关于zabbix守护进程例子分析

很好的一个守护进程例子，贴出来，分析一下！
/******************************************************************************
 *                                                                            *
 * Function: daemon_start                                                     *
 *                                                                            *
 * Purpose: init process as daemon                                            *
 *                                                                            *
 * Parameters: allow_root - allow root permision for application              *
 *                                                                            *
 * Return value:                                                              *
 *                                                                            *
 * Author: Alexei Vladishev                                                   *
 *                                                                            *
 * Comments: it doesn't allow running under 'root' if allow_root is zero      *
 *                                                                            *
 ******************************************************************************/

int    daemon_start(int allow_root)
{
    pid_t            pid;
    struct passwd        *pwd;
    struct sigaction    phan;
    char            user[7] = "zabbix";

    /* running as root ?*/
    if((0 == allow_root) && (0 == getuid() || 0 == getgid()))
    {
        pwd = getpwnam(user); //从密码文件中取得指定帐号的数据
        if (NULL == pwd)
        {
            zbx_error("User %s does not exist.",
                user);
            zbx_error("Cannot run as root !");
            exit(FAIL);
        }
        if(setgid(pwd->pw_gid) ==-1) //设置真实组识别码
        {
            zbx_error("Cannot setgid to %s [%s].",
                user,
                strerror(errno));
            exit(FAIL);
        }
#ifdef HAVE_FUNCTION_INITGROUPS
        if(initgroups(user, pwd->pw_gid) == -1) //初始化组清单，/etc/group中
        {
            zbx_error("Cannot initgroups to %s [%s].",
                user,
                strerror(errno));
            exit(FAIL);
        }
#endif /* HAVE_FUNCTION_INITGROUPS */
        if(setuid(pwd->pw_uid) == -1) //设置用户识别码
        {
            zbx_error("Cannot setuid to %s [%s].",
                user,
                strerror(errno));
            exit(FAIL);
        }

#ifdef HAVE_FUNCTION_SETEUID
        if( (setegid(pwd->pw_gid) ==-1) || (seteuid(pwd->pw_uid) == -1) )
        {//重新设置当前进程的有效用户，组识别码
            zbx_error("Cannot setegid or seteuid to zabbix [%s].", strerror(errno));
            exit(FAIL);
        }
#endif /* HAVE_FUNCTION_SETEUID */

    }

    if( (pid = zbx_fork()) != 0 ) //创建进程
    {
        exit( 0 );
    }

    setsid(); //创建一个新的对话期

    signal( SIGHUP, SIG_IGN ); //设置信号

    if( (pid = zbx_fork()) !=0 )
    {
        exit( 0 );
    }

    /* This is to eliminate warning: ignoring return value of chdir */
    if(-1 == chdir("/")) //设置工作目录
    {
        assert(0);
    }
    umask(0002); //设置新建文件的权限遮罩

    redirect_std(CONFIG_LOG_FILE);

#ifdef HAVE_SYS_RESOURCE_SETPRIORITY

    if(setpriority(PRIO_PROCESS,0,5)!=0) //设置程序进程执行优先权
    {
        zbx_error("Unable to set process priority to 5. Leaving default.");
    }

#endif /* HAVE_SYS_RESOURCE_SETPRIORITY */

/*------------------------------------------------*/

    if( FAIL == create_pid_file(APP_PID_FILE)) //pid 文件
    {
        exit(FAIL);
    }

/*    phan.sa_handler = child_signal_handler;*/
    phan.sa_sigaction = child_signal_handler;
    sigemptyset(&phan.sa_mask);
    phan.sa_flags = SA_SIGINFO;

    sigaction(SIGINT,    &phan, NULL);
    sigaction(SIGQUIT,    &phan, NULL);
    sigaction(SIGTERM,    &phan, NULL);
    sigaction(SIGPIPE,    &phan, NULL);

    zbx_setproctitle("main process");

    return MAIN_ZABBIX_ENTRY(); //进入开始调用的主函数
}