Not really necessary to use an intermediate buffer, but won't hurt. I'm not sure why you are using a circular buffer, though it shouldn't hurt either.
I wrote another code sometime back, to asychcronously playback data from a wav file, where I copied the contents of the wav file to a master buffer (like in this code) and then pumped data to the callback till the end of the buffer was reached. This code works fine -- I m trying to get the circular buffer to work now as part of an application I am writing that will allow realtime communication over the network between two hosts.
A circular buffer thus seems like a good idea, as in the case of real time communication I wont really know how much data is going to come through the network.
so now instead of copying from the wave file, i m copying data from
the master buffer to the circular buffer.. however, as per your suggestion, if i copy the data in the while loop in main, then, it might not always be copied between 2 consecutive callbacks, but maybe more, as the callbacks are asynchronous.
Well, your computer is creating the data that the callback function is going to send to alsa. If the callback function can fill the buffer and send it to alsa and keep up with the play rate, then if you fill the buffer in your main routine it will work even better. Your program is sleeping instead of doing anything. You can put in place some kind of interprocess communication so that they don't tread on each other in the buffer. With any modern processor that isn't heavily loaded, I don't think this will matter anyway.
One question I had about this interprocess communication was whether we can pass more data to the callback function apart from the pcm handler. From what I can see in the ALSA libraries, the snd_async_add_pcm_handler function only allows you to pass one parameter in the form of void * private_data.
however, the hardware still does not ask for more data after
executing the callback function about twice -- is there a way to flush the hardware buffer before beginning playback? I have pasted my code below
The program output would still be helpful. I implemented this a few years ago. I'll go look at the code and see how it differs from yours. I seem to recall that there was a sample of exactly this on the alsa website, www.alsa-project.org. You could look at that and it might trigger an Aha! moment.
I think you re talking about the pcm.c which implements asynchronous playback which in my case works -- but it doesnt implement a circular buffer. Another query that I have regarding this is that I get a "Bad File Descriptor" error from the snd_pcm_start function, but my asynchronous playback code works despite this error -- I dont think thats the reason for this code failing, but its a mystry all the same!
Here is the output of my program:
i=49
error in starting snd pcm start err :File descriptor in bad state infinite loop inside callback, available frames = 364 going out of callback, ind = 1 infinite loop inside callback, available frames = 577 going out of callback, ind = 2 infinite loop inside callback, available frames = 790 underrun recovery error from snd_pcm_prepare is: 0 going out of callback, ind = 3 infinite loop infinite loop infinite loop
Regards,
Ashlesha.
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