socket编程头文件分析

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[];       /* 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[];
};

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 = ,    /* Dummy protocol for TCP.  */
#define IPPROTO_IP      IPPROTO_IP
    IPPROTO_ICMP = ,      /* Internet Control Message Protocol.  */
#define IPPROTO_ICMP        IPPROTO_ICMP
    IPPROTO_IGMP = ,      /* Internet Group Management Protocol. */
#define IPPROTO_IGMP        IPPROTO_IGMP
    IPPROTO_IPIP = ,      /* IPIP tunnels (older KA9Q tunnels use 94).  */
#define IPPROTO_IPIP        IPPROTO_IPIP
    IPPROTO_TCP = ,       /* Transmission Control Protocol.  */
#define IPPROTO_TCP     IPPROTO_TCP
    IPPROTO_EGP = ,       /* Exterior Gateway Protocol.  */
#define IPPROTO_EGP     IPPROTO_EGP
    IPPROTO_PUP = ,      /* PUP protocol.  */
#define IPPROTO_PUP     IPPROTO_PUP
    IPPROTO_UDP = ,      /* User Datagram Protocol.  */
#define IPPROTO_UDP     IPPROTO_UDP
    IPPROTO_IDP = ,      /* XNS IDP protocol.  */

    IPPROTO_IPV6 = ,     /* 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 = ,        /* Echo service.  */
    IPPORT_DISCARD = ,     /* Discard transmissions service.  */
    IPPORT_SYSTAT = ,     /* System status service.  */
    IPPORT_DAYTIME = ,    /* Time of day service.  */
    IPPORT_NETSTAT = ,    /* Network status service.  */
    IPPORT_FTP = ,        /* File Transfer Protocol.  */
    IPPORT_TELNET = ,     /* Telnet protocol.  */
    IPPORT_SMTP = ,       /* Simple Mail Transfer Protocol.  */
    IPPORT_TIMESERVER = , /* Timeserver service.  */
    IPPORT_NAMESERVER = , /* Domain Name Service.  */
    IPPORT_WHOIS = ,      /* Internet Whois service.  */
    IPPORT_MTP = ,

    IPPORT_TFTP = ,       /* Trivial File Transfer Protocol.  */
    IPPORT_RJE = ,
    IPPORT_FINGER = ,     /* Finger service.  */
    IPPORT_TTYLINK = ,
    IPPORT_SUPDUP = ,     /* SUPDUP protocol.  */

    IPPORT_EXECSERVER = ,    /* execd service.  */
    IPPORT_LOGINSERVER = ,   /* rlogind service.  */
    IPPORT_CMDSERVER = ,
    IPPORT_EFSSERVER = ,

    /* UDP ports.  */
    IPPORT_BIFFUDP = ,
    IPPORT_WHOSERVER = ,
    IPPORT_ROUTESERVER = ,

    /* Ports less than this value are reserved for privileged processes.  */
    IPPORT_RESERVED = ,

    /* Ports greater this value are reserved for (non-privileged) servers.  */
    IPPORT_USERRESERVED =
  };

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[] /* 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);

6. sys/un.h--unix域头文件

un.h中包含Unix域套接字的地址结构体sockaddr_un。

#include <sys/un.h>

#define UNIX_PATH_MAX   108

struct sockaddr_un {
    __kernel_sa_family_t sun_family; /* AF_UNIX */
    char sun_path[UNIX_PATH_MAX];   /* pathname */
};

实际地址长度为：offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1