Access Violation Exception mystery

Question

I've been working with EMGU+OpenCV for quite some time and ran into this AccessViolationException mystery.

First thing first, the code:

class AVE_Simulation     {         public static int Width = 7500;         public static int Height = 7500;         public static Emgu.CV.Image<Rgb, float>[] Images;          static void Main(string[] args)         {             int N = 50;             int Threads = 5;              Images = new Emgu.CV.Image<Rgb, float>[N];             Console.WriteLine("Start");              ParallelOptions po = new ParallelOptions();             po.MaxDegreeOfParallelism = Threads;             System.Threading.Tasks.Parallel.For(0, N, po, new Action<int>((i) =>             {                 Images[i] = GetRandomImage();                 Console.WriteLine("Prossing image: " + i);                 Images[i].SmoothBilatral(15, 50, 50);                 GC.Collect();             }));             Console.WriteLine("End");         }          public static Emgu.CV.Image<Rgb, float> GetRandomImage()         {             Emgu.CV.Image<Rgb, float> im = new Emgu.CV.Image<Rgb, float>(Width, Height);              float[, ,] d = im.Data;             Random r = new Random((int)DateTime.Now.Ticks);              for (int y = 0; y < Height; y++)             {                 for (int x = 0; x < Width; x++)                 {                     d[y, x, 0] = (float)r.Next(255);                     d[y, x, 1] = (float)r.Next(255);                     d[y, x, 2] = (float)r.Next(255);                 }             }             return im;         }      } 

The code is simple. Allocate an array of images. Generate a random image and populate it with random numbers. Execute bilateral filter over the image. That's it.

If I execute this program in a single thread, (Threads=1) everything seems to work normally with no problem. But, if I raise the number of concurrent threads to 5 I get an AccessViolationException very quickly.

I've went over OpenCV code and verified that there are no allocations on the OpenCV side and also went over the EMGU code searching for un-pinned objects or other problems and everything seems correct.

Some notes:

1. If you remove the GC.Collect() you will get the AccessViolationException less often but it will eventually happen.
2. This happens only when executed in Release mode. In Debug mode I didn't experience any exceptions.
3. Although each Image is 675MB there is no problem with allocation (I have ALLOT of memory) and a 'OutOfMemoryException' is thrown in case the system ran out of memory.
4. I used bilateral filter but I get this exception with other filters/functions as well.

Any help would be appreciated. I've been trying to fix this for more than a week.

i7 (no overclock), Win7 64bit, 32GB RAM, VS 2010, Framework 4.0, OpenCV 2.4.3

Stack:

Start Prossing image: 20 Prossing image: 30 Prossing image: 40 Prossing image: 0 Prossing image: 10 Prossing image: 21  Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.    at Emgu.CV.CvInvoke.cvSmooth(IntPtr src, IntPtr dst, SMOOTH_TYPE type, Int32 param1, Int32 param2, Double param3, Double param4)    at TestMemoryViolationCrash.AVE_Simulation.<Main>b__0(Int32 i) in C:\branches\1.1\TestMemoryViolationCrash\Program.cs:line 32    at System.Threading.Tasks.Parallel.<>c__DisplayClassf`1.<ForWorker>b__c()    at System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask)    at System.Threading.Tasks.Task.<>c__DisplayClass10.<ExecuteSelfReplicating>b__f(Object param0)    at System.Threading.Tasks.Task.Execute()    at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)    at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)    at System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)    at System.Threading.Tasks.ThreadPoolTaskScheduler.TryExecuteTaskInline(Task task, Boolean taskWasPreviouslyQueued)    at System.Threading.Tasks.TaskScheduler.TryRunInline(Task task, Boolean taskWasPreviouslyQueued)    at System.Threading.Tasks.Task.InternalRunSynchronously(TaskScheduler scheduler, Boolean waitForCompletion)    at System.Threading.Tasks.Parallel.ForWorker[TLocal](Int32 fromInclusive, Int32 toExclusive, ParallelOptions parallelOptions, Action`1 body, Action`2 bodyWithState, Func`4 bodyWithLocal, Func`1 loc alInit, Action`1 localFinally)    at System.Threading.Tasks.Parallel.For(Int32 fromInclusive, Int32 toExclusive, ParallelOptions parallelOptions, Action`1 body)    at TestMemoryViolationCrash.AVE_Simulation.Main(String[] args) in C:\branches\1.1\TestMemoryViolationCrash\Program.cs:line 35  Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.    at Emgu.CV.CvInvoke.cvSmooth(IntPtr src, IntPtr dst, SMOOTH_TYPE type, Int32 param1, Int32 param2, Double param3, Double param4)    at TestMemoryViolationCrash.AVE_Simulation.<Main>b__0(Int32 i) in C:\branches\1.1\TestMemoryViolationCrash\Program.cs:line 32    at System.Threading.Tasks.Parallel.<>c__DisplayClassf`1.<ForWorker>b__c()    at System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask)    at System.Threading.Tasks.Task.<>c__DisplayClass10.<ExecuteSelfReplicating>b__f(Object param0)    at System.Threading.Tasks.Task.Execute()    at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)    at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)    at System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)    at System.Threading.ThreadPoolWorkQueue.Dispatch()  Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.    at Emgu.CV.CvInvoke.cvSmooth(IntPtr src, IntPtr dst, SMOOTH_TYPE type, Int32 param1, Int32 param2, Double param3, Double param4)    at TestMemoryViolationCrash.AVE_Simulation.<Main>b__0(Int32 i) in C:\branches\1.1\TestMemoryViolationCrash\Program.cs:line 32    at System.Threading.Tasks.Parallel.<>c__DisplayClassf`1.<ForWorker>b__c()    at System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask)    at System.Threading.Tasks.Task.<>c__DisplayClass10.<ExecuteSelfReplicating>b__f(Object param0)    at System.Threading.Tasks.Task.Execute()    at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)    at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)    at System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)    at System.Threading.ThreadPoolWorkQueue.Dispatch() Press any key to continue . . . 

Answer 1

Your example doesn't keep a reference to the result image from Image.SmoothBilatral. The input images are rooted in a static array so are fine.

An Emgu.CV Image's data array is pinned to a GCHandle inside the actual image, this is no different from the fact that image contains the array and doesn't prevent collection while the GCHandle's pointer is in use by unmanaged code (in the abscence of a managed root to the image).

Because the Image.SmoothBilatral method doesn't do anything with its temporary result image other than pass its pointer and return it, I think it gets optimised away to the extent that the result image can be collected while the smooth is processing.

Because there's no finalizer for this class, opencv will not get called upon to release it's unmanaged image header (which has a pointer to the managed image data) so opencv still thinks it has a usable image structure.

You can fix it by taking a reference to the result of SmoothBilatral and doing something with it (like disposing it).

This extension method would also work (i.e. allow it to be called successfuly for benchmarking without the result being used):

public static class BilateralExtensionFix {     public static Emgu.CV.Image<testchannels, testtype> SmoothBilateral(this Emgu.CV.Image<testchannels, testtype> image, int p1, int p2 , int p3)     {         var result = image.CopyBlank();         var handle = GCHandle.Alloc(result);         Emgu.CV.CvInvoke.cvSmooth(image.Ptr, result.Ptr, Emgu.CV.CvEnum.SMOOTH_TYPE.CV_BILATERAL, p1, p1, p2, p3);         handle.Free();         return result;     } } 

I think what EmguCV should be doing is only pinning pointers to pass to opencv while making an interop call.

p.s The OpenCv bilateral filter crashes (producing a very similar error to your problem) on any kind of float image passed with zero variation (min() = max()) across all channels. I think because of how it builds it's binned exp() lookup table.

This can be reproduced with:

    // create new blank image     var zeroesF1 = new Emgu.CV.Image<Rgb, float>(75, 75);     // uncomment next line for failure     zeroesF1.Data[0, 0, 0] += 1.2037063600E-035f;     zeroesF1.SmoothBilatral(15, 50, 50); 

This was confusing me as I was actually sometimes getting this error due to a bug in my test code...