socket通信的几个关键步骤

转自点击打开链接

面向连接的socket通信就像与对方打电话，首先需要通过电话建立一个连接，连接建立好之后，彼此才能双向通信。它有几个关键步骤

服务器端通常以守护进程的方式实现:

1: 创建守护进程

2：获取或注册服务

3：创建socket并绑定地址

4：开始监听

5：接收客户端连接请求

6：进行数据传输

客户端

1：获取或注册服务

2：创建socket

3：发送连接请求

示例代码

服务器端的实现（以端口的形式提供给用户使用）

#include <stdio.h>
    #include <stdlib.h>
    #include <netdb.h>
    #include <arpa/inet.h>
    #include <stddef.h>        /* for offsetof */
    #include <string.h>        /* for convenience */
    #include <unistd.h>        /* for convenience */
    #include <signal.h>        /* for SIG_ERR */

    #include <errno.h>
    #include <syslog.h>
    #include <sys/socket.h>

    #include <fcntl.h>
    #include <sys/resource.h>

    #define BUFLEN    128
    #define QLEN 10

    #ifndef HOST_NAME_MAX
    #define HOST_NAME_MAX 256
    #endif

    void daemonize(const char *cmd)
    {
        int                    i, fd0, fd1, fd2;
        pid_t                pid;
        struct rlimit        rl;
        struct sigaction    sa;

        /*
         * Clear file creation mask.
         */
        umask(0);

        /*
         * Get maximum number of file descriptors.
         */
        if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
            printf("%s: can't get file limit", cmd);

        /*
         * Become a session leader to lose controlling TTY.
         */
        if ((pid = fork()) < 0)
            printf("%s: can't fork", cmd);
        else if (pid != 0) /* parent */
            exit(0);
        setsid();

        /*
         * Ensure future opens won't allocate controlling TTYs.
         */
        sa.sa_handler = SIG_IGN;
        sigemptyset(&sa.sa_mask);
        sa.sa_flags = 0;
        if (sigaction(SIGHUP, &sa, NULL) < 0)
            printf("%s: can't ignore SIGHUP");
        if ((pid = fork()) < 0)
            printf("%s: can't fork", cmd);
        else if (pid != 0) /* parent */
            exit(0);

        /*
         * Change the current working directory to the root so
         * we won't prevent file systems from being unmounted.
         */
        if (chdir("/") < 0)
            printf("%s: can't change directory to /");

        /*
         * Close all open file descriptors.
         */
        if (rl.rlim_max == RLIM_INFINITY)
            rl.rlim_max = 1024;
        for (i = 0; i < rl.rlim_max; i++)
            close(i);

        /*
         * Attach file descriptors 0, 1, and 2 to /dev/null.
         */
        fd0 = open("/dev/null", O_RDWR);
        fd1 = dup(0);
        fd2 = dup(0);

        /*
         * Initialize the log file.
         */
        openlog(cmd, LOG_CONS, LOG_DAEMON);
        if (fd0 != 0 || fd1 != 1 || fd2 != 2) {
            syslog(LOG_ERR, "unexpected file descriptors %d %d %d",
             fd0, fd1, fd2);
            exit(1);
        }
    }

    int initserver(int type, const struct sockaddr *addr, socklen_t alen,
      int qlen)
    {
        int fd;
        int err = 0;

        if ((fd = socket(addr->sa_family, type, 0)) < 0)
            return(-1);
        if (bind(fd, addr, alen) < 0) {
            err = errno;
            goto errout;
        }
        if (type == SOCK_STREAM || type == SOCK_SEQPACKET) {
            if (listen(fd, qlen) < 0) {
                err = errno;
                goto errout;
            }
        }
        return(fd);

    errout:
        close(fd);
        errno = err;
        return(-1);
    }

    void serve(int sockfd)
    {
        int        clfd;
        FILE    *fp;
        char    buf[BUFLEN];

        for (;;) {
            clfd = accept(sockfd, NULL, NULL);
            if (clfd < 0) {
                syslog(LOG_ERR, "ruptimed: accept error: %s",
                 strerror(errno));
                exit(1);
            }
            if ((fp = popen("/usr/bin/uptime", "r")) == NULL) {
                sprintf(buf, "error: %s\n", strerror(errno));
                send(clfd, buf, strlen(buf), 0);
            } else {
                while (fgets(buf, BUFLEN, fp) != NULL)
                    send(clfd, buf, strlen(buf), 0);
                pclose(fp);
            }
            close(clfd);
        }
    }

    int main(int argc, char *argv[])
    {
        struct addrinfo    *ailist, *aip;
        struct addrinfo    hint;
        int                sockfd, err, n;
        char            *host;

        if (argc != 1)
            printf("usage: ruptimed");
    #ifdef _SC_HOST_NAME_MAX
        n = sysconf(_SC_HOST_NAME_MAX);
        if (n < 0)    /* best guess */
    #endif
            n = HOST_NAME_MAX;
        host = malloc(n);
        if (host == NULL)
            printf("malloc error");
        if (gethostname(host, n) < 0)
            printf("gethostname error");
        printf("hostname=%s\n",host);

        daemonize("ruptimed");
        hint.ai_flags = AI_PASSIVE;
        hint.ai_family = 0;
        hint.ai_socktype = SOCK_STREAM;
        hint.ai_protocol = 0;
        hint.ai_addrlen = 0;
        hint.ai_canonname = NULL;
        hint.ai_addr = NULL;
        hint.ai_next = NULL;
        if ((err = getaddrinfo(host, "2001", &hint, &ailist)) != 0) {
            syslog(LOG_ERR, "ruptimed: getaddrinfo error: %s",gai_strerror(err));
            exit(1);
        }
        for (aip = ailist; aip != NULL; aip = aip->ai_next) {
            if ((sockfd = initserver(SOCK_STREAM, aip->ai_addr,aip->ai_addrlen, QLEN)) >= 0) {
                printf("initserver ok !\n");
                serve(sockfd);
                exit(0);
            }
        }
        printf("server err !\n");
        exit(1);
    }

客户端的实现

    #include <stdio.h>
    #include <stdlib.h>
    #include <netdb.h>
    #include <arpa/inet.h>
    #include <unistd.h>        /* for convenience */

    #include <sys/resource.h>
    #include <errno.h>
    #include <sys/socket.h>

    #define MAXADDRLEN    256
    #define BUFLEN        128

    #define MAXSLEEP 128

    int connect_retry(int sockfd, const struct sockaddr *addr, socklen_t alen)
    {
        int nsec;

        /*
         * Try to connect with exponential backoff.
         */
        for (nsec = 1; nsec <= MAXSLEEP; nsec <<= 1) {
            if (connect(sockfd, addr, alen) == 0) {
                /*
                 * Connection accepted.
                 */
                return(0);
            }

            /*
             * Delay before trying again.
             */
            if (nsec <= MAXSLEEP/2)
                sleep(nsec);
        }
        return(-1);
    }

    void print_uptime(int sockfd)
    {
        int        n;
        char    buf[BUFLEN];

        while ((n = recv(sockfd, buf, BUFLEN, 0)) > 0)
            write(STDOUT_FILENO, buf, n);
        if (n < 0)
            printf("recv error");
    }

    int main(int argc, char *argv[])
    {
        struct addrinfo    *ailist, *aip;
        struct addrinfo    hint;
        int                sockfd, err;

        if (argc != 2)
            printf("usage: ruptime hostname");
        hint.ai_flags = AI_CANONNAME;
        hint.ai_family = 0;
        hint.ai_socktype = SOCK_STREAM;
        hint.ai_protocol = 0;
        hint.ai_addrlen = 0;
        hint.ai_canonname = NULL;
        hint.ai_addr = NULL;
        hint.ai_next = NULL;
        if ((err = getaddrinfo(argv[1], "2001", &hint, &ailist)) != 0)
            printf("getaddrinfo error: %s", gai_strerror(err));
        for (aip = ailist; aip != NULL; aip = aip->ai_next) {
            if ((sockfd = socket(aip->ai_family, SOCK_STREAM, 0)) < 0)
                err = errno;
            if (connect_retry(sockfd, aip->ai_addr, aip->ai_addrlen) < 0) {
                err = errno;
            } else {
                print_uptime(sockfd);
                exit(0);
            }
        }
        fprintf(stderr, "can't connect to %s: %s\n", argv[1],strerror(err));
        exit(1);
    }

测试流程：服务器端： 修改/etc/hosts如下

192.168.1.11 X64-server

127.0.0.1 localhost

#127.0.1.1 X64-server

客户端 修改/etc/hosts如下

127.0.0.1 localhost

127.0.1.1 ubuntu

192.168.1.11 X64-server



说明：若服务器端以服务的形式提供给用的使用，服务端和客户端都需要在/etc/services中注册服务，但它们的端口号可以不一样，如：ruptime 2001/tcp，然后将getaddrinfo函数中端口号改为服务名就可以了

面向无连接Socket通信

而向无连接的socket通信就向发电子邮件一样，数据通信之前不需要建立连接的。