struct type conversion in C

Question

In the below snippet , I found a sockaddr_in is type converted as sockaddr as '(struct sockaddr *) &sin' . I just want to know what are the variables that are in sockaddr_in will be mapped correspondingly to sockaddr . Below is the snippet.

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

// IPv4 AF_INET sockets:

struct sockaddr_in {
short            sin_family;   // e.g. AF_INET, AF_INET6
unsigned short   sin_port;     // e.g. htons(3490)
struct in_addr   sin_addr;     // see struct in_addr, below
char             sin_zero[8];  // zero this if you want to
};

struct sockaddr_in sin;

sin.sin_family = AF_INET;
sin.sin_port = htons(floodport);
sin.sin_addr.s_addr = inet_addr(argv[1]);

//type conversion

(struct sockaddr *) &sin  // what values of sockaddr_in would be mapped to sockaddr ?

this conversion is used in sendto() in socket programming as below .

sendto(s, datagram, iph->tot_len,0, (struct sockaddr *) &sin,sizeof(sin))

Thanks in advance .

glglgl glglgl · Accepted Answer · 2012-12-05T13:16:36

The only purpose for struct sockaddr is to have one type to pass around to functions like sendto() etc.

In fact, you don't use it for other purposes, there you have other structs such as

struct sockaddr_in for the legacy IPv4
struct sockaddr_in6 for IPv6
struct sockaddr_un for Unix sockets
struct sockaddr_bth for Bluetooth
struct sockaddr_storage which is as large as the largest in your architecture. Can neatly be used for storing addresses whose type you don't know.

struct type conversion in C

2 Answers