Question

I could not come to the reason why i m getting compilation error to this code?Would be great help if someone clarified it.

int main()
{
  struct xx
  {
    int x;
    struct yy
   {
    int z;
    struct xx *p;
    };
   struct yy *q;
  };
}

Answer 1

In C language you are not allowed to declare struct types inside other struct types without immediately introducing a data field. I.e. struct declaration and data field declaration should be done in one step (as one declaration). You violated that rule: your struct yy definition just sits inside struct xx definition for no reason whatsoever.

Either pull the struct yy definition outside of struct xx, or make sure that struct yy definition immediately declares a field of xx.

For example, this implements the same intent, but does it correctly

  struct xx
  {
    int x;
    struct yy
    {
      int z;
      struct xx *p;
    } *q; // `struct yy` definition is immediately used to declare field `q`
  };

However, usually in C language there’s no reason to create nested struct definitions. It is usually a better idea to define structs at the same level, without any nesting

struct yy
{
  int z;
  struct xx *p;
};

struct xx
{
  int x;
  struct yy *q;
};

Why didn’t you do it that way from the beginning? What was the point of defining these structs in “nested” fasion?