snd_ctl_file的作用什么呢？（求高手解答！！！！！！！）

chen_q07

struct snd_ctl_file {
        struct list_head list;                /* list of all control files */
        struct snd_card *card;
        struct pid *pid;
        int prefer_pcm_subdevice;
        int prefer_rawmidi_subdevice;
        wait_queue_head_t change_sleep;
        spinlock_t read_lock;
        struct fasync_struct *fasync;
        int subscribed;                        /* read interface is activated */
        struct list_head events;        /* waiting events for read */
};
ALSA