Q:

What is the difference between structure and function scope in C?

Consider this piece of code
   int main(void)
   {
       typedef struct {
           int i;
       } s;

       struct {
           s s;
       } t;

       return 0;
   }

It compiles fine. Now take a look at this one
   int main(void)
   {
       typedef struct {
           int i;
       } s;

       s s;
       return 0;
   }

This code will not compile -    
‘s’ redeclared as different kind of symbol.

Question: Why is it correct to have "s s;" as a declaration inside a structure, but not correct to have this definition inside a function?

A:

In upper example member s is a local to struct. You cannot use it without using t.s syntax, so there is no conflict with structure type s.
In lower example structure type s, and variable s are in the same scope, so it is unclear which you are referring to.

A:

As a struct member, the identifier s is unambiguous, because you'll always address it as somestruct.s or someptr->s.

