socket网络编程快速上手（二）——细节问题（2）

2.TCP数据包接收问题

对初学者来说，很多都会认为：客户端与服务器最终的打印数据接收或者发送条数都该是一致的，1000条发送打印，1000条接收打印，长度都为1000。但是，事实上并不是这样，发送打印基本不会有什么问题（只是一般情况，如果发生调度或者其他情况，有可能导致差别，因此也要注意封装），接收打印却不是固定的，下面是测试代码：

测试客户端程序：

 #include <stdio.h>

 #include <stdlib.h>

 #include <unistd.h>

 #include <string.h>

 #include <sys/types.h>

 #include <sys/socket.h>

 #include <netinet/in.h>

 #include <netdb.h>

 #define  PORT        1234

 #define  MAXDATASIZE 1000

 int main(int argc, char *argv[])

 {

     int  sockfd, num;

     char  buf[MAXDATASIZE + ] = {};

     struct sockaddr_in server;

     int   iCount = ;

     if (argc != )

     {

         printf("Usage:%s <IP Address>\n", argv[]);

         exit();

     }

     if ((sockfd=socket(AF_INET, SOCK_STREAM, )) == -)

     {

         printf("socket()error\n");

         exit();

     }

     bzero(&server, sizeof(server));

     server.sin_family = AF_INET;

     server.sin_port = htons(PORT);

     server.sin_addr.s_addr = inet_addr(argv[]);

     if (connect(sockfd, (struct sockaddr *)&server, sizeof(server)) == -)

     {

         printf("connect()error\n");

         exit();

     }

     while ()

     {

         memset(buf, , sizeof(buf));

         if ((num = recv(sockfd, buf, MAXDATASIZE,)) == -)

         {

             printf("recv() error\n");

             exit();

         }

         buf[num - ]='\0';

         printf("%dth Recv Length: %d\n", iCount++, num);

     }

     close(sockfd);

     return ;

 }

TCP客户端

测试服务器程序：

 #include <stdio.h>

 #include <stdlib.h>

 #include <string.h>

 #include <unistd.h>

 #include <sys/types.h>

 #include <sys/socket.h>

 #include <netinet/in.h>

 #include <arpa/inet.h>

 #include <signal.h>

 #define  PORT         1234

 #define  BACKLOG      5

 #define  MAXDATASIZE  1000

 int main()

 {

     int  listenfd, connectfd;

     struct  sockaddr_in server;

     struct  sockaddr_in client;

     socklen_t  addrlen;

     char    szbuf[MAXDATASIZE] = {};

     int     iCount = ;

     int     iLength = ;

     if ((listenfd = socket(AF_INET, SOCK_STREAM, )) == -)

     {

         perror("Creating  socket failed.");

         exit();

     }

     int opt = SO_REUSEADDR;

     setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

     bzero(&server, sizeof(server));

     server.sin_family = AF_INET;

     server.sin_port = htons(PORT);

     server.sin_addr.s_addr = htonl(INADDR_ANY);

     if (bind(listenfd, (struct sockaddr *)&server, sizeof(server)) == -)

     {

         perror("Bind()error.");

         exit();

     }

     if (listen(listenfd, BACKLOG) == -)

     {

         perror("listen()error\n");

         exit();

     }

     addrlen = sizeof(client);

     if ((connectfd = accept(listenfd, (struct sockaddr*)&client, &addrlen)) == -)

     {

         perror("accept()error\n");

         exit();

     }

     printf("You got a connection from cient's ip is %s, prot is %d\n", inet_ntoa(client.sin_addr), htons(client.sin_port));

     memset(szbuf, 'a', sizeof(szbuf));

     while (iCount < )

     {

         iLength = send(connectfd, szbuf, sizeof(szbuf), );

         printf("%dth Server Send Length %d\n", iCount++, iLength);

     }

     printf("send over!\n");

     sleep();

     close(connectfd);

     close(listenfd);

     return ;

 }

TCP服务器程序

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客户端接收打印片段

服务器发送打印片段整理时发现丢失了，大家可以自己试试，没有问题。

不难发现，服务器发送正常，客户端在接收时却和我们想的很不一样，但发送和接收的总数据量是一致的，就是说数据没有丢失。如果编程者认为TCP情况下发送和接收的数据长度都一致的，那就极有可能在代码中体现出这一思想，最终出现问题。

其实，这就是所谓的“粘包”现象，Stevens很明确地已经指出了这一点，他说，“UDP是长度固定的、无连接的不可靠报文传输；TCP是有序、可靠、双向的面向连接字节流”。他没说TCP是长度固定的，有没有？当然我更倾向于这样的理解，UDP是面向报文的，报文在传输时是不能被分割的（只是从应用层来看）；TCP是面向字节流的，接收多少数据完全取决于发送和接收的速度了，有多少数据recv就返回多少，数据长度并不和send保持一致，也没这个必要。

那么这个问题怎么解决呢？其实，我们只要将recv封装一层就可以了，那就是我们熟悉的readn函数（该函数不是系统调用），代码如下：

 int readn(int connfd, void *vptr, int n)

 {

     int    nleft;

     int    nread;

     char *ptr;

     struct timeval     select_timeout;

     fd_set rset;

     ptr = vptr;

     nleft = n;

     while (nleft > )

     {

         FD_ZERO(&rset);

         FD_SET(connfd, &rset);

         select_timeout.tv_sec = ;

         select_timeout.tv_usec = ;

         if (select(connfd+, &rset, NULL, NULL, &select_timeout) <= )

         {

             return -;

         }

         if ((nread = recv(connfd, ptr, nleft, )) < )

         {

             if(errno == EINTR)

             {

                 nread = ;

             }

             else

             {

                 return -;

             }

         }

         else if (nread == )

         {

             break;

         }

         nleft -= nread;

         ptr   += nread;

     }

     return(n - nleft);

 }

readn

相应的也有writen函数

 int writen(int connfd, void *vptr, size_t n)

 {

     int nleft, nwritten;

      char    *ptr;

     ptr = vptr;

     nleft = n;

     while(nleft>)

     {

         if((nwritten = send(connfd, ptr, nleft, )) == ERROR)

         {

             if(errnoGet() == EINTR)

             {

                 //PRT_ERR(("EINTR\n"));

                 nwritten = ;

             }

             else

             {

                 //PRT_ERR(("Send() error, 0x%x\n", errnoGet()));

                 return ERROR;

             }

         }

         nleft -= nwritten;

         ptr   += nwritten;

     }

     return(n);

 }

writen

函数中为什么对EINTR进行处理后面再说，也是必不可少的。

在处理TCP发送和接收部分时，可以说必须要使用上述封装，否则等到造成数据不完整或者不一致后再去找问题，可能就麻烦了。这个是必不可少滴。

socket网络编程快速上手（二）——细节问题（2）的相关教程结束。