Exploring what sockaddr and sockaddr_in structures are,
with "gcc -E sockaddr.c | less", where sockaddr.c is

---------[ sockaddr.c ]---------
#include <stdio.h>
#include <sys/socket.h>  // struct sockaddr
#include <netinet/in.h>  // strict sockaddr_in  

int main()
{
     printf("sizeof(sa_family_t): %d\n", sizeof(sa_family_t));
     printf("sizeof(sockaddr): %d\n", sizeof(struct sockaddr));
}
--------------------------------

Note that on Linux it gives 2 and 16, but on MacOS 1 and 16!
Why? The answer is in the header files.

Note: It's possible to grep the header files directly for these
definitions, but their nesting and macros quite become tiring.
Instead, we rely on "gcc -E" to resolve and pre-process them all,
so that we see the definitions as the compiler itself gets them.

So:

-----------------[ Darwin ]-----------------
# 93 "/usr/include/sys/socket.h" 3 4
# 1 "/usr/include/sys/_types/_sa_family_t.h" 1 3 4
# 30 "/usr/include/sys/_types/_sa_family_t.h" 3 4
typedef __uint8_t sa_family_t;

...

# 283 "/usr/include/sys/socket.h" 3 4
struct sockaddr {
 __uint8_t sa_len;
 sa_family_t sa_family;
 char sa_data[14];
};

...

# 30 "/usr/include/sys/_types/_in_addr_t.h" 3 4
typedef __uint32_t in_addr_t;

...

# 301 "/usr/include/netinet/in.h" 3 4
struct in_addr {
 in_addr_t s_addr;
};
# 374 "/usr/include/netinet/in.h" 3 4
struct sockaddr_in {
 __uint8_t sin_len;
 sa_family_t sin_family;
 in_port_t sin_port;
 struct in_addr sin_addr;
 char sin_zero[8];
};

-----------------[ Linux ]-----------------

# 28 "/usr/include/bits/sockaddr.h" 3 4
typedef unsigned short int sa_family_t;
# 168 "/usr/include/bits/socket.h" 2 3 4

struct sockaddr
  {
    sa_family_t sa_family;
    char sa_data[14];
  };

...

# 23 "/usr/include/netinet/in.h" 2 3 4

typedef uint32_t in_addr_t;
struct in_addr
  {
    in_addr_t s_addr;
  };

...

# 239 "/usr/include/netinet/in.h" 3 4
struct sockaddr_in
  {
    sa_family_t sin_family;
    in_port_t sin_port;
    struct in_addr sin_addr;

    unsigned char sin_zero[sizeof (struct sockaddr) -
      (sizeof (unsigned short int)) -
      sizeof (in_port_t) -
      sizeof (struct in_addr)];
  };

-------------------------------------------------------

So we see that: sockaddr is just a generic "base" type (in C++/Java
terms), and it doesn't even match between Linux and Darwin/MacOS.
 The sa_family_t, in particular, is 1 byte on Darwin, 2 bytes on
Linux, and Darwin's sockaddr has a 1-byte sa_len field that Linux's
doesn't!

Sockaddr exists for the sake of passing (struct sockaddr *) pointers
to many functions that take different socket structures, not just
IPv4's sockaddr_in (e.g., there are also sockaddr_in6, which contains
16-byte IPv6 addresses---a single address in longer that the 14 bytes
allowed for by sa_data!) So the actual struct passed around through a
(struct sockaddr *) pointer may actually be longer than sizeof(struct
sockaddr), and this size is useless for safety checks.

But, when cast to sockaddr_in, the properly specialized address struct
for Internet addresses that have fields for ports and IPs, these
fields and their offsets match up across systems. That's a relief.