本篇分析usb_linux_client.c中的usb_init()，它的代码如下：

void usb_init()
{
    if (access(USB_FFS_ADB_EP0, F_OK) == 0)
    {
        usb_ffs_init();
		
    }
    else{
        usb_adb_init();
		
    }
}

调用usb_adb_init():

static void usb_adb_init()
{
    usb_handle *h;
    adb_thread_t tid;
    int fd;

    h = calloc(1, sizeof(usb_handle));

    h->write = usb_adb_write;
    h->read = usb_adb_read;
    h->kick = usb_adb_kick;
    h->fd = -1;

    adb_cond_init(&h->notify, 0);
    adb_mutex_init(&h->lock, 0);

    // Open the file /dev/android_adb_enable to trigger 
    // the enabling of the adb USB function in the kernel.
    // We never touch this file again - just leave it open
    // indefinitely so the kernel will know when we are running
    // and when we are not.
    fd = unix_open("/dev/android_adb_enable", O_RDWR);
    if (fd < 0) {
       D("failed to open /dev/android_adb_enable\n");
    } else {
        close_on_exec(fd);
    }

    D("[ usb_init - starting thread ]\n");
    if(adb_thread_create(&tid, usb_adb_open_thread, h)){
        fatal_errno("cannot create usb thread");
    }
}

它初始化了usb_handle，并把它作为参数创建usb_adb_open_thread()线程。这里不能打开/dev/android_adb_enable。h->fd的值会在线程usb_adb_open_thread中赋值，并把它做为h->write(), h->read(), h->kick()的文件句柄，h->kick()函数功能是把h->fd置为-1。

usb_adb_open_thread的代码如下：

static void *usb_adb_open_thread(void *x)
{
    struct usb_handle *usb = (struct usb_handle *)x;
    int fd;

    while (1) {
        // wait until the USB device needs opening
        adb_mutex_lock(&usb->lock);
        while (usb->fd != -1)
            adb_cond_wait(&usb->notify, &usb->lock);
        adb_mutex_unlock(&usb->lock);

        D("[ usb_thread - opening device ]\n");
        do {
            /* XXX use inotify? */
            fd = unix_open("/dev/android_adb", O_RDWR);
            if (fd < 0) {
                // to support older kernels
                fd = unix_open("/dev/android", O_RDWR);
            }
            if (fd < 0) {
                adb_sleep_ms(1000);
            }
        } while (fd < 0);
        D("[ opening device succeeded ]\n");

        close_on_exec(fd);
        usb->fd = fd;

        D("[ usb_thread - registering device ]\n");
        register_usb_transport(usb, 0, 0, 1);
    }

    // never gets here
    return 0;
}

这个线程的作用是一进入立即打开/dev/android_adb或/dev/android，如果成功，则调用register_usb_transport()后再次循环，并阻塞在以下代码处

while(usb->fd != -1)

       adb_cond_wait(&usb->notify, &usb->lock);

当usb->kick()调用后fd的值被赋为-1，并发送cond唤醒上面的代码。