几个容易混淆的数据结构

套接字编程常遇到这几个数据结构：sockaddr、sockaddr_in和in_addr，很容易让人范糊涂。现将这几个结构的定义摘录于下：、

/*
 * Structure used by kernel to store most
 * addresses.
 */
struct sockaddr {
        u_short sa_family;              /* address family */
        char    sa_data[14];            /* up to 14 bytes of direct address */
};

/*
 * Socket address, internet style.
 */
struct sockaddr_in {
        short   sin_family;
        u_short sin_port;
        struct  in_addr sin_addr;
        char    sin_zero[8];
};

//
// IPv4 Internet address
// This is an ‘on-wire‘ format structure.
//
typedef struct in_addr {
        union {
                struct { UCHAR s_b1,s_b2,s_b3,s_b4; } S_un_b;
                struct { USHORT s_w1,s_w2; } S_un_w;
                ULONG S_addr;
        } S_un;
#define s_addr  S_un.S_addr /* can be used for most tcp & ip code */
#define s_host  S_un.S_un_b.s_b2    // host on imp
#define s_net   S_un.S_un_b.s_b1    // network
#define s_imp   S_un.S_un_w.s_w2    // imp
#define s_impno S_un.S_un_b.s_b4    // imp #
#define s_lh    S_un.S_un_b.s_b3    // logical host
} IN_ADDR, *PIN_ADDR, FAR *LPIN_ADDR;

首先sockaddr是通用的套接字地址，具体代表的含义要看sa_family成员变量，sockaddr_in是针对internet设计的数据结构，如果sa_family是AF_INET，可直接将sockaddr指针转换成sockaddr_in指针。in_addr仅用于表示IPV4网络地址，也就是常说的IP地址。