Socket Address Structures
Most socket functions require a pointer to a socket address structure as an argument. Each supported protocol suite defines its own socket address structure. The names of these structures begin with sockaddr_ and end with a unique suffix for each protocol suite.
IPv4 Socket Address Structure
An IPv4 socket address structure, commonly called an "Internet socket address structure," is named sockaddr_in and is defined by including the <netinet/in.h> header. Figure 3.1 shows the POSIX definition.
Figure 3.1 The Internet (IPv4) socket address structure: sockaddr_in.
struct in_addr {
in_addr_t s_addr; /* 32-bit IPv4 address */
/* network byte ordered */
};
struct sockaddr_in {
uint8_t sin_len; /* length of structure (16) */
sa_family_t sin_family; /* AF_INET */
in_port_t sin_port; /* 16-bit TCP or UDP port number */
/* network byte ordered */
struct in_addr sin_addr; /* 32-bit IPv4 address */
/* network byte ordered */
char sin_zero[8]; /* unused */
};
There are several points we need to make about socket address structures in general using this example:
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The length member, sin_len, was added with 4.3BSD-Reno, when support for the OSI protocols was added (Figure 1.15). Before this release, the first member was sin_family, which was historically an unsigned short. Not all vendors support a length field for socket address structures and the POSIX specification does not require this member. The datatype that we show, uint8_t, is typical, and POSIX-compliant systems provide datatypes of this form (Figure 3.2).
Figure 3.2. Datatypes required by the POSIX specification.
