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+6. libao2: this control audio playing
+
+  As in libvo (see 5.) also here are some drivers, based on the same API:
+
+static int control(int cmd, int arg);
+  This is for reading/setting driver-specific and other special parameters.
+  Not really used for now.
+
+static int init(int rate,int channels,int format,int flags);
+  The init of driver, opens device, sets sample rate, channels, sample format
+  parameters.
+  Sample format: usually AFMT_S16_LE or AFMT_U8, for more definitions see
+  dec_audio.c and linux/soundcards.h files!
+
+static void uninit(void);
+  Guess what.
+  Ok I help: closes the device, not (yet) called when exit.
+
+static void reset(void);
+  Resets device. To be exact, it's for deleting buffers' contents,
+  so after reset() the previously received stuff won't be output.
+  (called if pause or seek)
+
+static int get_space(void);
+  Returns how many bytes can be written into the audio buffer without
+  blocking (making caller process wait). MPlayer occasionally checks the
+  remaining space and tries to fill the buffer with play() if there's free
+  space. The buffer size used should be sane; a buffer that is too small
+  could run empty before MPlayer tries filling it again (normally once per
+  video frame), a buffer that is too big would force MPlayer decode the file
+  far ahead trying to find enough audio data to fill it.
+
+static int play(void* data,int len,int flags);
+  Plays a bit of audio, which is received throught the "data" memory area, with
+  a size of "len". It has to copy the data, because they can be overwritten
+  after the call is made. Doesn't have to use all the bytes; it has to
+  return the number of bytes used used (copied to buffer). If
+  flags|AOPLAY_FINAL_CHUNK is true then this is the last audio in the file.
+  The purpose of this flag is to tell aos that round down the audio played
+  from "len" to a multiple of some chunksize that this "len" should not be
+  rounded down to 0 or the data will never be played (as MPlayer will never
+  call play() with a larger len).
+
+static float get_delay(void);
+  Returns how long time it will take to play the data currently in the
+  output buffer. Be exact, if possible, since the whole timing depends
+  on this! In the worst case, return the maximum delay.
+
+!!! Because the video is synchronized to the audio (card), it's very important
+!!! that the get_delay function is correctly implemented!
+
+static void audio_pause(void);
+  Pause playing but do not delete buffered data if possible.
+
+static void audio_resume(void);
+  Continue playing after audio_pause().