but 
it puked. I believe that it behaved this way because the structure 
element in this discussion is really an element in a dynamically 
allocated linked list, so the structure name is really a pointer. I'm 
not copying the entire linked list, but only those elements that meet 
certain criteria. In essence, I'm splitting off a group of elements from 
the main list. The pointer to the next element is reassigned so there is 
no problem with a global copy of the original.

Thanks for your excellent response!

Byron A Jeff wrote:

>>Good Morning ALE'rs:
>>
>>This is a question for the C programmers out there. I have a situation 
>>where I need to copy a rather large data structure to another memory 
>>location.
>>
>
>OK.
>
>>Is there a C function that I can use to do this rather than 
>>going through the arduous task of allocating memory and coping each 
>>element manually?
>>
>
>Each element manually? In one of very strange quirks of C, while arrays
>generally cannot be copied wholesale by the assignment operator (due to
>the definition of the name of an array), structures can. So this is perfectly
>legal:
>
>// Structure copy example-------------------------------------------------
>struct mystruct one,two;
>
>// ...
>
>two = one; // Copy an entire struct including all the elements.
>
>// End of example---------------------------------------------------------
>
>>In addition, since this project is still in 
>>development, the makeup of the structure may change and I don't want to 
>>worry about having to keep my manual routine updated.
>>
>
>Agreed. But no matter. The assignment will copy all of the elements.
>
>>The structure 
>>contains a combination of 
>>character arrays,
>>
>
>No problem.
>
>>integer values
>>
>
>No problem.
>
>>and pointers to other structures.
>>
>
>It depends here. If it's OK for both structures, the original and the copy, to
>point to same 'other struct' (this is called a shallow copy in C++ speak), then
>no problem. You'd have to do more if you wanted to clone a copy of the 'other
>struct'. Simple assignment won't do that.
>
>
>>Since this is a structure, is all the allocated 
>>memory for the structure in one contiguous chunk?
>>
>
>Yes. But there's no guarantee that the individual elements are contiguous.
>Often compilers will insert padding between the elements to maintain alignment.
>
>>If so, could I then 
>>use something like memcpy to do the job?
>>
>
>Sure. But why when a simple assignment does the job so well.
>
>Here are the major differences between arrays and structs:
>
>* The name of an array is defined as a pointer to the first element. A struct's
>  name defines the struct itself.
>
>* Assignment on arrays is illegal because array names cannot be assigned with
>  new values. Struct assignment copies a struct as you would expect.
>
>* Array parameters are pass by reference, since it implicitly passes a pointer
>  to the array. Struct parameters are copied. One needs to watch that when
>  passing a very large struct. It generally better to pass a pointer to the
>  struct, even when you have no intention of changing it (use a const...)
>
>* Arrays cannot be returned as the result of a function. Structs can.
>
>In the end structs are much more like simple scalars and behave in very much
>the same manner.
>
>So just assign the structs and don't worry about it. It'll do the same thing
>as the memcpy, but will be much more intuitive.
>
>It's kind of funny how C's quirkiness with arrays throws folks off when 
>structs come behind it...
>
>BAJ
>

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