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Linux学习之socket编程（二）

1.C/S模型——UDP

UDP处理模型

　　由于UDP不需要维护连接，程序逻辑简单了很多，但是UDP协议是不可靠的，实际上有很多保证通讯可靠性的机制需要在应用层实现。多保证通讯可靠性的机制需要在应用层实现。编译运行server，在两个终端里各开一个client与server交互，看看server是否具有并发服务的能力。用Ctrl+C关闭server，然后再运行server，看此时client还能否和server联系上。和前面TCP程序的运行结果相比较，体会无连接的含义。

/* server.c */#include <stdio.h>
#include <string.h> #include <netinet/in.h> #include "wrap.h" #define MAXLINE 80 #define SERV_PORT 8000 int main(void) { struct sockaddr_in servaddr, cliaddr;//用于IPv4的地址  socklen_t cliaddr_len; int sockfd;//文件描述符 char buf[MAXLINE]; char str[INET_ADDRSTRLEN];//16 Bytes int i, n; /*构造用于UDP通信的套接字*/ sockfd = socket(AF_INET, SOCK_DGRAM, 0);//  bzero(&servaddr, sizeof(servaddr));//将地址清零 //设置地址 servaddr.sin_family = AF_INET;/*IPv4*/ servaddr.sin_addr.s_addr = htonl(INADDR_ANY);//网络字节数，本地任意IP servaddr.sin_port = htons(SERV_PORT); bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)); printf("Accepting connections ...\n"); while (1) { cliaddr_len = sizeof(cliaddr); n = recvfrom(sockfd, buf, MAXLINE, 0, (struct sockaddr *)&cliaddr, &cliaddr_len); if (n == -1) perr_exit("recvfrom error"); printf("received from %s at PORT %d\n", inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)), ntohs(cliaddr.sin_port)); for (i = 0; i < n; i++) buf[i] = toupper(buf[i]);//小写转大写 //发送数据 n = sendto(sockfd, buf, n, 0, (struct sockaddr *)&cliaddr, sizeof(cliaddr)); if (n == -1) perr_exit("sendto error"); } } 

/* client.c */
#include <stdio.h>
#include <string.h> #include <unistd.h> #include <netinet/in.h> #include "wrap.h" #define MAXLINE 80 #define SERV_PORT 8000 int main(int argc, char *argv[]) { struct sockaddr_in servaddr; int sockfd, n; char buf[MAXLINE]; char str[INET_ADDRSTRLEN]; socklen_t servaddr_len; sockfd = Socket(AF_INET, SOCK_DGRAM, 0); bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr); servaddr.sin_port = htons(SERV_PORT); while(fgets(buf, MAXLINE, stdin) != NULL) { n = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&servaddr, sizeof(servaddr)); if (n == -1) perr_exit ("sendto error"); n = recvfrom(sockfd, buf, MAXLINE, 0, NULL, 0); if (n == -1) perr_exit("recvfrom error"); write(STDOUT_FILENO, buf, n);//写到屏幕上  } close(sockfd); return 0; } 

2.出错处理封装函数 　　上面的例子不仅功能简单，而且简单到几乎没有什么错误处理，我们知道，系统调用不能保证每次都成功，必须进行出错处理，这样一方面可以保证程序逻辑正常，另一方面可以迅速得到故障信息。为使错误处理的代码不影响主程序的可读性，我们把与socket相关的一些系统函数加上错误处理代码包装成新的函数，做成一个模块wrap.c：

#include <stdlib.h>
#include <errno.h>
#include <sys/socket.h>
void perr_exit(const char *s) { perror(s); exit(1); } int Accept(int fd, struct sockaddr *sa, socklen_t *salenptr) { int n; again: if ( (n = accept(fd, sa, salenptr)) < 0) { if ((errno == ECONNABORTED) || (errno == EINTR)) goto again; else perr_exit("accept error"); } return n; } void Bind(int fd, const struct sockaddr *sa, socklen_t salen) { if (bind(fd, sa, salen) < 0) perr_exit("bind error"); } void Connect(int fd, const struct sockaddr *sa, socklen_t salen) { if (connect(fd, sa, salen) < 0) perr_exit("connect error"); } void Listen(int fd, int backlog) { if (listen(fd, backlog) < 0) perr_exit("listen error"); } int Socket(int family, int type, int protocol) { int n; if ( (n = socket(family, type, protocol)) < 0) perr_exit("socket error"); return n; } ssize_t Read(int fd, void *ptr, size_t nbytes) { ssize_t n; again: if ( (n = read(fd, ptr, nbytes)) == -1) { if (errno == EINTR) goto again; else return -1; } return n; } ssize_t Write(int fd, const void *ptr, size_t nbytes) { ssize_t n; again: if ( (n = write(fd, ptr, nbytes)) == -1) { if (errno == EINTR) goto again; else return -1; } return n; } void Close(int fd) { if (close(fd) == -1) perr_exit("close error"); } ssize_t Readn(int fd, void *vptr, size_t n)//指定读够n个字节返回 { size_t nleft; ssize_t nread; char *ptr; ptr = vptr; nleft = n; while (nleft > 0) { if ( (nread = read(fd, ptr, nleft)) < 0) { if (errno == EINTR) nread = 0; else return -1; } else if (nread == 0) break; nleft -= nread; ptr += nread; } return n - nleft; } ssize_t Writen(int fd, const void *vptr, size_t n) { size_t nleft; ssize_t nwritten; const char *ptr; ptr = vptr; nleft = n; while (nleft > 0) { if ( (nwritten = write(fd, ptr, nleft)) <= 0) { if (nwritten < 0 && errno == EINTR) nwritten = 0; else return -1; } nleft -= nwritten; ptr += nwritten; } return n; } static ssize_t my_read(int fd, char *ptr) { static int read_cnt; static char *read_ptr; static char read_buf[100]; if (read_cnt <= 0) { again: if ( (read_cnt = read(fd, read_buf, sizeof(read_buf))) < 0) { if (errno == EINTR) goto again; return -1; } else if (read_cnt == 0) return 0; read_ptr = read_buf; } read_cnt--; *ptr = *read_ptr++; return 1; } ssize_t Readline(int fd, void *vptr, size_t maxlen)//一次读一行 { ssize_t n, rc; char c, *ptr; ptr = vptr; for (n = 1; n < maxlen; n++) { if ( (rc = my_read(fd, &c)) == 1) { *ptr++ = c; if (c == '\n'