tcp是基于字节流的，udp是基于报文即数据包的，所以tcp会产生一个叫做粘包的问题，而udp不会产生。
我们这节主要讨论粘包问题：
先看一下粘包问题的原因：
总结如下：
1、应用进程的缓冲区和Socket缓冲区的大小不一定相吻合。
2、tcp传输段有mss限制。
3、链路层有个mtu限制。
粘包的解决方案：
1、设置定长包，定长接受。
2、在包尾加上\r\n，例如ftp就是这样实现的。
3、包头加上包体长度。
4、更复杂的应用层协议。
下面就封装一下readn和writen来实现解决粘包问题，改进一下先前的那个回射程序：
服务器端：
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#define ERR_EXIT(m) do { perror(m); exit(EXIT_FAILURE); } while(0)
struct packet //新定义包的结构，包体长度和包体
{
int len;
char buf[1024];
};
ssize_t readn(int fd, void *buf, size_t count) //readn的封装
{
size_t nleft = count;
ssize_t nread;
char *bufp = (char*)buf;
while (nleft > 0)
{
if ((nread = read(fd, bufp, nleft)) < 0)//这里小于零两种情况一种是信号打断一种就是失败
{
if (errno == EINTR)
continue;
return -1;
}
else if (nread == 0)
return count - nleft;
bufp += nread;
nleft -= nread;
}
return count;
}
ssize_t writen(int fd, const void *buf, size_t count)//writen的封装
{
size_t nleft = count;
ssize_t nwritten;
char *bufp = (char*)buf;
while (nleft > 0)
{
if ((nwritten = write(fd, bufp, nleft)) < 0)
{
if (errno == EINTR)
continue;
return -1;
}
else if (nwritten == 0)
continue;
bufp += nwritten;
nleft -= nwritten;
}
return count;
}
void do_service(int conn)
{
struct packet recvbuf;
int n;
while (1)
{
memset(&recvbuf, 0, sizeof(recvbuf));
int ret = readn(conn, &recvbuf.len, 4); //先接受包头长度
if (ret == -1)
ERR_EXIT("read");
else if (ret < 4)
{
printf("client close\n");
break;
}
n = ntohl(recvbuf.len); //这里有个字节序的转换，因为是要在网络上传输
ret = readn(conn, recvbuf.buf, n); //再接受包体
if (ret == -1)
ERR_EXIT("read");
else if (ret < n)
{
printf("client close\n");
break;
}
fputs(recvbuf.buf, stdout);
writen(conn, &recvbuf, 4+n); //回写要包体长度加包体
}
}
int main(void)
{
int listenfd;
if ((listenfd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
/* if ((listenfd = socket(PF_INET, SOCK_STREAM, 0)) < 0)*/
ERR_EXIT("socket");
struct sockaddr_in servaddr;
memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(5188);
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
/*servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");*/
/*inet_aton("127.0.0.1", &servaddr.sin_addr);*/
int on = 1;
if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) < 0)
ERR_EXIT("setsockopt");
if (bind(listenfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0)
ERR_EXIT("bind");
if (listen(listenfd, SOMAXCONN) < 0)
ERR_EXIT("listen");
struct sockaddr_in peeraddr;
socklen_t peerlen = sizeof(peeraddr);
int conn;
pid_t pid;
while (1)
{
if ((conn = accept(listenfd, (struct sockaddr*)&peeraddr, &peerlen)) < 0)
ERR_EXIT("accept");
printf("ip=%s port=%d\n", inet_ntoa(peeraddr.sin_addr), ntohs(peeraddr.sin_port));
pid = fork();
if (pid == -1)
ERR_EXIT("fork");
if (pid == 0)
{
close(listenfd);
do_service(conn);
exit(EXIT_SUCCESS);
}
else
close(conn);
}
return 0;
}
客户端：
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#define ERR_EXIT(m) do { perror(m); exit(EXIT_FAILURE); } while(0)
struct packet
{
int len;
char buf[1024];
};
ssize_t readn(int fd, void *buf, size_t count)
{
size_t nleft = count;
ssize_t nread;
char *bufp = (char*)buf;
while (nleft > 0)
{
if ((nread = read(fd, bufp, nleft)) < 0)
{
if (errno == EINTR)
continue;
return -1;
}
else if (nread == 0)
return count - nleft;
bufp += nread;
nleft -= nread;
}
return count;
}
ssize_t writen(int fd, const void *buf, size_t count)
{
size_t nleft = count;
ssize_t nwritten;
char *bufp = (char*)buf;
while (nleft > 0)
{
if ((nwritten = write(fd, bufp, nleft)) < 0)
{
if (errno == EINTR)
continue;
return -1;
}
else if (nwritten == 0)
continue;
bufp += nwritten;
nleft -= nwritten;
}
return count;
}
int main(void)
{
int sock;
if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
ERR_EXIT("socket");
struct sockaddr_in servaddr;
memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(5188);
servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
if (connect(sock, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0)
ERR_EXIT("connect");
struct packet sendbuf;
struct packet recvbuf;
memset(&sendbuf, 0, sizeof(sendbuf));
memset(&recvbuf, 0, sizeof(recvbuf));
int n;
while (fgets(sendbuf.buf, sizeof(sendbuf.buf), stdin) != NULL)
{
n = strlen(sendbuf.buf);
sendbuf.len = htonl(n);
writen(sock, &sendbuf, 4+n);
int ret = readn(sock, &recvbuf.len, 4);
if (ret == -1)
ERR_EXIT("read");
else if (ret < 4)
{
printf("client close\n");
break;
}
n = ntohl(recvbuf.len);
ret = readn(sock, recvbuf.buf, n);
if (ret == -1)
ERR_EXIT("read");
else if (ret < n)
{
printf("client close\n");
break;
}
fputs(recvbuf.buf, stdout);
memset(&sendbuf, 0, sizeof(sendbuf));
memset(&recvbuf, 0, sizeof(recvbuf));
}
close(sock);
return 0;
}
这种解决粘包的方法特别实在网络中传输的时候十分必要。
原文：http://www.cnblogs.com/DamonBlog/p/4458627.html