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Segfault with smem module
Posted: 30 May 2010 12:59
by wetneb
Yesterday evening on #videolan I talked about a segfault I get with the smem module.
The normal behaviour should be :
1. The prerender callback allocates a buffer with the right size
2. Smem uses memcpy to fill the buffer
3. The postrender callback uses the buffer (and deletes it, if needed)
My code is :
Code: Select all
void StreamPlayer::prepareRender( void* p_audio_data, uint8_t** pp_pcm_buffer , unsigned int size )
{
cout << "Prerender" << endl;
cout << " pp_pcm_buffer = ";
cout << pp_pcm_buffer << endl;
cout << " *pp_pcm_buffer = " << flush;
cout << *pp_pcm_buffer << endl; // Here I get a segfault. Why ?
// This pointer has been passed from another thread, from SendAudio in smem.
// I should be able to access to it since I'm supposed to modify it !
(*pp_pcm_buffer) = new uint8_t[size]; // Dummy behaviour : each time, we delete and reallocate, which should take a while
cout << "Prerender ended." << endl << endl;
}
void StreamPlayer::handleStream(void* p_audio_data, uint8_t* p_pcm_buffer, unsigned int channels, unsigned int rate, unsigned int nb_samples, unsigned int bits_per_sample, unsigned int size, int64_t pts )
{
cout << "Postrender" << endl;
delete p_pcm_buffer;
cout << "Postrender ended." << endl << endl;
}
Does anybody wrote a code using this module, which runs correctly ?
Thanks
Re: Segfault with smem module
Posted: 30 May 2010 13:06
by Jean-Baptiste Kempf
vlmc does
Re: Segfault with smem module
Posted: 30 May 2010 14:00
by Rémi Denis-Courmont
Your code is patently broken. VLC does not initialize the data pointer, since that's the very job of the prerender function. In other words, pp_pcm_buffer is valid (it should point a few byte up the stack), but *pp_pcm_buffer is garbage.
cout << *pp_pcm_buffer << endl; tries to print a string contained at *pp_pcm_buffer, i.e. it will read **pp_pcm_buffer. Since this is a garbage address, it is likely to segfault.
Re: Segfault with smem module
Posted: 30 May 2010 14:33
by wetneb
*pp_pcm_buffer should have the same value as p_pcm_buffer in the SendAudio function, since pp_pcm_buffer has been initialized with &p_pcm_buffer.
So I should be able to read it.
If I write this code, everything works :
uint16_t *test;
uint16_t **pp = &test;
cout << pp << endl;
cout << *pp << endl;
Or let's remove this line. I still get a segfault when I allocate my memory (in the following line).
Re: Segfault with smem module
Posted: 01 Jun 2010 18:47
by wetneb
Here is the code used by VLMC :
Code: Select all
void
VideoClipWorkflow::lock( VideoClipWorkflow *cw, void **pp_ret, int size )
{
Q_UNUSED( size );
LightVideoFrame* lvf = NULL;
cw->m_renderLock->lock();
if ( cw->m_availableBuffers.isEmpty() == true )
{
lvf = new LightVideoFrame( cw->m_width, cw->m_height );
}
else
lvf = cw->m_availableBuffers.dequeue();
cw->m_computedBuffers.enqueue( lvf );
*pp_ret = (*(lvf))->frame.octets;
}
void
VideoClipWorkflow::unlock( VideoClipWorkflow *cw, void *buffer, int width,
int height, int bpp, int size, qint64 pts )
{
Q_UNUSED( buffer );
Q_UNUSED( width );
Q_UNUSED( height );
Q_UNUSED( bpp );
Q_UNUSED( size );
cw->computePtsDiff( pts );
LightVideoFrame *lvf = cw->m_computedBuffers.last();
(*(lvf))->ptsDiff = cw->m_currentPts - cw->m_previousPts;
cw->commonUnlock();
cw->m_renderWaitCond->wakeAll();
cw->m_renderLock->unlock();
}
Maybe I need to use mutexes ? Currently, my code doesn't access to all this data, except from the lock and unlock functions, so I don't see why my code would be responsible for this segfault…
Re: Segfault with smem module
Posted: 02 Jun 2010 19:39
by Rémi Denis-Courmont
I think you're not getting the point. Your code is basically doing this:
Code: Select all
uint8_t *p; // uninitialized
uint8_t **pp = &p; // in VLC smem plugin
cout << *pp << endl; // in your callback
which is equivalent to this:
Code: Select all
unsigned char *p; // uninitialized
cout << p << endl;
but is
completely different from this:
Code: Select all
unsigned char *p; // uninitialized
cout << (void *)p << endl;
All code snippets are undefined, as they use an undefined value. But the first two snippets try to print a nul-terminated string at the undefined location "p". This expectedly results in a segmentation fault. The third snippet just prints a pointer value, which should result in an hexadecimal number.
Re: Segfault with smem module
Posted: 02 Jun 2010 20:47
by wetneb
I didn't think that the type of the pointer could be the problem. But now if i add this cast (void*), it doesn't work better.
The first two snippets you gave run well here (actually, the addresses aren't displayed correctly, but there's no segfault at all).
Re: Segfault with smem module
Posted: 02 Jun 2010 21:02
by Rémi Denis-Courmont
The VLC code is correct, otherwise VLMC could not use it. Whether you need to acquire a mutex is dependent on your application, there is no magic yes/no answer.
Re: Segfault with smem module
Posted: 02 Jun 2010 21:08
by wetneb
Ok, thanks for your help
.