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：
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <unistd.h> 4 #include <string.h> 5 #include <sys/types.h> 6 #include <sys/socket.h> 7 #include <netinet/in.h> 8 #include <netdb.h> 9 10 #define PORT 1234 11 #define MAXDATASIZE 1000 12 13 int main(int argc, char *argv[]) 14 { 15 int sockfd, num; 16 char buf[MAXDATASIZE + 1] = {0}; 17 struct sockaddr_in server; 18 int iCount = 0; 19 20 if (argc != 2) 21 { 22 printf("Usage:%s <IP Address> ", argv[0]); 23 exit(1); 24 } 25 26 if ((sockfd=socket(AF_INET, SOCK_STREAM, 0)) == -1) 27 { 28 printf("socket()error "); 29 exit(1); 30 } 31 bzero(&server, sizeof(server)); 32 server.sin_family = AF_INET; 33 server.sin_port = htons(PORT); 34 server.sin_addr.s_addr = inet_addr(argv[1]); 35 if (connect(sockfd, (struct sockaddr *)&server, sizeof(server)) == -1) 36 { 37 printf("connect()error "); 38 exit(1); 39 } 40 41 while (1) 42 { 43 memset(buf, 0, sizeof(buf)); 44 if ((num = recv(sockfd, buf, MAXDATASIZE,0)) == -1) 45 { 46 printf("recv() error "); 47 exit(1); 48 } 49 buf[num - 1]=' '; 50 printf("%dth Recv Length: %d ", iCount++, num); 51 } 52 53 close(sockfd); 54 55 return 0; 56 }
测试服务器程序：
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <string.h> 4 #include <unistd.h> 5 #include <sys/types.h> 6 #include <sys/socket.h> 7 #include <netinet/in.h> 8 #include <arpa/inet.h> 9 #include <signal.h> 10 11 #define PORT 1234 12 #define BACKLOG 5 13 #define MAXDATASIZE 1000 14 15 int main() 16 { 17 int listenfd, connectfd; 18 struct sockaddr_in server; 19 struct sockaddr_in client; 20 socklen_t addrlen; 21 char szbuf[MAXDATASIZE] = {0}; 22 int iCount = 0; 23 int iLength = 0; 24 25 if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) 26 { 27 perror("Creating socket failed."); 28 exit(1); 29 } 30 31 int opt = SO_REUSEADDR; 32 setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)); 33 34 bzero(&server, sizeof(server)); 35 server.sin_family = AF_INET; 36 server.sin_port = htons(PORT); 37 server.sin_addr.s_addr = htonl(INADDR_ANY); 38 if (bind(listenfd, (struct sockaddr *)&server, sizeof(server)) == -1) 39 { 40 perror("Bind()error."); 41 exit(1); 42 } 43 if (listen(listenfd, BACKLOG) == -1) 44 { 45 perror("listen()error "); 46 exit(1); 47 } 48 49 addrlen = sizeof(client); 50 if ((connectfd = accept(listenfd, (struct sockaddr*)&client, &addrlen)) == -1) 51 { 52 perror("accept()error "); 53 exit(1); 54 } 55 printf("You got a connection from cient's ip is %s, prot is %d ", inet_ntoa(client.sin_addr), htons(client.sin_port)); 56 57 memset(szbuf, 'a', sizeof(szbuf)); 58 while (iCount < 1000) 59 { 60 iLength = send(connectfd, szbuf, sizeof(szbuf), 0); 61 printf("%dth Server Send Length %d ", iCount++, iLength); 62 } 63 64 printf("send over! "); 65 sleep(10); 66 67 close(connectfd); 68 close(listenfd); 69 70 return 0; 71 }
客户端接收打印片段如下：
1 936th Recv Length: 1000 2 937th Recv Length: 1000 3 938th Recv Length: 1000 4 939th Recv Length: 1000 5 940th Recv Length: 1000 6 941th Recv Length: 1000 7 942th Recv Length: 384 8 943th Recv Length: 616 9 944th Recv Length: 1000 10 945th Recv Length: 1000 11 946th Recv Length: 1000 12 947th Recv Length: 1000 13 948th Recv Length: 1000 14 949th Recv Length: 1000 15 950th Recv Length: 1000 16 951th Recv Length: 1000 17 952th Recv Length: 1000 18 953th Recv Length: 1000 19 954th Recv Length: 1000 20 955th Recv Length: 1000 21 956th Recv Length: 1000 22 957th Recv Length: 1000 23 958th Recv Length: 1000 24 959th Recv Length: 1000 25 960th Recv Length: 1000 26 961th Recv Length: 1000 27 962th Recv Length: 384 28 963th Recv Length: 616 29 964th Recv Length: 1000 30 965th Recv Length: 1000 31 966th Recv Length: 1000 32 967th Recv Length: 1000 33 968th Recv Length: 1000 34 969th Recv Length: 1000 35 970th Recv Length: 1000 36 971th Recv Length: 1000 37 972th Recv Length: 1000 38 973th Recv Length: 1000 39 974th Recv Length: 1000 40 975th Recv Length: 1000 41 976th Recv Length: 1000 42 977th Recv Length: 1000 43 978th Recv Length: 1000 44 979th Recv Length: 1000 45 980th Recv Length: 1000 46 981th Recv Length: 1000 47 982th Recv Length: 384 48 983th Recv Length: 616 49 984th Recv Length: 1000 50 985th Recv Length: 1000 51 986th Recv Length: 1000 52 987th Recv Length: 1000 53 988th Recv Length: 1000 54 989th Recv Length: 1000 55 990th Recv Length: 1000 56 991th Recv Length: 1000 57 992th Recv Length: 1000 58 993th Recv Length: 1000 59 994th Recv Length: 1000 60 995th Recv Length: 1000 61 996th Recv Length: 1000 62 997th Recv Length: 1000 63 998th Recv Length: 1000 64 999th Recv Length: 1000 65 1000th Recv Length: 1000 66 1001th Recv Length: 1000 67 1002th Recv Length: 384 68 1003th Recv Length: 616 69 1004th Recv Length: 1000 70 1005th Recv Length: 1000 71 1006th Recv Length: 1000 72 1007th Recv Length: 1000 73 1008th Recv Length: 1000 74 1009th Recv Length: 1000 75 1010th Recv Length: 1000 76 1011th Recv Length: 1000 77 1012th Recv Length: 1000 78 1013th Recv Length: 1000 79 1014th Recv Length: 1000 80 1015th Recv Length: 1000 81 1016th Recv Length: 1000 82 1017th Recv Length: 1000 83 1018th Recv Length: 1000 84 1019th Recv Length: 1000 85 1020th Recv Length: 1000 86 1021th Recv Length: 1000 87 1022th Recv Length: 384 88 1023th Recv Length: 616 89 1024th Recv Length: 1000 90 1025th Recv Length: 1000 91 1026th Recv Length: 1000 92 1027th Recv Length: 1000 93 1028th Recv Length: 1000 94 1029th Recv Length: 1000 95 1030th Recv Length: 1000 96 1031th Recv Length: 1000 97 1032th Recv Length: 1000 98 1033th Recv Length: 1000 99 1034th Recv Length: 1000 100 1035th Recv Length: 1000 101 1036th Recv Length: 1000 102 1037th Recv Length: 1000 103 1038th Recv Length: 1000 104 1039th Recv Length: 1000 105 1040th Recv Length: 1000 106 1041th Recv Length: 1000 107 1042th Recv Length: 384 108 1043th Recv Length: 616 109 1044th Recv Length: 1000 110 1045th Recv Length: 1000 111 1046th Recv Length: 1000 112 1047th Recv Length: 1000 113 1048th Recv Length: 1000 114 1049th Recv Length: 1000 115 1050th Recv Length: 1000
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：
1 int readn(int connfd, void *vptr, int n) 2 { 3 int nleft; 4 int nread; 5 char *ptr; 6 struct timeval select_timeout; 7 fd_set rset; 8 9 ptr = vptr; 10 nleft = n; 11 12 while (nleft > 0) 13 { 14 FD_ZERO(&rset); 15 FD_SET(connfd, &rset); 16 select_timeout.tv_sec = 5; 17 select_timeout.tv_usec = 0; 18 if (select(connfd+1, &rset, NULL, NULL, &select_timeout) <= 0) 19 { 20 return -1; 21 } 22 if ((nread = recv(connfd, ptr, nleft, 0)) < 0) 23 { 24 if(errno == EINTR) 25 { 26 nread = 0; 27 } 28 else 29 { 30 return -1; 31 } 32 } 33 else if (nread == 0) 34 { 35 break; 36 } 37 nleft -= nread; 38 ptr += nread; 39 } 40 return(n - nleft); 41 }
相应的也有writen函数
1 int writen(int connfd, void *vptr, size_t n) 2 { 3 int nleft, nwritten; 4 char *ptr; 5 6 ptr = vptr; 7 nleft = n; 8 9 while(nleft>0) 10 { 11 if((nwritten = send(connfd, ptr, nleft, 0)) == ERROR) 12 { 13 if(errnoGet() == EINTR) 14 { 15 //PRT_ERR(("EINTR ")); 16 nwritten = 0; 17 } 18 else 19 { 20 //PRT_ERR(("Send() error, 0x%x ", errnoGet())); 21 return ERROR; 22 } 23 } 24 nleft -= nwritten; 25 ptr += nwritten; 26 } 27 28 return(n); 29 }
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。