ALSA programming - segmantation fault

anuel

Hello

I write simple program which I need to implement in my MA thesis. It have to read data from audio card by some time an then write them to that device but it doesn't work, because I get Segmentation fault. I think it's associated with handle, because I was testing it and noticed that ALSA functions which have only handle as an argument give segmentation fault (but maybe reason is lies deeper). I don't know about I/O programming - ALSA programming too - so I'm asking you for help. My code is attached.

mfillpot

I am sorry that I have not touched on audio development yet and cannot help you with the code.
If you have no luck in here, then checkout my post at http://www.linux.com/community/forums?func=view&catid=17&id=5637 to be directed to our sister site for Linux Developers in which you can get better responses for development related questions.

woboyle

I took a very quick look at your code. In your function inicjuj_PCM() you pass the address of an snd_pcm_t object as *pcm_handle, and then you pass the address of that to snd_pcm_open() as &pcm_handle. This may be a cause of your problem, since the actual address of the pointer itself (not the contents) is not available. You need to change your code to pas a snd_pcm_t** to inicjuj_PCM() and then just pass that along unmodified to snd_pcm_open(). Then you need to fix the rest of your code to deal with the double-indirection of pcm_handle. The reason for this is that the ALSA functions may modify the value of the pointer address and there is no way to do that as you have written it. Hence, the core dump / segfault.

anuel

It works what you said. Thank you. I thought the same but I was so bored my thesis that I just gave up and didn't try my speculation.

woboyle

Happy to help and glad that this did the trick. Post more questions when you have coding problems. I've been programming in C and C++ for about 30 years now. I've made most of the possible errors one can make already... :-)

marc

woboyle wrote:

Happy to help and glad that this did the trick. Post more questions when you have coding problems. I've been programming in C and C++ for about 30 years now. I've made most of the possible errors one can make already... :-)

That sentence might get you in trouble: I'm learning C++ and Qt right now.

anuel

So you know C almost as long as I live . I will remember it. Can you tell me one more thing?? When I want to read something from audio card and something send to it, do I have to close handler not used in same time as other handler, for instance close capture handler when sending data to playback handler. Or it's not so importent?? If it has matter in Alsa programming context.

woboyle

Well, I may have been programming in C/C++ for almost 30 years, but I haven't gotten into the ALSA and Linux sound API's yet since I haven't had the need to. That said, it is doubtful that you need to close one stream before you use the other. However, depending upon whether the streams are buffered or not, you may want to use a thread to handle each separately. That would be appropriate if you have an unbuffered input stream and don't want to lose any incoming data.

anuel

In my case there is no need to catch all data from stream because I "remember" only this fragment of signal which different from silence and there put it to output stream to know what was recorded and then I compute these data (or however to call it - sorry, I'm not from anglo-phone country ). Ok, thank you for your advice.

woboyle

marc wrote:

woboyle wrote:

Happy to help and glad that this did the trick. Post more questions when you have coding problems. I've been programming in C and C++ for about 30 years now. I've made most of the possible errors one can make already... :-)

That sentence might get you in trouble: I'm learning C++ and Qt right now.

Well, I am available on a consulting basis. My rates are $200USD per hour ($175 for contracts of 100+ hours) :-)