SMTP

	/*
     * _domain 套接字使用的协议族信息
     * _type 套接字的传输类型
     * __protocol 通信协议
     * */
     int socket (int __domain, int __type, int __protocol) __THROW;

/* 
* __fd:socket描述字，也就是socket引用
* myaddr:要绑定给sockfd的协议地址
* __len:地址的长度
*/
int bind (int __fd, const struct sockaddr* myaddr, socklen_t __len)  __THROW;

struct sockaddr
  {
    sa_family_t sin_family;	/* Common data: address family and length.  */
    char sa_data[14];		/* Address data.  */
  };

struct sockaddr_in{
   sa_family_t sin_family;		//前面介绍的地址族
   uint16_t sin_port;			//16位的TCP/UDP端口号
   struct in_addr sin_addr;	//32位的IP地址
   char sin_zero[8];			//不使用
}

/* Internet address.  */
typedef uint32_t in_addr_t;
struct in_addr
{
	in_addr_t s_addr;
};

struct sockaddr_in serv_addr;
...
bind(serv_socket, (struct sockaddr*)&serv_addr, sizeof(serv_addr)；

in_addr_t inet_addr (const char *__cp) __THROW;
//或者
int inet_aton (const char *__cp, struct in_addr *__inp) __THROW;	//windows无此函数

/*Functions to convert between host and network byte order.

   Please note that these functions normally take `unsigned long int' or
   `unsigned short int' values as arguments and also return them.  But
   this was a short-sighted decision since on different systems the types
   may have different representations but the values are always the same.  */

// h代表主机字节序
// n代表网络字节序
// s代表short(4字节)
// l代表long(8字节)
extern uint32_t ntohl (uint32_t __netlong) __THROW __attribute__ ((__const__));
extern uint16_t ntohs (uint16_t __netshort)
     __THROW __attribute__ ((__const__));
extern uint32_t htonl (uint32_t __hostlong)
     __THROW __attribute__ ((__const__));
extern uint16_t htons (uint16_t __hostshort)

/* Prepare to accept connections on socket FD.
   N connection requests will be queued before further requests are refused.
   Returns 0 on success, -1 for errors.  */
extern int listen (int __fd, int __n) __THROW;

/* Await a connection on socket FD.
   When a connection arrives, open a new socket to communicate with it,
   set *ADDR (which is *ADDR_LEN bytes long) to the address of the connecting
   peer and *ADDR_LEN to the address's actual length, and return the
   new socket's descriptor, or -1 for errors.

   This function is a cancellation point and therefore not marked with
   __THROW.  */
extern int accept (int __fd, struct sockaddr *addr, socklen_t *addr_len);

/* Write N bytes of BUF to FD.  Return the number written, or -1.

   This function is a cancellation point and therefore not marked with
   __THROW.  */
 ssize_t write (int __fd, const void *__buf, size_t __n) ;

ssize_t send (int sockfd, const void *buf, size_t nbytes, int flag) ;

/* Read NBYTES into BUF from FD.  Return the
   number read, -1 for errors or 0 for EOF.

   This function is a cancellation point and therefore not marked with
   __THROW.  */
ssize_t read (int __fd, void *__buf, size_t __nbytes);

ssize_t recv(int sockfd, void *buf, size_t nbytes, int flag) ;

/* Close the file descriptor FD.

   This function is a cancellation point and therefore not marked with
   __THROW.  */
int close (int __fd);

/* Open a connection on socket FD to peer at ADDR (which LEN bytes long).
   For connectionless socket types, just set the default address to send to
   and the only address from which to accept transmissions.
   Return 0 on success, -1 for errors.

   This function is a cancellation point and therefore not marked with
   __THROW.  */
int connect (int socket, struct sockaddr* servaddr, socklen_t addrlen);

/* Put the current value for socket FD's option OPTNAME at protocol level LEVEL
   into OPTVAL (which is *OPTLEN bytes long), and set *OPTLEN to the value's
   actual length.  Returns 0 on success, -1 for errors.  */
extern int getsockopt (int sock, int __level, int __optname,
		       void *__optval, socklen_t *optlen) __THROW;

/* Set socket FD's option OPTNAME at protocol level LEVEL
   to *OPTVAL (which is OPTLEN bytes long).
   Returns 0 on success, -1 for errors.  */
extern int setsockopt (int sock, int __level, int __optname,
		       const void *__optval, socklen_t __optlen) __THROW;