Anonymous structures and anonymous unions

Anonymous structures and anonymous unions are type definitions without names, and they are usually used in other types as a nested type. It is easier to explain them with an example. Here, you can see a type that has both an anonymous structure and an anonymous union in one place, displayed in Code Box 12-10:

typedef struct {
  union {
    struct {
      int x;
      int y;
    };
    int data[2];
  };
} point_t;

Code Box 12-10: Example of an anonymous structure together with an anonymous union

The preceding type uses the same memory for the anonymous structure and the byte array field data. The following code box shows how it can be used in a real example:

#include <stdio.h>
typedef struct {
  union {
    struct {
      int x;
      int y;
    };
    int data[2];
  };
} point_t;
int main(int argc, char** argv) {
  point_t p;
  p.x = 10;
  p.data[1] = -5;
  printf("Point (%d, %d) using an anonymous structure inside an...