SOCKADDR结构

winsock中应用通过SOCKADDR结构来指定IP地址和服务器端口信息：  
  struct   sockaddr_in  
  {  
      short     sin_family;  
      u_short   sin_port;  
      struct   in_addr   sin_addr;  
      char         sin_zero[8];  
  }

struct   in_addr   {  
      union   {  
                      struct   {   u_char   s_b1,s_b2,s_b3,s_b4;   }       S_un_b;  
                      struct   {   u_short   s_w1,s_w2;   }                       S_un_w;  
                      u_long                                                                  S_addr;  
      }   S_un;  
  };

//--------------------------------------------------------------------------------------------------------

struct sockaddrand pals

Structures for handling internet addresses

Prototypes

include // All pointers to socket address structures are often cast to pointers
// to this type before use in various functions and system calls:struct sockaddr {unsigned short sa_family; // address family, AF_xxxchar sa_data[14]; // 14 bytes of protocol address
};// IPv4 AF_INET sockets:struct sockaddr_in {short sin_family; // e.g. AF_INET, AF_INET6unsigned short sin_port; // e.g. htons(3490)struct in_addr sin_addr; // see struct in_addr, belowchar sin_zero[8]; // zero this if you want to
};struct in_addr {unsigned long s_addr; // load with inet_pton()
};// IPv6 AF_INET6 sockets:struct sockaddr_in6 {u_int16_t sin6_family; // address family, AF_INET6u_int16_t sin6_port; // port number, Network Byte Orderu_int32_t sin6_flowinfo; // IPv6 flow informationstruct in6_addr sin6_addr; // IPv6 addressu_int32_t sin6_scope_id; // Scope ID
};struct in6_addr {unsigned char s6_addr[16]; // load with inet_pton()
};// General socket address holding structure, big enough to hold either
// struct sockaddr_in or struct sockaddr_in6 data:struct sockaddr_storage {sa_family_t ss_family; // address family// all this is padding, implementation specific, ignore it:char __ss_pad1[_SS_PAD1SIZE];int64_t __ss_align;char __ss_pad2[_SS_PAD2SIZE];
};

Description

These are the basic structures for all syscalls and functions that deal with internet addresses. Often you&＃39;ll use getaddinfo() to fill these structures out, and then will read them when you have to.

In memory, the struct sockaddr_inand struct sockaddr_in6share the same beginning structure as struct sockaddr, and you can freely cast the pointer of one type to the other without any harm, except the possible end of the universe.

Just kidding on that end-of-the-universe thing...if the universe does end when you cast a struct sockaddr_in*to a struct sockaddr*, I promise you it&＃39;s pure coincidence and you shouldn&＃39;t even worry about it.

So, with that in mind, remember that whenever a function says it takes a struct sockaddr*you can cast your struct sockaddr_in*, struct sockaddr_in6*, or struct sockadd_storage*to that type with ease and safety.

struct sockaddr_inis the structure used with IPv4 addresses (e.g. "192.0.2.10"). It holds an address family (AF_INET), a port in sin_port, and an IPv4 address in sin_addr.

There&＃39;s also this sin_zerofield in struct sockaddr_inwhich some people claim must be set to zero. Other people don&＃39;t claim anything about it (the Linux documentation doesn&＃39;t even mention it at all), and setting it to zero doesn&＃39;t seem to be actually necessary. So, if you feel like it, set it to zero using memset().

Now, that struct in_addris a weird beast on different systems. Sometimes it&＃39;s a crazy union with all kinds of #defines and other nonsense. But what you should do is only use the s_addr field in this structure, because many systems only implement that one.

struct sockadd_in6and struct in6_addrare very similar, except they&＃39;re used for IPv6.

struct sockaddr_storageis a struct you can pass to accept() or recvfrom() when you&＃39;re trying to write IP version-agnostic code and you don&＃39;t know if the new address is going to be IPv4 or IPv6. The struct sockaddr_storagestructure is large enough to hold both types, unlike the original small struct sockaddr.

Example

// IPv4:struct sockaddr_in ip4addr;
int s;ip4addr.sin_family = AF_INET;
ip4addr.sin_port = htons(3490);
inet_pton(AF_INET, "10.0.0.1", &ip4addr.sin_addr);s = socket(PF_INET, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)&ip4addr, sizeof ip4addr);

// IPv6:struct sockaddr_in6 ip6addr;
int s;ip6addr.sin6_family = AF_INET6;
ip6addr.sin6_port = htons(4950);
inet_pton(AF_INET6, "2001:db8:8714:3a90::12", &ip6addr.sin6_addr);s = socket(PF_INET6, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)&ip6addr, sizeof ip6addr);