struct sockaddr_in

Forming Internet (IPv4) Socket Addresses

The most commonly used address family under Linux is the AF_INET family. This gives a socket an IPv4 socket address to allow it to communicate with other hosts over a TCP/IP network. The include file that defines the structure sockaddr_in is defined by the C language statement:

#include <netinet/in.h>

Example

Listing 2.7 shows an example of the structure sockaddr_in which is used for Internet addresses. An additional structure in_addr is also shown, because the sockaddr_in structure uses it in its definition.

Listing 2.7: The sockaddr_in Structure

struct sockaddr_in {

sa_family_t sin_family;    /* Address Family */

uint16_t sin_port;         /* Port number */

struct in_addr sin_addr;   /* Internet address */

unsigned char sin_zero[8]; /* Pad bytes */

};

struct in_addr {

uint32_ t s_ addr;         /* Internet address */

};

Listing 2.7 can be described as follows:

• The sin_family member occupies the same storage area that sa_family does in the generic socket definition. The value of sin_family is initialized to the value of AF_INET. 
• The sin_port member defines the TCP/IP port number for the socket address. This value must be in network byte order (this will be elaborated upon later). 
• The sin_addr member is defined as the structure in_addr, which holds the IP number in network byte order. If you examine the structure in_addr, you will see that it consists of one 32- bit unsigned integer. 
• Finally, the remainder of the structure is padded to 16 bytes by the member sin_zero[8] for 8 bytes. This member does not require any initialization and is not used.

Now turn your attention to Figure 2.3 to visualize the physical layout of the address.

Figure 2.3:

Here is the structure sockaddr_in physical layout.

In Figure 2.3, you see that the sin_port member uses two bytes, whereas the sin_addr member uses four bytes. Both of these members show a tag on them indicating that these values must be in network byte order.

TIP

Information about IPv4 Internet addresses can be obtained by examining the ip(4) man page.