socket编程入门（三）

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下面给出开发简单客户回射服务器程序迭代过程。

第一版

程序见上一篇博客末尾，由于服务器关闭后处于TIMEWAIT状态，因此存在一个问题：短时间内服务器程序不能重启并使用。

第二版（服务器端使用了REUSEADDR选项）

使用这个选项后，即使服务器处于TIMEWAIT状态，也可以重新启用。
下面给出实现代码。（默认makefile就是上篇博客末尾的makefile）
cli：同第一版的cli程序。
srv：

#include <stdio.h>#include <sys/types.h>#include <sys/socket.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <netinet/in.h>#include <arpa/inet.h>#include <unistd.h>#define ERR_EXIT(m)\    do\    {\        perror(m);\        exit(EXIT_FAILURE);\    }while(0)int main(int argc,char* argv[]){    int listenfd;    if((listenfd = 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 = 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;    if((conn = accept(listenfd,(struct sockaddr*)&peeraddr,&peerlen)) < 0)        ERR_EXIT("accept");    printf("ip=%s portr=%d\n",inet_ntoa(peeraddr.sin_addr),ntohs(peeraddr.sin_port));       char recvbuf[1024];    while(1)    {        memset(recvbuf,0,sizeof(recvbuf));        int ret = read(conn,recvbuf,sizeof(recvbuf));        fputs(recvbuf,stdout);        write(conn,recvbuf,ret);    }    return 0;}

第三版（增加并发）

父子进程做处理：父进程接受accept，子进程处理连接。
（cli和makefile同第二版）
srv：

#include <stdio.h>#include <sys/types.h>#include <sys/socket.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <netinet/in.h>#include <arpa/inet.h>#include <unistd.h>#define ERR_EXIT(m)\    do\    {\        perror(m);\        exit(EXIT_FAILURE);\    }while(0)void do_service(int fd);ssize_t readn(int fd,void* buf,size_t count);int main(int argc,char* argv[]){    int listenfd;    if((listenfd = 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 = 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 portr=%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);        /*break out then close child process*/            exit(EXIT_SUCCESS);        }else        {            close(conn);/*parent and child share the fds*/        }    }    return 0;}void do_service(int fd){    char recvbuf[1024];    while(1)    {        memset(recvbuf,0,sizeof(recvbuf));        int ret = read(fd,recvbuf,sizeof(recvbuf));        if(ret == 0)/*client has closed*/        {            printf("client closed\n");            break;        }else if(ret == -1)        {            ERR_EXIT("read");        }        fputs(recvbuf,stdout);        write(fd,recvbuf,ret);    }}

第四版（增加对粘包的处理）

粘包来源
* TCP协议本身（传输层）不能传递消息边界，原因在于基于字节流的TCP是不能保证对等方一次读操作返回多少字节的，因而存在粘包问题。
处理粘包的常见解决办法：
* 定长包
* 包尾加\r\n（ftp）
* 包头加上包体长度

第一次尝试

先尝试定长包的方式来完善之前的第三版程序，思路是封装readn和writen函数。

代码

cli：

#include <stdio.h>#include <sys/types.h>#include <sys/socket.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <netinet/in.h>#include <arpa/inet.h>#include <unistd.h>#define ERR_EXIT(m)\    do\    {\        perror(m);\        exit(EXIT_FAILURE);\    }while(0)ssize_t readn(int fd,void* buf,size_t count);ssize_t writen(int fd,void* buf,size_t count);int main(int argc,char* argv[]){    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 = htonl("127.0.0.1");    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");    //inet_aton("127.0.0.1",&servaddr.sin_addr);    if(connect(sock,(struct sockaddr*)&servaddr,sizeof(servaddr)) < 0)        ERR_EXIT("connect");    char sendbuf[1024] = {0};    char recvbuf[1024] = {0};    while(fgets(sendbuf,sizeof(sendbuf),stdin)!=NULL)    {        writen(sock,sendbuf,sizeof(sendbuf));        readn(sock,recvbuf,sizeof(recvbuf));        fputs(recvbuf,stdout);        memset(sendbuf,0,sizeof(sendbuf));        memset(recvbuf,0,sizeof(recvbuf));    }    return 0;}ssize_t readn(int fd,void* buf,size_t count){    size_t nleft = count;    size_t nread = 0;    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,void* buf,size_t count){    size_t nleft = count;    size_t nwritten = 0;    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;}

srv：

#include <stdio.h>#include <sys/types.h>#include <sys/socket.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <netinet/in.h>#include <arpa/inet.h>#include <unistd.h>#define ERR_EXIT(m)\    do\    {\        perror(m);\        exit(EXIT_FAILURE);\    }while(0)void do_service(int fd);ssize_t readn(int fd,void* buf,size_t count);ssize_t writen(int fd,void* buf,size_t count);int main(int argc,char* argv[]){    int listenfd;    if((listenfd = 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 = 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 portr=%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);        /*break out then close child process*/            exit(EXIT_SUCCESS);        }else        {            close(conn);/*parent and child share the fds*/        }    }    return 0;}void do_service(int fd){    char recvbuf[1024];    while(1)    {        memset(recvbuf,0,sizeof(recvbuf));        int ret = readn(fd,recvbuf,sizeof(recvbuf));        if(ret == 0)/*client has closed*/        {            printf("client closed\n");            break;        }else if(ret == -1)        {            ERR_EXIT("read");        }        fputs(recvbuf,stdout);        writen(fd,recvbuf,ret);    }}ssize_t readn(int fd,void* buf,size_t count){    size_t nleft = count;/*count:bytes which need to read*/    ssize_t nread;/*has readed*/    char* bufp = (char*)buf;    while(nleft > 0)    {        if((nread = read(fd,bufp,nleft)) < 0)        {            /*if signal interrupt*/            if(errno == EINTR)                continue;            return -1;        }        else if(nread == 0)/*peer has been closed*/        {        //  break;        return count - nleft;        }         bufp += nread;        nleft -= nread;    }    return count;}ssize_t writen(int fd,void* buf,size_t count){    size_t nleft = count;/*count:bytes which need to read*/    ssize_t nwritten;/*has readed*/    char* bufp = (char*)buf;    while(nleft > 0)    {        if((nwritten = write(fd,bufp,nleft)) < 0)        {            /*if signal interrupt*/            if(errno == EINTR)                continue;            return -1;        }        else if(nwritten == 0)/*peer has been closed*/        {            continue;        }         bufp += nwritten;        nleft -= nwritten;    }    return count;}

定长包虽然可以解决粘包问题，但是其每次发生都需要一个缓冲区，有效数据可能在其中所占比重非常小，举一个极端例子，每次只发送一个字符。每次发送数据时，大量无效数据充斥于网络，无形中增大了网络负担，因而还需要进一步改进。

第二次尝试

增加一个包头，包头存放每次传输的数据长度。
这样可舍弃定长包。

代码