The VideoLAN Forums

Posted: **01 Dec 2011 17:58**

I am trying to use libvlc to receive frames in a video processing application.

I have been working from the samples on the wiki. I am able to display a video using the libvlc_media_player_play function. Now I am attempting to uses the call backs so that I can have direct access to the data.

I added three function (lock, unlock and display) which are all void at this time.

Then I call
libvlc_video_set_callbacks(pVlcMediaPlayer, Lock, unlock, display, 0) ;
libvlc_video_set_format(pVlcMediaPlayer, "RV16", VIDEOWIDTH, VIDEOHEIGHT,VIDEOWIDTH*2);
The code in its entirety can be found below.

The applications runs and crashes in with a memory access violation in libswscale_plugin.dll
I am running this on windows using VS2008 and the call stack locks as follows:

libswscale_plugin.dll!6b8ce795()
[Frames below may be incorrect and/or missing, no symbols loaded for libswscale_plugin.dll]
libswscale_plugin.dll!6b8d6f30()
libswscale_plugin.dll!6b8c07ee()
libswscale_plugin.dll!6b8b2a1c()
libvmem_plugin.dll!68a112f3()
libvlccore.dll!6feebdac()
libvout_wrapper_plugin.dll!68a217c2()
libvlccore.dll!6fea637d()
ntdll.dll!774cf965()
KernelBase.dll!753204d6()
libvlccore.dll!6feef86d()
msvcrt.dll!754d1287()
msvcrt.dll!754d1328()
kernel32.dll!75ce339a()
ntdll.dll!774e9ed2()
ntdll.dll!774e9ea5()

Please let me know if there is any other information that would be useful to help solve this issue.

Code: Select all

extern "C"
{
  #include <vlc/vlc.h>
}

#include <wx/app.h>
using namespace std;

int VIDEOWIDTH = 100;
int VIDEOHEIGHT = 200;

extern "C" {
//*****************************************************************************
//*****************************************************************************
static void* Lock(void *data, void **p_pixels)
{
  return NULL;
}

//*****************************************************************************
//*****************************************************************************
static void display(void *data, void *id)
{
}

//*****************************************************************************
//*****************************************************************************
static void unlock(void *data, void *id, void *const *p_pixels)
{
}
}
//*****************************************************************************
//*****************************************************************************
int main()
{
  libvlc_instance_t* pLibVlc;
  libvlc_media_player_t* pVlcMediaPlayer;
  libvlc_media_t* pVlcMedia;

  /* Load the VLC engine */
  pLibVlc = libvlc_new(0, NULL);

  /* Create a new item */
  pVlcMedia = libvlc_media_new_path(
    pLibVlc,
    "C:\\DataLocal\\Video\\Simpsons.mp4");

  /* Create a media player playing environement */
  pVlcMediaPlayer = libvlc_media_player_new_from_media(pVlcMedia);

  /* No need to keep the media now */
  libvlc_media_release(pVlcMedia);

  libvlc_video_set_callbacks(
    pVlcMediaPlayer,
    Lock,
    unlock,
    display,
    0);

  libvlc_video_set_format(
    pVlcMediaPlayer,
    "RV16",
    VIDEOWIDTH,
    VIDEOHEIGHT,
    VIDEOWIDTH*2);

  /* play the media_player */
  libvlc_media_player_play(pVlcMediaPlayer);

  ::wxMilliSleep(30000); /* Let it play a bit */

  /* Stop playing */
  libvlc_media_player_stop(pVlcMediaPlayer);

  /* Free the media_player */
  libvlc_media_player_release(pVlcMediaPlayer);

  libvlc_release(pLibVlc);

  return 0;
}

Posted: **01 Dec 2011 18:36**

Lock is not allowed to fail. You must set p_pixels to point to a memory buffer for the picture. If you hit a fatal error allocating memory, then don't start the playback.

What you see is the scaler trying to output the picture to your undefined invalid memory area.

Posted: **04 Dec 2011 00:06**

Thank You. That solved the issue.

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