Compatible types and structures in C

Question

I have the following code:

int main(void)
{
    struct { int x; } a, b;
    struct { int x; } c;
    struct { int x; } *p;

    b = a;   /* OK */
    c = a;          


        

Answer 1

struct { int x; } is a anonymous structure tag, two anonymous structures cannot have "the same name", which is a necessary condition for type compatibility. You can declare types that are compatible with a non-anonymous structure using typedef.

struct tmp { int x; }; // declare structure tag
typedef struct tmp type1;
typedef struct tmp type2; // declare 3 types compatible with struct tmp
typedef struct tmp type3; // and with each other

type1 a, b;
type2 c;
type3 *p;
b = a;
c = a;
p = &a;