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
客户端接收打印片段如下：
936th Recv Length:
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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。