void service_start(struct service *svc, const char *dynamic_args)
{   /// ******************************  start service 的第一個階段
    struct stat s;
    pid_t pid;
    int needs_console;
    int n;
    char *scon = NULL;
    int rc;

        /* starting a service removes it from the disabled or reset
         * state and immediately takes it out of the restarting
         * state if it was in there
         */
    svc->flags &= (~(SVC_DISABLED|SVC_RESTARTING|SVC_RESET|SVC_RESTART|SVC_DISABLED_START)); // 這個service即將被啟動，將其從disable或reset的狀態給移除掉，置其為重新執行的狀態
    svc->time_started = 0;

        /* running processes require no additional work -- if
         * they're in the process of exiting, we've ensured
         * that they will immediately restart on exit, unless
         * they are ONESHOT
         */
    if (svc->flags & SVC_RUNNING) {  // 如果這個service仍然是執行態的話，即return
        return;
    }

    needs_console = (svc->flags & SVC_CONSOLE) ? 1 : 0;
    if (needs_console && (!have_console)) {
        ERROR("service '%s' requires console\n", svc->name);
        svc->flags |= SVC_DISABLED;
        return;
    } // 如果這個service的flags是初始console，但是這個已經啟動了的話，就會設定當前的flags為disabled

    if (stat(svc->args[0], &s) != 0) {   // 如果要執行的這個service的start的command不存在的話，返回error
        ERROR("cannot find '%s', disabling '%s'\n", svc->args[0], svc->name);
        svc->flags |= SVC_DISABLED;
        return;
    }

    if ((!(svc->flags & SVC_ONESHOT)) && dynamic_args) { // 因為dynamic_args為null，所以這邊不會進入這個判斷
        ERROR("service '%s' must be one-shot to use dynamic args, disabling\n",
               svc->args[0]);
        svc->flags |= SVC_DISABLED;
        return;
    }
   // ***********************************************************這裡我們可以認為是第二個階段，selinux是資訊保安相關的操作，這邊我們忽略掉
    if (is_selinux_enabled() > 0) {
        if (svc->seclabel) {
            scon = strdup(svc->seclabel);
            if (!scon) {
                ERROR("Out of memory while starting '%s'\n", svc->name);
                return;
            }
        } else {
            char *mycon = NULL, *fcon = NULL;

            INFO("computing context for service '%s'\n", svc->args[0]);
            rc = getcon(&mycon);
            if (rc < 0) {
                ERROR("could not get context while starting '%s'\n", svc->name);
                return;
            }

            rc = getfilecon(svc->args[0], &fcon);
            if (rc < 0) {
                ERROR("could not get context while starting '%s'\n", svc->name);
                freecon(mycon);
                return;
            }

            rc = security_compute_create(mycon, fcon, string_to_security_class("process"), &scon);
            if (rc == 0 && !strcmp(scon, mycon)) {
                ERROR("Warning!  Service %s needs a SELinux domain defined; please fix!\n", svc->name);
            }
            freecon(mycon);
            freecon(fcon);
            if (rc < 0) {
                ERROR("could not get context while starting '%s'\n", svc->name);
                return;
            }
        }
    }
    // ***************************************** selinux的操作結束，進入到第三個階段
    NOTICE("starting '%s'\n", svc->name);

    pid = fork();   // fork一個自程序，即所有從init.rc啟動的service，都是一個子程序

    if (pid == 0) {  // pid = 0, 進入到子程序中
        struct socketinfo *si;
        struct svcenvinfo *ei;
        char tmp[32];
        int fd, sz;

        umask(077);
        if (properties_inited()) {  
            get_property_workspace(&fd, &sz);  // 得到屬性儲存空間的資訊並加入到環境變數中
            sprintf(tmp, "%d,%d", dup(fd), sz);
            add_environment("ANDROID_PROPERTY_WORKSPACE", tmp);
        }

        for (ei = svc->envvars; ei; ei = ei->next)  // 將service自己宣告的env加入到環境變數中
            add_environment(ei->name, ei->value);

        for (si = svc->sockets; si; si = si->next) {  // 根據socket info設定socket
            int socket_type = (
                    !strcmp(si->type, "stream") ? SOCK_STREAM :
                        (!strcmp(si->type, "dgram") ? SOCK_DGRAM : SOCK_SEQPACKET));
            int s = create_socket(si->name, socket_type,
                                  si->perm, si->uid, si->gid, si->socketcon ?: scon);
            if (s >= 0) {
                publish_socket(si->name, s);
            }
        }

        freecon(scon);
        scon = NULL;

        if (svc->ioprio_class != IoSchedClass_NONE) {
            if (android_set_ioprio(getpid(), svc->ioprio_class, svc->ioprio_pri)) {
                ERROR("Failed to set pid %d ioprio = %d,%d: %s\n",
                      getpid(), svc->ioprio_class, svc->ioprio_pri, strerror(errno));
            }
        }

        if (needs_console) {
            setsid();
            open_console();
        } else {
            zap_stdio();
        }

#if 0
        for (n = 0; svc->args[n]; n++) {
            INFO("args[%d] = '%s'\n", n, svc->args[n]);
        }
        for (n = 0; ENV[n]; n++) {
            INFO("env[%d] = '%s'\n", n, ENV[n]);
        }
#endif

        setpgid(0, getpid());

    /* as requested, set our gid, supplemental gids, and uid */
        if (svc->gid) { // 設定gid
            if (setgid(svc->gid) != 0) {
                ERROR("setgid failed: %s\n", strerror(errno));
                _exit(127);
            }
        }
        if (svc->nr_supp_gids) {
            if (setgroups(svc->nr_supp_gids, svc->supp_gids) != 0) {
                ERROR("setgroups failed: %s\n", strerror(errno));
                _exit(127);
            }
        }
        if (svc->uid) {  // 設定uid
            if (setuid(svc->uid) != 0) {
                ERROR("setuid failed: %s\n", strerror(errno));
                _exit(127);
            }
        }
        if (svc->seclabel) {
            if (is_selinux_enabled() > 0 && setexeccon(svc->seclabel) < 0) {
                ERROR("cannot setexeccon('%s'): %s\n", svc->seclabel, strerror(errno));
                _exit(127);
            }
        }

        if (!dynamic_args) { // 因為dynamic_args設定的為null，我們在第一次從init.rc啟動的時候，一定會進入到這個判斷。
            if (execve(svc->args[0], (char**) svc->args, (char**) ENV) < 0) {  // ！！！ 執行當前的service的啟動的命令，也就是說從這邊開始，我們就可以理解為已經從init程序中，去像kernel執行init一樣，就去執行各個service所對應的啟動函數了！
                ERROR("cannot execve('%s'): %s\n", svc->args[0], strerror(errno));
            }
        } else {
            char *arg_ptrs[INIT_PARSER_MAXARGS+1];
            int arg_idx = svc->nargs;
            char *tmp = strdup(dynamic_args);
            char *next = tmp;
            char *bword;

            /* Copy the static arguments */
            memcpy(arg_ptrs, svc->args, (svc->nargs * sizeof(char *)));

            while((bword = strsep(&next, " "))) {
                arg_ptrs[arg_idx++] = bword;
                if (arg_idx == INIT_PARSER_MAXARGS)
                    break;
            }
            arg_ptrs[arg_idx] = '\0';
            execve(svc->args[0], (char**) arg_ptrs, (char**) ENV);
        }
        _exit(127);
    }

    freecon(scon);

    if (pid < 0) {
        ERROR("failed to start '%s'\n", svc->name);
        svc->pid = 0;
        return;
    }

    svc->time_started = gettime();
    svc->pid = pid;
    svc->flags |= SVC_RUNNING;

    if (properties_inited())
        notify_service_state(svc->name, "running");
}