在socket网络编程中经常用到一些宏定义、结构和函数，这些经常包含在相关的头文件中，使用时直接include相关头文件即可。下面简单描述下相关的一些结构及头文件。
1. sockaddr / bits/socket.h
socket编程最基本的就是socket地址，其定义在bits/socket.h中。但sys/socket.h包含bits/socket.h，且bits/socket.h明确指出该头文件不能直接使用，应使用sys/socket.h代替。
struct sockaddr
{
__SOCKADDR_COMMON (sa_); /* Common data: address family and length. */
char sa_data[14]; /* Address data. */
};
其中__SOCKADDR_COMMON()宏定义在bits/sockaddr.h中
#define __SOCKADDR_COMMON(sa_prefix) \
sa_family_t sa_prefix##family
#define __SOCKADDR_COMMON_SIZE (sizeof (unsigned short int))
替换后为
struct sockaddr{
unsigned short sa_family;
char sa_data[14];
};
在bits/socket.h中还定义了协议族（Protocol families，PF_xxx）和地址族（Address families，AF_xx），两者一一对应，因此可以互换，应用场合根据概念稍有不同。
#define PF_UNSPEC 0 /* Unspecified. */
#define PF_LOCAL 1 /* Local to host (pipes and file-domain). */
#define PF_UNIX PF_LOCAL /* POSIX name for PF_LOCAL. */
#define PF_FILE PF_LOCAL /* Another non-standard name for PF_LOCAL. */
#define PF_INET 2 /* IP protocol family. */
#define PF_AX25 3 /* Amateur Radio AX.25. */
#define PF_IPX 4 /* Novell Internet Protocol. */
#define PF_APPLETALK 5 /* Appletalk DDP. */
#define PF_INET6 10 /* IP version 6. */
#define PF_LLC 26 /* Linux LLC. */
#define PF_CAN 29 /* Controller Area Network. */
#define PF_NFC 39 /* NFC sockets. */
#define PF_VSOCK 40 /* vSockets. */
#define PF_MAX 41 /* For now.. */
2. sys/socket.h
该头文件包含常用的socket编程函数接口，如：
socket() bind() listen() connect() accept() shutdown()
send() recv() sendto() recvfrom() sendmsg() recvmsg() sendmmsg() recvmmsg()
socketpair() getsockname() getpeername()
getsockopt() setsockopt()
除此外，sys/socket.h包含bits/socket.h，需要bits/socket.h的函数直接include sys/socket.h即可。
3. 网络socket地址 sockaddr_in -- netinet/in.h
与internet网络相关的头文件大部分在netinet目录下，如in.h ip.h(ip协议相关数据结构） tcp.h（tcp协议） udp.h（udp协议） ip_icmp.h（icmp协议） ip6.h icmp6.h等。
在网络编程中用到的socket地址是经过转换的网络socket地址sockaddr_in:
/* Structure describing an Internet socket address. */
struct sockaddr_in
{
__SOCKADDR_COMMON (sin_); /* unsigned short sin_family; */
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)];
};
网络地址
/* Internet address. */
typedef uint32_t in_addr_t;
struct in_addr
{
in_addr_t s_addr;
};
网络端口
/* Type to represent a port. */
typedef uint16_t in_port_t;
据此也可推断出：唯一确定一个连接（socket）的五要素：目的IP，目的端口，源IP，源端口，协议。
经典的4个主机网络字节序转换函数
/* 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. */
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)
__THROW __attribute__ ((__const__));
netinet/in.h定义了标准协议enum：
enum
{
IPPROTO_IP = 0, /* Dummy protocol for TCP. */
#define IPPROTO_IP IPPROTO_IP
IPPROTO_ICMP = 1, /* Internet Control Message Protocol. */
#define IPPROTO_ICMP IPPROTO_ICMP
IPPROTO_IGMP = 2, /* Internet Group Management Protocol. */
#define IPPROTO_IGMP IPPROTO_IGMP
IPPROTO_IPIP = 4, /* IPIP tunnels (older KA9Q tunnels use 94). */
#define IPPROTO_IPIP IPPROTO_IPIP
IPPROTO_TCP = 6, /* Transmission Control Protocol. */
#define IPPROTO_TCP IPPROTO_TCP
IPPROTO_EGP = 8, /* Exterior Gateway Protocol. */
#define IPPROTO_EGP IPPROTO_EGP
IPPROTO_PUP = 12, /* PUP protocol. */
#define IPPROTO_PUP IPPROTO_PUP
IPPROTO_UDP = 17, /* User Datagram Protocol. */
#define IPPROTO_UDP IPPROTO_UDP
IPPROTO_IDP = 22, /* XNS IDP protocol. */
IPPROTO_IPV6 = 41, /* IPv6 header. */
#define IPPROTO_IPV6 IPPROTO_IPV6
IPPROTO_RAW = 255, /* Raw IP packets. */
#define IPPROTO_RAW IPPROTO_RAW
IPPROTO_MAX}；
标准端口enum
enum
{
IPPORT_ECHO = 7, /* Echo service. */
IPPORT_DISCARD = 9, /* Discard transmissions service. */
IPPORT_SYSTAT = 11, /* System status service. */
IPPORT_DAYTIME = 13, /* Time of day service. */
IPPORT_NETSTAT = 15, /* Network status service. */
IPPORT_FTP = 21, /* File Transfer Protocol. */
IPPORT_TELNET = 23, /* Telnet protocol. */
IPPORT_SMTP = 25, /* Simple Mail Transfer Protocol. */
IPPORT_TIMESERVER = 37, /* Timeserver service. */
IPPORT_NAMESERVER = 42, /* Domain Name Service. */
IPPORT_WHOIS = 43, /* Internet Whois service. */
IPPORT_MTP = 57,
IPPORT_TFTP = 69, /* Trivial File Transfer Protocol. */
IPPORT_RJE = 77,
IPPORT_FINGER = 79, /* Finger service. */
IPPORT_TTYLINK = 87,
IPPORT_SUPDUP = 95, /* SUPDUP protocol. */
IPPORT_EXECSERVER = 512, /* execd service. */
IPPORT_LOGINSERVER = 513, /* rlogind service. */
IPPORT_CMDSERVER = 514,
IPPORT_EFSSERVER = 520,
/* UDP ports. */
IPPORT_BIFFUDP = 512,
IPPORT_WHOSERVER = 513,
IPPORT_ROUTESERVER = 520,
/* Ports less than this value are reserved for privileged processes. */
IPPORT_RESERVED = 1024,
/* Ports greater this value are reserved for (non-privileged) servers. */
IPPORT_USERRESERVED = 5000
};
ABCD四类网络，常用IP地址
#define IN_CLASSA(a) ((((in_addr_t)(a)) & 0x80000000) == 0)
#define IN_CLASSB(a) ((((in_addr_t)(a)) & 0xc0000000) == 0x80000000)
#define IN_CLASSC(a) ((((in_addr_t)(a)) & 0xe0000000) == 0xc0000000)
#define IN_CLASSD(a) ((((in_addr_t)(a)) & 0xf0000000) == 0xe0000000)
/* Address to accept any incoming messages. */
#define INADDR_ANY ((in_addr_t) 0x00000000)
/* Address to send to all hosts. */
#define INADDR_BROADCAST ((in_addr_t) 0xffffffff)
/* Address indicating an error return. */
#define INADDR_NONE ((in_addr_t) 0xffffffff)
/* Network number for local host loopback. */
#define IN_LOOPBACKNET 127
还有IPv6、filter相关结构及函数。
4. 网络地址二进制与字符串转换 -- arpa/inet.h
推荐使用inet_pton()或inet_aton(),而非inet_addr()或inet_network(),因为返回值-1代表IP地址255.255.255.255。
extern int inet_aton (const char *__cp, struct in_addr *__inp) __THROW;
extern char *inet_ntoa (struct in_addr __in) __THROW;
/* Convert from presentation format of an Internet number in buffer
starting at CP to the binary network format and store result for
interface type AF in buffer starting at BUF. */
extern int inet_pton (int __af, const char *__restrict __cp,
void *__restrict __buf) __THROW;
/* Convert a Internet address in binary network format for interface
type AF in buffer starting at CP to presentation form and place
result in buffer of length LEN astarting at BUF. */
extern const char *inet_ntop (int __af, const void *__restrict __cp,
char *__restrict __buf, socklen_t __len) __THROW;
#define servip "192.168.1.103"
inet_pton(AF_INET, servip, &servaddr.sin_addr.s_addr);
实现二进制网络ip与字符串ip之间的转换，均以inet开头。
此外arpa还包含ftp.h tftp.h telnet.h nameser.h nameser_compat.h。
5. netdb.h -- network data base library
/* All data returned by the network data base library are supplied in
host order and returned in network order (suitable for use in
system calls). */
提供主机/域名等结构体或函数。
/* Description of data base entry for a single host. */
struct hostent
{
char *h_name; /* Official name of host. */
char **h_aliases; /* Alias list. */
int h_addrtype; /* Host address type. */
int h_length; /* Length of address. */
char **h_addr_list; /* List of addresses from name server. */
#if defined __USE_MISC || defined __USE_GNU
# define h_addr h_addr_list[0] /* Address, for backward compatibility.*/
#endif
};
/* Description of data base entry for a single service. */
struct servent
{
char *s_name; /* Official service name. */
char **s_aliases; /* Alias list. */
int s_port; /* Port number. */
char *s_proto; /* Protocol to use. */
};
/* Description of data base entry for a single service. */
struct protoent
{
char *p_name; /* Official protocol name. */
char **p_aliases; /* Alias list. */
int p_proto; /* Protocol number. */
};
提供的常用函数：
extern struct hostent *gethostent (void);
extern struct hostent *gethostbyaddr (const void *__addr, __socklen_t __len,
int __type);
extern struct hostent *gethostbyname (const char *__name);
extern struct servent *getservbyname (const char *__name, const char *__proto);
extern struct servent *getservbyport (int __port, const char *__proto);
extern struct protoent *getprotobyname (const char *__name);
extern struct protoent *getprotobynumber (int __proto);