在c中这样的结构声明是否可能

Question

Hii ALL, i am new to c and learing structure.I came across to one of structure declaration and have a doubt on its validation ...

struct a
{ 
int x;
struct a b;
}

is it fine to have such structure declaration and if not then why??

Answer 1

No. A structure cannot contain itself. This simply would not make any sense, since the resultant type will be infinitely large.

In other words, your structure includes an instance of itself, which in turn also includes an instance of itself, which in turn also includes an instance of itself, and so on and so forth ad infinitum...

Form the formal point of view, you are trying to declare a structure member b that has incomplete type . It is illegal to declare struct members of incomplete type in C.

Answer 2

This will not work because the compiler cannot determine the size of your structure if it is self contained.

What works is:

struct a
{
int x;
struct a* b;
}

Then you have a pointer to the same structure inside your structure. This can be used eg for linked lists.

Answer 3

Compiling this you'll get an error similar to:

src.c: In function `main':
src.c:4: field `b' has incomplete type

Using struct a *b , however, is perfectly fine.

http://www.crasseux.com/books/ctutorial/Nested-structures.html