Linux下C Socket编程基础API

文章目录

• 1. 地址类API
• • 1.1 字节序
  • 1.2 通用Socket地址
  • 1.3 专用socket地址
  • • 1.3.1 UNIX 本地域协议族
    • 1.3.2 IPv4
    • 1.3.3 IPv6
  • 1.4 IP 地址转换函数
  • • 1.4.1 适用于 IPv4 地址
    • 1.4.2 适用于 IPv4 和 IPv6 地址
• 2. Socket API
• • 2.1 创建Socket
  • 2.2 命名Socket
  • 2.3 监听Socket
  • 2.4 接受连接
  • 2.5 发起连接
  • 2.6 关闭Socket
  • 2.7 数据读写
  • • 2.7.1 TCP 数据读写
    • 2.7.2 UDP 数据读写
    • 2.7.3 通用数据读写函数
  • 2.8 带外标记
  • 2.9 地址信息函数
  • 2.10 Socket 选项
  • • 2.10.1 SO_REUSEADDR 选项
    • 2.10.2 SO_RCVBUF 和 SO_SNDBUF 选项
    • 2.10.3 SO_RCVLOWAT 和 SO_SNDLOWAT 选项
    • 2.10.4 SO_LINGER 选项
• 3 网络信息API
• • 3.1 gethostbyname 和 gethostbyaddr
  • 3.2 getservbyname 和 getservbyport
  • 3.3 getaddrinfo
  • 3.4 getnameinfo

1. 地址类API

1.1 字节序

分为大端字节序和小端字节序，区别是大端字节序更符合人类的视觉，即高位字节存储在低位地址处，低位字节存储在高位字节处，而小端序符合计算机的口味，即高位字节存储在高位地址处，低位字节存储在低位地址处。

一般PC使用小端字节序，所以小端字节序也称为主机字节序；网络数据传送使用大端字节序，所以大端字节序也称为网络字节序。

一个简单的判断程序：

void byteorder()
{union byteorder {short value;char  union_byte[sizeof(short)];} test;test.value = 0x0102;if ((test.union_byte[0] == 1) && (test.union_byte[1] == 2))printf("Big endian\n");else if ((test.union_byte[0] == 2) && (test.union_byte[1] == 1))printf("Little endian\n");elseprintf("Unknown\n");
}

1.2 通用Socket地址

#include 
struct sockaddr
{sa_family_t sa_family; 			// 地址族类型，通常与协议族类型对应，char 		sa_data[14]; 		// Family-specific address info, 存放socket地址
};

常见的协议族（protocol family，也称domain）

AF_*与PF_*的宏都定义在bits/socket.h中，两者值相同，所以可以混用。
协议族及其地址值

由上，sockaddr 放不下多数协议族地址，而设计了下面的sockaddr_storage。

#include 
struct sockaddr_storage
{sa_famiy_t 		  sa_family;unsigned long int __ss_align;char 			  __ss_padding[128 - sizeof __ss_align];
};

不仅提供了足够大的空间用于存放地址，而且是内对齐的。

但是因为函数都传递struct sockaddr*类型参数，所以都需要进行强制类型转换，而使用domain判断具体是使用的哪一个。

1.3 专用socket地址

1.3.1 UNIX 本地域协议族

#include 
struct sockaddr_un
{sa_family_t sin_family;	// 地址族：AF_UNIXchar sun_path[108];		// 文件路径名
};

1.3.2 IPv4

struct sockaddr_in
{sa_family_t		sin_family; // 地址族：AF_INETu_int16_t 		sin_port;	// 端口号，要用网络字节序表示struct in_addr	sin_addr;	// IPv4地址结构体
};
struct in_addr 
{u_int32_t s_addr;	// IPv4 地址，要用网络字节序表示
};

1.3.3 IPv6

struct sockaddr_in6
{sa_family_t 	sin6_family;	// 地址族：AF_INET6u_int16_t 		sin6_port;		// 端口号，要用网络字节序表示u_int32_t		sin6_flowinfo;	// 流信息，应设置为0struct in6_addr	sin6_addr;		// IPv6 地址结构体u_int32_t		sin6_scope_id;	// socpe ID, 尚处于实验阶段
};
struct in6_addr
{unsigned char sa_addr[16];		// IPv6 地址，要用网络字节序表示
};

1.4 IP 地址转换函数

1.4.1 适用于 IPv4 地址

#include 
/** inet_addr: Internet address manipulation routines，converts the Internet host address cp * 		from IPv4 numbers-and-dots notation into binary  data  in  network  byte  order. * 		【用点分十进制字符串表示的IPv4 地址转化为用网络字节序整数表示的IPv4地址。】* @params* 	strptr：	IPv4地址* @return:* 	失败返回INADDR_NONE（其实是-1，但-1对应地址255.255.255.255）*/
in_addr_t inet_addr(const char* strptr);/** inet_aton: converts the Internet host address cp from the IPv4 numbers-and-dots notation*		into binary form (in network byte order) and stores it in the structure that inp points to.*      【点分十进制的IPv4地址转换为二进制形式，其实就是网络字节序的整数形式】* @params：具体形式请查看Man Pages*  cp: 		IPv4点分十进制形式* 	inp: 		IPv4网络字节序形式* @return:*  returns nonzero if the address is valid, zero if not.* 		【地址参数有效返回非0，否则返回0】*/
int inet_aton(const char* cp, struct in_addr*inp);
/** inet_ntoa: converts the Internet host address in, given in network byte order,*      to a string in IPv4 dotted-decimal notation. *     【网络字节序的IPv4地址转换为点分十进制】* @params:* 	in: 	IPv4网络字节序地址（struct in_addr）* @return:* 	char*: The string is returned in  a  statically  allocated* 	 	buffer, which subsequent calls will overwrite.* 		【IPv4点分十进制字节序，静态内存，不可重入】*/
char* inet_ntoa(struct in_addr in);

1.4.2 适用于 IPv4 和 IPv6 地址

#include 
/** inet_pton: converts  the character string src into a network address structure in the af*      address family, then copies the network address structure to dst. * 		【把src 字符串类型转换为af指定的网络地址类型，并保存在dst】* @params:* 	af: 	address family, AF_INET or AF_INET6【地址族】*  src: 	字符串类型的IP地址*  dst: 	网络字节序类型的IP地址* @return:* 	returns 1 on success (network address was successfully converted).  0 is returned*      if src does not contain a character string representing a valid network address in the speci‐*      fied address family.  If af does not contain a valid address family, -1 is returned and errno*      is set to EAFNOSUPPORT.*		【成功1，src不对0，af不对-1】*/
int inet_pton(int af, const char* src, void* dst);/** inet_ntop: converts  the  network  address structure src in the af address family into a*       character string. * @params:* 	af: 	address family, AF_INET or AF_INET6【地址族】* 	src: 	网络字节序类型的IP地址* 	dst: 	字符串类型的IP地址* 	size: 	The caller specifies the number of bytes available in this buffer in*      	the argument size.* 			【指定目标存储单元的大小】* @return:* 	On success, inet_ntop() returns a non-null pointer to dst.  * 	NULL is returned if there was an error, * 		with errno set to indicate the error.* 		【成功返回目标存储单元地址，失败NULL，并设置errno】*/	
const char *inet_ntop(int af, const void *src, char *dst, socklen_t size);

2. Socket API

因为 UNIX 哲学：所有东西都是文件，Socket 本质也是一个文件。

2.1 创建Socket

#include 
#include 
/** socket: create an endpoint for communication* 		【创建通信端点】* @params* 	domain: 	specifies a communication domain【指定协议族】* 		AF_INET(IPv4), AF_INET6(IPv6), AF_UNSPEC(IPv4或IPv6), ...* 	type: 		specifies the communication semantics【指定服务类型】* 		SOCK_STREAM, SOCK_DGRAM, ...* 	protocol: 	specifies a particular protocol to be used with the socket.【指定协议类型，一般都设置为0，表示使用默认协议】* 		IPPROTO_TCP, IPPROTO_UDP, ...* @return：* 	On success, a file descriptor for the new socket is returned.  * 		【成功，返回一个Socket文件描述符】* 	On error, -1 is returned, and errno is set appropriately.* 		【失败，返回-1，并设置errno】*/
int socket(int domain, int type, int protocol);

2.2 命名Socket

#include 
#include 
/** bind: bind a name to a socket* 		【命名socket：将一个socket与socket地址绑定】* @params:* 	sockfd: 	socket file descriptor【socket文件描述符】* 	addr: 		address【给socket绑定的地址】* 	addrlen: 	specifies the size, in bytes, of the address structure pointed to by addr.【地址长度】* @return:* 	On success, zero is returned.  * 		【成功，返回0】* 	On error, -1 is returned, and errno is set appropriately.* 		【失败，返回-1，并设置errno】*/
int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

2.3 监听Socket

#include           /* See NOTES */
#include 
/** listen: marks the socket referred to by sockfd as a passive socket, that is, as a socket*      that will be used to accept incoming connection requests using accept(2).* 		【监听socket】* @params:* 	sockfd: 	socket file descriptor，socket文件描述符* 	backlog: 	提示内核监听队列的最大长度，典型值为5* @return:* 	On success, zero is returned.  * 	On error, -1 is returned, and errno is set appropriately.*/
int listen(int sockfd, int backlog);

2.4 接受连接

#include 
#include 
/** accept: accept a connection on a socket. * 		used with connection-based socket types * 		(SOCK_STREAM, SOCK_SEQPACKET). * 		【用于有连接的类型】* @params:* 	sockfd:		socket file descriptor* 	addr:		address【被接受连接的远端socket地址】* 	addrlen:	length of address【socket地址长度】* @return:* 	On success, these system calls return a nonnegative integer * 		that is a file descriptor for the accepted socket.  * 	On error, -1 is returned, and errno is set appropriately.*/
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

2.5 发起连接

/** connect - initiate a connection on a socket* 		【初始化Socket连接，本函数会自动绑定一个未使用的端口。】* 		【不过一般也就是在Client用，Server提供的服务不会动态分配端口的。】* @params:* 	sockfd: 	socket file descriptor【Socket文件描述符，是socket()函数的返回值】* 	addr: 		address【地址】	* 	addrlen: 	the size of addr【地址的大小】* @return:* 	If the connection or binding succeeds, zero is returned.  * 		【成功，返回0】* 	On error, -1 is returned, and errno is set appropriately.* 		【失败，返回-1，并设置errno】*/
int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen)

2.6 关闭Socket

/** close: close a file descriptor* 		【关闭文件，其实就是文件I/O的close()函数，不过并非立即关闭，* 		而是文件的引用计数减一，减为0才关闭】* @params:* 	fd: 		file descriptor* @return:* 	Returns zero on success.  * 		【成功返回0】* 	On error, -1 is returned, and errno is set appropriately.* 		【失败返回-1，并设置errno】*/
#include 
int close(int fd);/** shutdown: shut down part of a full-duplex connection，立即终止连接* @params:* 	sockfd: 	socket file descriptor* 	how: * 		If how is SHUT_RD, further receptions will be disallowed.   * 			【关闭读】* 		If how is SHUT_WR, further transmissions will be disallowed. * 			【关闭写】* 		If how is SHUT_RDWR, further receptions and transmissions will be disallowed.* 			【关闭读和写】* @return:* 	On success, zero is returned.  * 	On error, -1 is returned, and errno is set appropriately.*/
#include 
int shutdown(int sockfd, int how);

2.7 数据读写

2.7.1 TCP 数据读写

#include 
#include 
/** send - transmit a message to another socket* 		【发送消息】* @params:* 	sockfd: socket file descriptor【Socket文件描述符，是socket()函数的返回值】* 	buf: 	buffer【发送数据的首地址】* 	len: 	buffer length【发送数据的长度】* 	flags: 	is the bitwise OR of zero or more of the following flags.【0：此时功能同write或以下标志】* 		MSG_CONFIRM*      MSG_DONTROUTE* 	    MSG_DONTWAIT*      MSG_EOR* 		MSG_MORE* 		MSG_NOSIGNAL*      MSG_OOB* @return:* 	On success, these calls return the number of bytes sent.  * 		【成功，返回发送的字节数，不一定发送完了bufLen】* 	On error, -1 is returned, and errno is set appropriately.* 		【失败，返回-1，并设置errno】*/
ssize_t send(int sockfd, const void *buf, size_t len, int flags);/** recv - receive a message from a socket* 		【接收消息】* @params:* 	同send() * @return:* 	These calls return the number of bytes received, or -1 if an error occurred.* 		【成功返回接收到的字节数；0,意味着通信对方关闭连接了；失败，返回-1，并设置errno】*/
ssize_t recv(int sockfd, void *buf, size_t len, int flags);

2.7.2 UDP 数据读写

#include 
#include 
/** sendto: send a message on a socket* @params:* 	sockfd: 	socket file descriptor* 	buf: 		buffer* 	len: 		length of buffer* 	flags: 		同上* 	dest_addr:* 	addrlen: * @return:*/
ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,const struct sockaddr *dest_addr, socklen_t addrlen);/** recvfrom: receive messages from a socket.* @params:* 	sockfd: socket file descriptor* 	buf: 		buffer，存储收到的信息* 	len: 		size of buffer* 	flags: 		同上* 	src_addr:* 		If src_addr is not NULL, and the underlying protocol provides the source address of the * 			message,  that  source  address  is  placed in the buffer pointed to by src_addr.* 		If  the caller is not interested in the source address,* 			 src_addr and addrlen should be specified as NULL.* 	addr_len: 	地址长度* @return:*  These calls return the number of bytes received, or -1 if an error occurred.  In the event of*      an error, errno is set to indicate the error.* 	Datagram  sockets in various domains (e.g., the UNIX and Internet domains) permit zero-length*      datagrams. When such a datagram is received, the return value is 0.* The value 0 may also be returned if the requested number of bytes to receive from a stream*     socket was 0.*/
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,struct sockaddr *src_addr, socklen_t *addrlen);

2.7.3 通用数据读写函数

#include 
/** sendmsg: send a message on a socket* @params:* 	sockfd: 	socket file descriptor* 	msg:		msghdr 结构类型指针* 				详情参见下面的struct msghdr* 	flags: 		同上* @return:*/
ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);struct msghdr {/** optional address【Socket地址】 * 	TCP协议中无用，必须设置为0*/void         *msg_name;/* * size of address【Socket地址长度】 */socklen_t     msg_namelen;    /* * scatter/gather array【分散的内存块】 * 	struct iovec封装了一块内存起始位置和长度，定义见下文 */struct iovec *msg_iov;        /* * # elements in msg_iov【分散内存块的数量】 */size_t        msg_iovlen;     /* * ancillary data, see below【指向辅助数据的起始位置】 */void         *msg_control;    /** ancillary data buffer len【辅助数据的大小】 */size_t        msg_controllen; /* * flags (unused)【复制函数中的flags参数，并在调用过程中更新 */int           msg_flags;      
};/** struct iovec: 封装了一块内存的起始位置和长度*/
struct iovec
{void* 	iov_base; 	/* 内存起始地址 */size_t	iov_len;	/* 这块内存的长度 */
};

2.8 带外标记

/** sockatmask: 判断 sockfd 是否处于带外标记，即下一个读取到的数据是否是带外数据* @params:* 	sockfd: 	socket file descriptor* @return:* 	是则返回1，可以使用带MSG_OOB标记的recv调用来接收带外数据* 	否则返回0*/
#include 
int sockatmask(int sockfd);

2.9 地址信息函数

#include 
/** getsockname: get socket name. * 		the current address to which the socket sockfd is bound, * 		in the buffer pointed to by addr. * 		【获取sockfd对应的本端socket地址，并将其存储在addr指定的内存中】* @params:* 	sockfd: 	socket file descriptor* 	addr: 		address of the peer connected to the socket sockfd* 	addrlen: 	indicate the amount of space pointed to by addr. * @return:* 	On success, zero is returned.  * 	On error, -1 is returned, and errno is set appropriately.*/
int getsockname(int sockfd, struct sockaddr *addr, socklen_t *addrlen);/** getpeername: get name of connected peer socket. * 		the address of the peer connected to the socket sockfd, * 		in the buffer pointed to by addr. * 		【获取sockfd对应的远端socket地址，并将其存储在addr指定的内存中】* @params:* 	sockfd: 	socket file descriptor* 	addr: 		address of the peer connected to the socket sockfd* 	addrlen: 	indicate the amount of space pointed to by addr. * @return:* 	On success, zero is returned.  * 	On error, -1 is returned, and errno is set appropriately.*/
int getpeername(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

2.10 Socket 选项

#include           /* See NOTES */
#include 
/* * getsockopt - get options on sockets* @params:* 	sockfd:socket file descriptor* 	level: 		指定要操作哪个协议的选项（即属性），如IPv4、IPv6、TCP等* 	optname: 	指定选项的名字见：表 socket选项* 	optval: 	值* 	optlen: 	值的长度* @return:* 	On success, zero is returned for the standard options.  * 	On error, -1 is returned, and errno is set appropriately.*  Netfilter allows the programmer to define custom socket options with associated handlers; * 		for such options, the return value on success is the value returned by the handler.*/
int getsockopt(int sockfd, int level, int optname,void *optval, socklen_t *optlen);/** setsockopt - set options on sockets* @params:* 	sockfd:socket file descriptor* 	level: 		指定要操作哪个协议的选项（即属性），如IPv4、IPv6、TCP等* 	optname: 	指定选项的名字见：表 socket选项。* 	optval: 	值* 	optlen: 	值的长度* @return:* 	On success, zero is returned for the standard options.  * 	On error, -1 is returned, and errno is set appropriately.*  Netfilter allows the programmer to define custom socket options with associated handlers;* 		for such options, the return value on success is the value returned by the handler.* 	*/
int setsockopt(int sockfd, int level, int optname,const void *optval, socklen_t optlen);

2.10.1 SO_REUSEADDR 选项

强制使用被处于TIME_WAIT状态的连接占用的socket地址。示例：

......
int sockfd = socket(PF_INET, SOCK_STREAM, 0);
if (scokfd < 0)
{perror("socket()");exit(1);
}int reuse = 1;
setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof reuse);struct sockaddr_in address;
memset(&address, 0, sizeof address);
address.sin_family = AF_INET;
inet_pton(AF_INET, ip, &address.sin_addr);
address.sin_port = htons(port);
int ret = bind(sock, (struct sockaddr*) &address, sizeof address);
......

2.10.2 SO_RCVBUF 和 SO_SNDBUF 选项

分别表示TCP接收缓冲区和发送缓冲区的大小。
TCP接收缓冲区最小值是256字节，发送缓冲区最小值是2048字节。

2.10.3 SO_RCVLOWAT 和 SO_SNDLOWAT 选项

分别表示TCP接收缓冲区和发送缓冲区的低水位标记。
被I/O复用系统调用用来判断socket是否可读或可写。

2.10.4 SO_LINGER 选项

控制close系统调用再关闭TCP连接时的行为。此时需要传递一个linger类型的结构体
linger类型的结构体

#include 
struct linger
{int	l_onoff; 	/* 开启（非0）还是关闭（0）该选项 */int l_linger; 	/* 滞留时间 */
};

• l_onoff 等于 0。此时SO_LINGER 选项不起作用，close用默认行为来关闭socket
• l_onoff不为 0，l_linger等于 0。此时close系统调用立即返回，TCP模块将丢弃被关闭的socket对应的TCP发送缓冲区中残留的数据，同时给对方发送一个复位报文段。因此这种情况给服务器提供了一个异常终止一个连接的方法。
• l_onoff不为 0，l_linger大于 0。此时close的行为取决于两个条件：
  • 一是被关闭的socket对应的TCP发送缓冲区中是否还有残留的数据；
  • 二是该socket是阻塞的还是非阻塞的。
    对于阻塞的socket，close将等待一段长为l_linger的时间，知道TCP模块发送完所有残留数据并得到对方的确认。如果这段时间内TCP模块没有发送完残留书并得到对方的确认，那么close系统调用将返回-1并设置errno位EWOULDBLOCK。如果socket非阻塞的，close将立即返回，此时我们需要根据其返回值和errno来判断残留数据是否已经发送完毕。

3 网络信息API

3.1 gethostbyname 和 gethostbyaddr

但是实际上这两个函数已经 obsolete。所以不再详细介绍了，推荐使用 getaddrinfo(3), getnameinfo(3), 和gai_strerror(3)。

#include 
struct hostent *gethostbyname(const char *name);
#include        /* for AF_INET */
struct hostent *gethostbyaddr(const void *addr,socklen_t len, int type);

3.2 getservbyname 和 getservbyport

#include 
/** getservbyname: returns a servent structure for the entry from the database * 		that matches the service name using protocol proto.* 		【根据名称获取某个服务的完整信息】* @params:* 	name: 		service name，目标服务名称* 	proto: 		protocol，服务类型* @return:* 	return a pointer to a statically allocated servent structure, or NULL if an*      error occurs or the end of the file is reached.* 	返回servent结构类型*/
struct servent *getservbyname(const char *name, const char *proto);/** getservbyport: returns  a  servent structure for the entry from the database * 		that matches the port port (given in network byte order) using protocol proto.* 		【根据端口获取某个服务的完整信息】* @params:* 	port: 		端口* 	proto: 		协议* @return:* 	return a pointer to a statically allocated servent structure, * 		or NULL if an error occurs or the end of the file is reached.*/
struct servent *getservbyport(int port, const char *proto);struct servent {char  *s_name;       /* official service name */char **s_aliases;    /* alias list */int    s_port;       /* port number */char  *s_proto;      /* protocol to use */
};

3.3 getaddrinfo

#include 
#include 
#include /** getaddrinfo: Given  node  and  service,  which identify an Internet host and a service, * 		getaddrinfo() returns one or more addrinfo structures, each of which* 		contains an Internet address that can be specified in a call to bind(2) or connect(2).* @params:* 	node: 		identify an Internet host【可以是主机名，也可以是IP地址】* 	service: 	identify an Internet service【接收服务名或者十进制端口号】* 	hints: 		points to an addrinfo structure that specifies criteria for * 				selecting the socket address structures returned in the list pointed to by res. * 				【提示，对getaddrinfo的输出进行更精确的控制，可以为NULL，表示允许getaddrinfo反馈任何可用的结果】* 	res: 		result, 【函数反馈的地址结果，可能不止一个】* @return:* 	On success, 0 is returned, and node and service names, if requested, are filled with * 		null-terminated strings, possibly  truncated  to  fit  the specified buffer lengths.  * 	On error, one of the following nonzero error codes is returned:* 		EAI_AGAIN		The name could not be resolved at this time.  Try again later.【调用临时失败，提示应用程序过后再试】*  	EAI_BADFLAGS	The flags argument has an invalid value.* 						【非法的ai_flags值】*		EAI_FAIL        A nonrecoverable error occurred.*						【名称解析失败】*		EAI_FAMILY      The address family was not recognized, or the address length *						was invalid for the specified family.*						【不支持的ai_family参数】*      EAI_MEMORY      Out of memory.* 						【内存分配失败】*      EAI_NONAME      The name does not resolve for the supplied arguments.  * 						NI_NAMEREQD is set and the host's name cannot be located, * 						or neither hostname nor service name were requested.* 						【非法的主机名或服务名】*      EAI_OVERFLOW    The buffer pointed to by host or serv was too small.* 						【用户提供的缓冲区溢出。仅发生在getnameinfo中】* 		EAI_SERVICE		【没有支持的服务，比如用数据报服务类型来查找ssh服务。因为ssh服务只能使用流服务】* 		EAI_SOCKTYPE	【不支持的服务类型。如果hints.ai_socktype和hints.ai_protocol不一致，则会触发此类错误】*      EAI_SYSTEM      A system error occurred.  The error code can be found in errno.* 						【系统错误，错误值存储在errno中】*/
int getaddrinfo(const char *node, const char *service,const struct addrinfo *hints, struct addrinfo **res);struct addrinfo {int              ai_flags;		/* 取值OR（按位或）或者表 ai_flags成员*/int              ai_family;		/* 地址族 */int              ai_socktype;	/* 服务类型，SOCK_STREAM或SOCK_DGRAM */int              ai_protocol;	/* 指具体的网络协议，通常为0 */socklen_t        ai_addrlen;	/* socket地址ai_addr的长度 */struct sockaddr *ai_addr;		/* 指向socket地址 */char            *ai_canonname;	/* 主机的别名 */struct addrinfo *ai_next;		/* 指向下一个sockinfo结构的对象 */
};

3.4 getnameinfo

#include 
#include /** getnameinfo: the inverse of getaddrinfo(3): it converts a socket address * 		to a corresponding host and service, in a protocol-independent manner.* 		【通过socket地址同时获得以字符串表示的主机名和服务名】* @params:* 	addr: 地址* 	addrlen: 	地址长度* 	host: 		保存返回的主机名* 	hostlen: 	主机名长度* 	serv: 		保存返回的服务名* 	servlen: 	服务名长度* 	flags: 		见表 flags参数* @return:* 	同getaddrinfo()*/
int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,char *host, socklen_t hostlen,char *serv, socklen_t servlen, int flags);

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