[LAD] About some interface of aplay

[新月如钩]

Hi All,
     Say hello to everyone!
     I'm a new member to learn how to write a ALSA application.
     I analyze the source code of aplay and very confused on follow interfaces:
            snd_pcm_playback_info()
            snd_pcm_playback_format()
            snd_pcm_playback_params()

     The aplay is belongs to alsa-utils that also is a application.
     It should be use interfaces of alsa-lib, but why i can't find these interfaces in alsa-lib ?
     Could you tell me where can find out these interface?

     Looking forward to your reply!

Best regards
Spring
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