IPv4套接字地址结构

   IPv4套接字地址结构通常也称为“网络套接字地址结构”，它以 sockaddr_in 命名，定义在<netinet/in.h>头文件中

/* Internet address. */
typedef uint32_t in_addr_t;
struct in_addr
{
    
    in_addr_t s_addr;                       /* 32-bit IPv4 address */
                                                                /* network byte ordered */
};

/* POSIX.1g specifies this type name for the `sa_family' member. */
typedef unsigned short int sa_family_t;
/* This macro is used to declare the initial common members of the data types used for socket addresses, `struct sockaddr', `struct sockaddr_in', `struct sockaddr_un', etc. */
#define __SOCKADDR_COMMON(sa_prefix) \ sa_family_t sa_prefix##family
  
/* Structure describing an Internet socket address. */
struct sockaddr_in
  {
    
    __SOCKADDR_COMMON (sin_);           /* AF_INET */
    in_port_t sin_port;			                       /* Port number. */
    struct in_addr sin_addr;		                  /* Internet address. */
    /* Pad to size of `struct sockaddr'. */
    unsigned char sin_zero[sizeof (struct sockaddr) -
			   __SOCKADDR_COMMON_SIZE -
			   sizeof (in_port_t) -
			   sizeof (struct in_addr)];
  };