UAC设备，通过alsa处理系统得音量设置

相关api

snd_ctl_open

snd_ctl_card_info_alloca

snd_ctl_event_alloca

snd_ctl_subscribe_events

snd_ctl_wait

函数说明

int snd_ctl_open(snd_ctl_t **ctl, const char *name, int mode);

获取控制声卡得句柄

#define snd_ctl_card_info_alloca(ptr) __snd_alloca(ptr, snd_ctl_card_info)

获取声卡得信息

        snd_ctl_card_info_get_card

        snd_ctl_card_info_get_id

        snd_ctl_card_info_get_driver

        snd_ctl_card_info_get_name

        snd_ctl_card_info_get_longname

        snd_ctl_card_info_get_mixername

        snd_ctl_card_info_get_components

snd_ctl_event_alloca+snd_ctl_elem_value_malloc

通过        snd_ctl_elem_value_set_numid+snd_ctl_elem_read可以获取对应id得当前状态

注册snd_ctl_subscribe_events event事件

snd_ctl_wait等待event事件发生然后进行处理，event事件发生后直接调用snd_ctl_read+snd_ctl_elem_value_set_numid+snd_ctl_elem_read + snd_ctl_elem_value_get_bytes获取触发event得id以及对应得值