第十三夜：网络编程1-网络协议及其数据结构

C语言的30个夜晚 14 分钟 读完 (约 2038 字)

没有人逼你学C语言，但它在复杂的计算世界中，曲径通幽。

socket相关的数据结构

sockaddr_in
sockaddr_in结构体定义在/usr/include/netinet/in.h中，它的定义是这样的：

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/* Structure describing an Internet socket address.  */
struct sockaddr_in
  {
    __SOCKADDR_COMMON (sin_);
    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)];
  };

这个结构体中又涉及到另外几个类型和宏定义，包括__SOCKADDR_COMMON、in_port_t、in_addr。

__SOCKADDR_COMMON

__SOCKADDR_COMMON定义在bits/sockaddr.h当中，它的作用是将sin_和family参数拼接为对应的sa_family_t类型的地址家族。

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/* POSIX.1g specifies this type name for the `sa_family' member.  */
typedef unsigned short int sa_family_t;

/* This macro is used to declare the initial common members
   of the data types used for socket addresses, `struct sockaddr',
   `struct sockaddr_in', `struct sockaddr_un', etc.  */

#define    __SOCKADDR_COMMON(sa_prefix) \
  sa_family_t sa_prefix##family

也就是说这个预处理宏定义的意思是将sin_和family拼接在一起构成sa_family_t类型的sin_family。

sa_family_t就是短整形，长度16 bits的整数。

所以sockaddr_in结构体的第一个元素其实是sa_family_t类型的sin_family参数。

in_port_t

in_port_t实际上是uint16_t，16位的无符号整数。

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typedef uint16_t in_port_t;

uint16_t其实就是__uint16_t，表示unsigned short int，它的长度是2个字节，能表示的范围是0-65535。

in_addr

这个显而易见，就是表示一个IPv4的地址：

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

而uint32_t也就是__uint32_t，表示unsigned int，一共4个字节。IP地址一般是拆成点分十进制看的，一个字节能表示的范围是0-255，所以的in_addr也就是能表示成0.0.0.0 ～ 255.255.255.255。

sin_zero

sin_zero的相关定义：

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/* Structure describing an Internet (IP) socket address. */
#if  __UAPI_DEF_SOCKADDR_IN
#define __SOCK_SIZE__    16        /* sizeof(struct sockaddr)    */
struct sockaddr_in {
  __kernel_sa_family_t    sin_family;    /* Address family        */
  __be16        sin_port;    /* Port number            */
  struct in_addr    sin_addr;    /* Internet address        */

  /* Pad to size of `struct sockaddr'. */
  unsigned char        __pad[__SOCK_SIZE__ - sizeof(short int) -
            sizeof(unsigned short int) - sizeof(struct in_addr)];
};
#define sin_zero    __pad        /* for BSD UNIX comp. -FvK    */
#endif

__pad声明了一个长素为8个字节的字符数组。

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__SOCK_SIZE__ - sizeof(short int) - sizeof(unsigned short int) - sizeof(struct in_addr)
= 16 - 2 - 2 - 4 = 8

sin_family的类型

在C语言实现网络通信中，最开始需要做的就是声明并且初始化sockaddr_in结构体。
其中sin_family的类型有这些：

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/* Protocol families.  */
#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_NETROM    6    /* Amateur radio NetROM.  */
#define PF_BRIDGE    7    /* Multiprotocol bridge.  */
#define PF_ATMPVC    8    /* ATM PVCs.  */
#define PF_X25        9    /* Reserved for X.25 project.  */
#define PF_INET6    10    /* IP version 6.  */
#define PF_ROSE        11    /* Amateur Radio X.25 PLP.  */
#define PF_DECnet    12    /* Reserved for DECnet project.  */
#define PF_NETBEUI    13    /* Reserved for 802.2LLC project.  */
#define PF_SECURITY    14    /* Security callback pseudo AF.  */
#define PF_KEY        15    /* PF_KEY key management API.  */
#define PF_NETLINK    16
#define PF_ROUTE    PF_NETLINK /* Alias to emulate 4.4BSD.  */
#define PF_PACKET    17    /* Packet family.  */
#define PF_ASH        18    /* Ash.  */
#define PF_ECONET    19    /* Acorn Econet.  */
#define PF_ATMSVC    20    /* ATM SVCs.  */
#define PF_RDS        21    /* RDS sockets.  */
#define PF_SNA        22    /* Linux SNA Project */
#define PF_IRDA        23    /* IRDA sockets.  */
#define PF_PPPOX    24    /* PPPoX sockets.  */
#define PF_WANPIPE    25    /* Wanpipe API sockets.  */
#define PF_LLC        26    /* Linux LLC.  */
#define PF_IB        27    /* Native InfiniBand address.  */
#define PF_MPLS        28    /* MPLS.  */
#define PF_CAN        29    /* Controller Area Network.  */
#define PF_TIPC        30    /* TIPC sockets.  */
#define PF_BLUETOOTH    31    /* Bluetooth sockets.  */
#define PF_IUCV        32    /* IUCV sockets.  */
#define PF_RXRPC    33    /* RxRPC sockets.  */
#define PF_ISDN        34    /* mISDN sockets.  */
#define PF_PHONET    35    /* Phonet sockets.  */
#define PF_IEEE802154    36    /* IEEE 802.15.4 sockets.  */
#define PF_CAIF        37    /* CAIF sockets.  */
#define PF_ALG        38    /* Algorithm sockets.  */
#define PF_NFC        39    /* NFC sockets.  */
#define PF_VSOCK    40    /* vSockets.  */
#define PF_KCM        41    /* Kernel Connection Multiplexor.  */
#define PF_QIPCRTR    42    /* Qualcomm IPC Router.  */
#define PF_SMC        43    /* SMC sockets.  */
#define PF_MAX        44    /* For now..  */


/* Address families.  */
#define AF_UNSPEC    PF_UNSPEC
#define AF_LOCAL    PF_LOCAL
#define AF_UNIX        PF_UNIX
#define AF_FILE        PF_FILE
#define AF_INET        PF_INET
#define AF_AX25        PF_AX25
#define AF_IPX        PF_IPX
#define AF_APPLETALK    PF_APPLETALK
#define AF_NETROM    PF_NETROM
#define AF_BRIDGE    PF_BRIDGE
#define AF_ATMPVC    PF_ATMPVC
#define AF_X25        PF_X25
#define AF_INET6    PF_INET6
#define AF_ROSE        PF_ROSE
#define AF_DECnet    PF_DECnet
#define AF_NETBEUI    PF_NETBEUI
#define AF_SECURITY    PF_SECURITY
#define AF_KEY        PF_KEY
#define AF_NETLINK    PF_NETLINK
#define AF_ROUTE    PF_ROUTE
#define AF_PACKET    PF_PACKET
#define AF_ASH        PF_ASH
#define AF_ECONET    PF_ECONET
#define AF_ATMSVC    PF_ATMSVC
#define AF_RDS        PF_RDS
#define AF_SNA        PF_SNA
#define AF_IRDA        PF_IRDA
#define AF_PPPOX    PF_PPPOX
#define AF_WANPIPE    PF_WANPIPE
#define AF_LLC        PF_LLC
#define AF_IB        PF_IB
#define AF_MPLS        PF_MPLS
#define AF_CAN        PF_CAN
#define AF_TIPC        PF_TIPC
#define AF_BLUETOOTH    PF_BLUETOOTH
#define AF_IUCV        PF_IUCV
#define AF_RXRPC    PF_RXRPC
#define AF_ISDN        PF_ISDN
#define AF_PHONET    PF_PHONET
#define AF_IEEE802154    PF_IEEE802154
#define AF_CAIF        PF_CAIF
#define AF_ALG        PF_ALG
#define AF_NFC        PF_NFC
#define AF_VSOCK    PF_VSOCK
#define AF_KCM        PF_KCM
#define AF_QIPCRTR    PF_QIPCRTR
#define AF_SMC        PF_SMC
#define AF_MAX        PF_MAX

其中最常用的AF_INET就是PF_INET，也就2所代表的IP协议家族。

sin_addr的赋值
sin_addr需要一个IPv4的地址作为值，in.h中定义了这样几个常用值，其中包括本地地址、多拨地址等：

写服务端socket最常用的INADDR_ANY地址就是((unsigned long int) 0x00000000)，即 0.0.0.0：

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/* Address to accept any incoming messages. */
#define    INADDR_ANY        ((unsigned long int) 0x00000000)

/* Address to send to all hosts. */
#define    INADDR_BROADCAST    ((unsigned long int) 0xffffffff)

/* Address indicating an error return. */
#define    INADDR_NONE        ((unsigned long int) 0xffffffff)

/* Network number for local host loopback. */
#define    IN_LOOPBACKNET        127

/* Address to loopback in software to local host.  */
#define    INADDR_LOOPBACK        0x7f000001    /* 127.0.0.1   */
#define    IN_LOOPBACK(a)        ((((long int) (a)) & 0xff000000) == 0x7f000000)

/* Defines for Multicast INADDR */
#define INADDR_UNSPEC_GROUP       0xe0000000U    /* 224.0.0.0   */
#define INADDR_ALLHOSTS_GROUP     0xe0000001U    /* 224.0.0.1   */
#define INADDR_ALLRTRS_GROUP    0xe0000002U    /* 224.0.0.2 */
#define INADDR_MAX_LOCAL_GROUP  0xe00000ffU    /* 224.0.0.255 */
#endif

socket的类型
关于socket的类型，在bits/socket_type.h中定义：

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/* Types of sockets.  */
enum __socket_type
{
  SOCK_STREAM = 1,        /* Sequenced, reliable, connection-based
                   byte streams.  */
#define SOCK_STREAM SOCK_STREAM
  SOCK_DGRAM = 2,        /* Connectionless, unreliable datagrams
                   of fixed maximum length.  */
#define SOCK_DGRAM SOCK_DGRAM
  SOCK_RAW = 3,            /* Raw protocol interface.  */
#define SOCK_RAW SOCK_RAW
  SOCK_RDM = 4,            /* Reliably-delivered messages.  */
#define SOCK_RDM SOCK_RDM
  SOCK_SEQPACKET = 5,        /* Sequenced, reliable, connection-based,
                   datagrams of fixed maximum length.  */
#define SOCK_SEQPACKET SOCK_SEQPACKET
  SOCK_DCCP = 6,        /* Datagram Congestion Control Protocol.  */
#define SOCK_DCCP SOCK_DCCP
  SOCK_PACKET = 10,        /* Linux specific way of getting packets
                   at the dev level.  For writing rarp and
                   other similar things on the user level. */
#define SOCK_PACKET SOCK_PACKET

  /* Flags to be ORed into the type parameter of socket and socketpair and
     used for the flags parameter of paccept.  */

  SOCK_CLOEXEC = 02000000,    /* Atomically set close-on-exec flag for the
                   new descriptor(s).  */
#define SOCK_CLOEXEC SOCK_CLOEXEC
  SOCK_NONBLOCK = 00004000    /* Atomically mark descriptor(s) as
                   non-blocking.  */
#define SOCK_NONBLOCK SOCK_NONBLOCK
};

socket相关的函数

主要是定义在sys/socket.h中的以下函数：

  • socket
  • bind
  • listen
  • accept
  • connect

关于这些函数的介绍暂不赘述，sys/socket.h里写的很详细。这里用一个socket基础的例子来演示其用法：

服务端server.c:

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/*
* 源码出处：https://www.cnblogs.com/uestc-mm/p/7630145.html
* 编译：gcc -m32 -o server server.c
*/

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(int argc, char *argv[]){
    int fd, new_fd, struct_len, numbytes;
    struct sockaddr_in server_addr; // 定义在netinet/in.h
    struct sockaddr_in client_addr;
    char buff[BUFSIZ];//BUFSIZ定义在stdio.h，默认8192

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8000);
    server_addr.sin_addr.s_addr = INADDR_ANY;
    bzero(&(server_addr.sin_zero), 8);
    struct_len = sizeof(struct sockaddr_in);

    fd = socket(AF_INET, SOCK_STREAM, 0); // 创建一个TCP类型的socket
    
    while(bind(fd, (struct sockaddr *)&server_addr, struct_len) == -1);
        printf("Bind Success!\n");
        
    while(listen(fd, 10) == -1);
        printf("Listening(%d)....\n", fd);
        
    printf("Ready for Accept...\n");
    
    while( new_fd = accept(fd, (struct sockaddr *)&client_addr, &struct_len) ){
        printf("Get the New Client: %d\n", new_fd);
    
        numbytes = send(new_fd,"Welcome to my server\n",21,0);
        
        while((numbytes = recv(new_fd, buff, BUFSIZ, 0)) > 0)
        {
            buff[numbytes] = '\0';
            printf("%s\n",buff);
                if(send(new_fd,buff,numbytes,0)<0)
                {
                    perror("write");
                    return 1;
                }
        }
        close(new_fd);
    }
    
    close(fd);
    return 0;
}

客户端client.c：

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/*
* 源码出处：https://www.cnblogs.com/uestc-mm/p/7630145.html
* 编译：gcc -m32 -o client client.c
*/
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(int argc,char *argv[])
{
    int sockfd,numbytes;
    char buf[BUFSIZ];
    struct sockaddr_in their_addr;
    printf("break!");
    while((sockfd = socket(AF_INET,SOCK_STREAM,0)) == -1);
    printf("We get the sockfd~\n");
    their_addr.sin_family = AF_INET;
    their_addr.sin_port = htons(8000);
    their_addr.sin_addr.s_addr=inet_addr("127.0.0.1");
    bzero(&(their_addr.sin_zero), 8);
    
    while(connect(sockfd,(struct sockaddr*)&their_addr,sizeof(struct sockaddr)) == -1);
    printf("Get the Server~Cheers!\n");
    numbytes = recv(sockfd, buf, BUFSIZ,0);//接收服务器端信息  
    buf[numbytes]='\0';  
    printf("%s",buf);
    while(1)
    {
        printf("Entersome thing:");
        scanf("%s",buf);
        numbytes = send(sockfd, buf, strlen(buf), 0);
            numbytes=recv(sockfd,buf,BUFSIZ,0);  
        buf[numbytes]='\0'; 
        printf("received:%s\n",buf);  
    }
    close(sockfd);
    return 0;
}

参考资料

Linux下C语言的socket网络编程
socketpair的用法和理解
进程间通信：管道和socketpair的区别

#编程 #C语言
第十四夜：网络编程2-Socket通信 第十二夜：使用C语言探索堆栈
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