socket通信的几个关键步骤

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。