casting between sockaddr and sockaddr_in

Question

I came across a socket programming tutorial in which it is quoted

\"a pointer to a struct sockaddr_in can be cast to a pointer to a struct sockad

Answer 1

They are of equal size, so no, you don't get any UB!

Proof:

#include 

struct sockaddr {
  unsigned short sa_family; // address family, AF_xxx
  char sa_data[14]; // 14 bytes of protocol address
};

// src: http://www.gta.ufrj.br/ensino/eel878/sockets/sockaddr_inman.html
struct in_addr {
    unsigned long s_addr;  // load with inet_aton()
};

struct sockaddr_in {
  short int sin_family; // Address family, AF_INET
  unsigned short int sin_port; // Port number
  struct in_addr sin_addr; // Internet address
  unsigned char sin_zero[8]; // Same size as struct sockaddr
};

int main (void) {
  printf("%zu\n", sizeof(unsigned short) + sizeof(char) * 14);
  printf("%zu\n", sizeof(short int) + sizeof(unsigned short int) + sizeof(struct in_addr) + sizeof(unsigned char) * 8);
  return 0;
}

Output:

16
16

A good comment: sockaddr: 2+14=16, sockaddr_in:2+2+4+8=16 – Amer Agovic

You may also want to take a look at this question: Why isn't sizeof for a struct equal to the sum of sizeof of each member?

---

Please, check this question: Is it possible to cast struct to another?

I am copying the answer of Benoit here too:

This is refered as Type Punning. Here, both structures have the same size, so there is no question of struct size. Although you can cast almost anything to anything, doing it with structures is error-prone.

and this one by Richard J. Ross III too:

This is C's form of "inheritance" (notice the quotes). This works because C does not care about the underlying data in an address, just what you represent it as.

The function determines what structure it actually is by using the sa_family field, and casting it into the proper sockaddr_in inside the function.