09Socket编程

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)   //可能是信号打断
				continue;
			return -1;            //失败
		}
		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。