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wantnon_cn

a question about OpenAL on iOS

9 posts in this topic

i am adding sound to my iOS game using OpenAL, i downloaded the sample code "MusicCube" at Apple Developer, i use the code directly in my game and it works fine.

but however, i have a question about the sample code, in the sample code, in file MusicCubePlayback.m i find the initBuffer function, for your convenience i paste it here?

- (void) initBuffer

{
ALenum  error = AL_NO_ERROR;
ALenum  format;
ALsizei size;
ALsizei freq;
 
NSBundle* bundle = [NSBundle mainBundle];
 
// get some audio data from a wave file
CFURLRef fileURL = (CFURLRef)[[NSURL fileURLWithPath:[bundle pathForResource:@"sound" ofType:@"wav"]] retain];
 
if (fileURL)
{
_data = MyGetOpenALAudioData(fileURL, &size, &format, &freq);
        NSLog(@"audioInfo:%d,%d,%d",size,format,freq);
CFRelease(fileURL);
 
if((error = alGetError()) != AL_NO_ERROR) {
printf("error loading sound: %x\n", error);
exit(1);
}
 
// use the static buffer data API
alBufferDataStaticProc(_buffer, format, _data, size, freq);
 
if((error = alGetError()) != AL_NO_ERROR) {
printf("error attaching audio to buffer: %x\n", error);
}
}
else
{
printf("Could not find file!\n");
_data = NULL;
}
}

my question is: after the alBuffer established, why didn't they free the _data?  i think the alBuffer should be a memory in audio card and through the initBuffer function we submit the audio data in _data to audio card(alBuffer) and then _data is no more needed so we can free it. but i tried to free _data and it cause the sound not play normal. 

i just wonder why it is not just as the process of creating textures in OpenGL: when creating texture, after the texture established we can free the image data. 

so, back to OpenAL and the MusicCube sample, did it freed the _data in some where else i did not notice? or it just not be freed? if _data not freed, may it cause memory inefficiency?  

 

 

 

 

Edited by wantnon_cn
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It looks like you are correct.  I don't see anywhere in their example code that they free the memory the allocated with malloc() except in the case where there is a failure in initialization.

 

My guess is that they are just leaking the memory, making the OS clean up after them.

 

I agree it is a bad practice.  You should take ownership of your objects and manage their lifetimes properly.

Edited by frob
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i thought, for iOS developing, Apple's official samples should be the most correct. but if this MusicCube is not so good, what else sample code should i refer to? 

and, is there any possibility that the MusicCube sample is no inefficiency or leaking problems as we thought and there is something we not fully understand about this code?

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The alBufferDataStaticProc function does not make a copy of the data and instead leaves the application with the responsibility of managing the associated memory. With this being the case, freeing the data while it is still in use would not be desirable :)

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The alBufferDataStaticProc function does not make a copy of the data and instead leaves the application with the responsibility of managing the associated memory. With this being the case, freeing the data while it is still in use would not be desirable smile.png

i also suspected the reason lies in the alBufferDataStaticProc function. but does this mean we use normal memory instead of audio card memory? if so, would it be inefficient on performance or space?

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Firstly, just because that sound buffer may exist until shut-down of the application, it is still correct to delete it yourself and your responsibility to do so.  Anything you let the OS clean up for you is nothing but sloppiness on your part.  You still need to free() it after you get rid of the sound device.

 

As for the location of the RAM, iOS uses a unified memory model, but I can’t find any references on whether or not that extends to the sound hardware.

I’ve never heard of there being any other form of RAM for any other component of the device, and the fact that it even can play sounds directly from main memory likely implies the UMM extends to the sound hardware, so my feeling would be that there will be no difference in performance.

 

 

L. Spiro

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iOS uses a unified memory model,..., so my feeling would be that there will be no difference in performance.

if indeed like that, my worry will relieve.

 

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The alBufferDataStaticProc function does not make a copy of the data and instead leaves the application with the responsibility of managing the associated memory. With this being the case, freeing the data while it is still in use would not be desirable :)

i also suspected the reason lies in the alBufferDataStaticProc function. but does this mean we use normal memory instead of audio card memory? if so, would it be inefficient on performance or space?
If you look at the full code sample on Apple's web site, you can see that they simply use malloc() to get the memory.

The memory is not coming from within the audio library, nor is it a hardware-specific buffer.

The audio library both can notify you when complete and also allows you to query if it is complete. So it was just lazy programming in the demo.
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now, i have got most of the sound effect done in my game (using a modified version of the sample code). glad to see your replies about the question. it seems the conclusion is the sample code is no problem, right?

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The memory is being allocated and freed in the MyGetOpenALAudioData function.  Right click(assuming you have this OS mouse setting activated) on MyGetOpenALAudioData and go to 'Jump to Definition'  it'll open the file where the function definition is.  It uses malloc and free.

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