前面学习了readn和writen封装函数的相关内容，接下来学习：read、write与recv、send的区别，readline实现，用readline实现回射客户/服务器以及getsockname、getpeername等一系列函数。
1.read、write与recv、send
（1）recv函数只能用于套接口I\O，而read函数可以用于任何I\O
（2）recv函数与read函数相比，多了一个flags选项。该选型可以指定接收的行为（常用的有MSG_OOB，MSG_PEEK）。
（3）MSG_PEEK：可以接收缓冲区中的数据，但是并没有将数据从缓冲区中清除。下面封装一个recv_peek函数：
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ssize_t recv_peek(int sockfd, void *buf, size_t len) { while(1) { int ret = recv(sockfd, buf, len, MSG_PEEK); if (ret == -1 && errno = EINTR) continue; return ret; } } |
2.readline实现
（1）使用上面封装的recv_peek函数来实现readline功能——按行读取。即，读到\n为一条消息。
使用readline也可以解决粘包问题，ftp就是采用类似的机制。
（2）实现按行读取标准的做法是一个字符一个字符的读取，并判断该字符是否为\n。但是，这种做法效率较低。这里采用“偷窥”的方法
（3）使用recv_peek函数来实现readline：
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ssize_t readline(int sockfd, void *buf, size_t maxline) { int ret; int nread; char *bufp = buf; int nleft = maxline; while(1) { ret = recv_peek(sockfd, bufp, nleft); if (ret < 0) return ret; else if (ret == 0) return ret; nread = ret; int i; for (i=0; i<nread; ++i) { if (bufp[i] == '\n') { ret = readn(sockfd, bufp, i+1); if (ret != i+1) exit(EXIT_FAILURE); return ret; } } if (nread > nleft) exit(EXIT_FAILURE); nleft -= nread; ret = readn(sockfd, bufp, nread); if (ret != nread) exit(EXIT_FAILURE); bufp += nread; } return -1; } |
1）首先调用recv_peek函数，来偷窥“接收”缓冲区中是否有\n。(recv_peek不清除缓冲区)
2）前一步偷窥到\n后，就可以利用readn函数将\n之前的数据(包括\n)读走。
3）若没有偷窥到\n字符，那么也可以断言recv_peek读到的这些数据也是包含在“发送数据”之内的，调用readn将其发送出去(同时清除了缓冲区)。然后，再进行下一次“偷窥”…
3.readline实现回射客户/服务器
echosrv.c
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#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) 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; } ssize_t recv_peek(int sockfd, void *buf, size_t len) { while(1){ int ret = recv(sockfd, buf ,len, MSG_PEEK); if (ret == -1 && errno == EINTR) continue; return ret; } } size_t readline(int sockfd, void *buf, size_t maxline) { int ret; int nread; char *bufp = buf; int nleft = maxline; while(1){ ret = recv_peek(sockfd, bufp, nleft); if(ret < 0) return ret; else if (ret == 0) return ret; nread = ret; int i; for (i=0; i<nread; i++){ if (bufp[i] == '\n'){ ret = readn(sockfd, bufp, i+1); if (ret != i+1) exit(EXIT_FAILURE); return ret; } } if (nread > nleft) exit(EXIT_FAILURE); nleft -= nread; ret = readn(sockfd, bufp, nread); if (ret != nread) exit(EXIT_FAILURE); bufp += nread; } return -1; } void do_service(int conn) { char recvbuf[1024]; while(1){ memset(recvbuf, 0, sizeof(recvbuf)); int ret = readline(conn, recvbuf,1024); if(ret == -1) ERR_EXIT("readline"); else if(ret ==0 ){ printf("client close\n"); break; } fputs(recvbuf, stdout); writen(conn, recvbuf, strlen(recvbuf)); } } int main (void) { 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(5581); servaddr.sin_addr.s_addr = htonl(INADDR_ANY); 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("liten"); 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; } |
echocli.c
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#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) 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; } ssize_t recv_peek(int sockfd, void *buf, size_t len) { while(1){ int ret = recv(sockfd, buf ,len, MSG_PEEK); if (ret == -1 && errno == EINTR) continue; return ret; } } size_t readline(int sockfd, void *buf, size_t maxline) { int ret; int nread; char *bufp = buf; int nleft = maxline; while(1){ ret = recv_peek(sockfd, bufp, nleft); if(ret < 0) return ret; else if (ret == 0) return ret; nread = ret; int i; for (i=0; i<nread; i++){ if (bufp[i] == '\n'){ ret = readn(sockfd, bufp, i+1); if (ret != i+1) exit(EXIT_FAILURE); return ret; } } if (nread > nleft) exit(EXIT_FAILURE); nleft -= nread; ret = readn(sockfd, bufp, nread); if (ret != nread) exit(EXIT_FAILURE); bufp += nread; } return -1; } 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(5581); servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); if(connect(sock, (struct sockaddr*) &servaddr, sizeof(servaddr)) < 0) ERR_EXIT("connect"); struct sockaddr_in localaddr; socklen_t addrlen = sizeof(localaddr); if(getsockname(sock, (struct sockaddr*)&localaddr, &addrlen) < 0) ERR_EXIT("getsockname"); printf("ip = %s port = %d\n", inet_ntoa(localaddr.sin_addr), ntohs(localaddr.sin_port)); char sendbuf[1024]= {0}; char recvbuf[1024]= {0}; while(fgets(sendbuf, sizeof(sendbuf), stdin) != NULL){ writen(sock, sendbuf, strlen(sendbuf)); int ret = readline (sock, recvbuf, sizeof(recvbuf)); if(ret == -1) ERR_EXIT("readline"); else if(ret ==0 ){ printf("server close\n"); break; } fputs(recvbuf,stdout); memset(sendbuf, 0, sizeof(sendbuf)); memset(recvbuf, 0, sizeof(recvbuf)); } close (sock); return 0; } |
4.getsockname，getpeername
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#include <sys/socket.h> int getsockname(int sockfd, struct sockaddr *localaddr, socklen_t *addrlen); int getpeername(int sockfd, struct sockaddr *peeraddr, socklen_t *addrlen); //返回值：若成功，返回0；失败返回-1 |
需要这两个函数的理由主要有：
（1）在一个没有调用bind的TCP客户上，connect成功返回后，getsockname用于返回由内核赋予该连接的本地IP地址和本地端口号。
（2）在以端口号0调用bind(告知内核去选择本地端口号)后，getsockname用于返回由内核赋予的本地端口号。
（3）getsockname可用于获取某个套接字的地址族。
（4）在一个以通配IP地址调用bind的TCP服务器上，与某个客户的连接一旦建立(accept成功返回)，getsockname就可以用于返回由内核赋予该连接的本地IP地址。在这样的调用中，套接字描述符参数必须是已连接套接字的描述符，而不是监听套接字的描述符。
如上客户端示例，getsockname()函数用于获取一个套接字的名字。它用于一个已捆绑或已连接套接字s，本地地址将被返回。