C# application with unmanaged DLL freezes whole system

Question

I'm currently writing a software in Visual Studio 2012 for communication with RFID-cards. I got a DLL written in Delphi to handle the communication with the card reader.

The problem is: My software is running fine on machines, that have VS2012 installed. On other systems it freezes itself or the whole system. I tried it on Win XP / 7 / 8 with x32 and x64 configuration. I'm using .NET 4.0.

After connecting to the reader, the software starts a backgroundWorker, which polls (at 200ms rate) the reader with a command to inventory cards in the readers RF-field. The crash usally happens ca. 10 to 20 seconds after the reader connect. Here is the code:

[DllImport("tempConnect.dll", CallingConvention = CallingConvention.StdCall)]
 private static extern int inventory(int maxlen, [In] ref int count, 
                                         IntPtr UIDs, UInt32 HFOffTime);

public String getCardID()
    {
        if (isConnectet())
        {
            IntPtr UIDs = IntPtr.Zero;
            int len = 2 * 8;
            Byte[] zero = new Byte[len];
            UIDs = Marshal.AllocHGlobal(len);
            Thread.Sleep(50);
            Marshal.Copy(zero, 0, UIDs, len);
            int count = 0;
            int erg;
            String ret;
            try
            {
                erg = inventory(len, ref count, UIDs, 50);
            }
            catch (ExternalException) // this doesn't catch anything (iI have set <legacyCorruptedStateExceptionsPolicy enabled="true"/>)
            {
                return "\0";
            }
            finally
            {
                ret = Marshal.PtrToStringAnsi(UIDs, len);
                IntPtr rslt = LocalFree(UIDs);
                GC.Collect();
            }
            if (erg == 0)
                return ret;
            else
                return zero.ToString();
        }
        else
            return "\0";
    }

The DLL is written in Delphi, the code DLL command is:

function inventory (maxlen: Integer; var count: Integer; 
                  UIDs: PByteArray; HFOffTime: Cardinal = 50): Integer; STDCALL;

I think there may be a memory leak somewhere, but I have no idea how to find it...

EDIT:

I added some ideas (explicit GC.Collect(), try-catch-finally) to my code above, but it still doesnt work.

Here is the code, that calls getCardID():

The action, that runs every 200ms:

if (!bgw_inventory.IsBusy)
   bgw_inventory.RunWorkerAsync();

Async backgroundWorker does:

private void bgw_inventory_DoWork(object sender, DoWorkEventArgs e)
    {
            if (bgw_inventory.CancellationPending)
        {
            e.Cancel = true;
            return;
        }
        else
        {
            String UID = reader.getCardID();
            if (bgw_inventory.CancellationPending)
            {
                e.Cancel = true;
                return;
            }
            if (UID.Length == 16 && UID.IndexOf("\0") == -1)
            {
                setCardId(UID);
                if (!allCards.ContainsKey(UID))
                {
                    allCards.Add(UID, new Card(UID));
                }
                if (readCardActive || deActivateCardActive || activateCardActive)
                {
                    if (lastActionCard != UID)
                        actionCard = UID;
                    else
                        setWorkingStatus("OK", Color.FromArgb(203, 218, 138));
                }
            }
            else
            {
                setCardId("none");
                if (readCardActive || deActivateCardActive || activateCardActive)
                    setWorkingStatus("waiting for next card", Color.Yellow);
            }
        }
   }

EDIT

Till now I have made some little reworks (updates above) at the code. Now only the App. crashes with 0xC00000FD (Stack overflow) at "tempConnect.dll". This does not happen on Systems with VS2012 installed or if I use the DLL with native Delphi! Do anyone have any other ideas ?

EDIT

Now I made the DLL logging it's stacksize and found something weird: If it's called and polled from my C# Programm, the stacksize is changing continuously up and down. If i do the same from a natural Deplhi Program the stacksize is constant! So I'll do further investigations, but I have no really idea, what I have to search for...

Just one doubt, I don't think it is related to your problem, but analyzing the code after calling the `inventory()` function, you are checking if UIDS is not null. This construct suggests that the value of UIDS might be set to null inside the `inventory` function. If that's the case - shouldn't you be pasing the UIDS by reference? — , Nov 01 '12 at 18:18
The current code might leak if there's an exception being thrown inside the innermost try-catch handler, before the call to `Marshal.FreeHGlobal()` is reached. I would suggest adding to this handler a `finally` clause where the call to `Marshal.FreeHGlobal()` is done. This way you ensure the allocated memory is freed even if something went wrong. I'd also suggest logging the error inside the catch clause, since at this moment you can't tell if a return value of `zero.ToString()` is a legitimate value returned by `inventory()`, or something went wrong. — , Nov 01 '12 at 18:42
Could you please post the code that is calling `getCardID()`? The reason I ask this is just to verify that there is no possibility that inventory() might be called concurrently by more than one thread. If it is not designed to be thread-safe, that could be a reason for the freezes you are experiencing. Cheers! — , Nov 01 '12 at 19:15
By freezes the whole system, you mean, you can't access the task manager or kill the hung process? — Warren P, Nov 01 '12 at 21:39
The problem is almost certainly at kernel level in the device driver. Not in that user mode code. — David Heffernan, Nov 01 '12 at 23:32
For everyone who is curious: I ended up with rewriting the *.dll that is responsible for the communication with the RFID reader in managed C#. Sorry for that disappointing resolution. — deR_Ed, May 04 '15 at 20:54

score 1 · Answer 1 · edited May 23 '17 at 12:28

I'm a little concerned about how're using that Marshal object. As you fear with the memory leak, it seems to be allocating memory quite often but I don't see it ever explicitly releasing it. The garbage collector should (operative word) be taking care of that, but you say yourself you have some unmanaged code in the mix. It is difficult with the posted information to tell where the unmanaged code begins.

Check out this question for some good techniques to finding memory leaks in .NET itself - this will give you a ton of information on how memory is being used in the managed end of your code (that is, the part you can directly control). Use the Windows Performance Monitor with breakpoints to keep an eye on the overall health of the system. If .NET appears to be behaving, but WPM is showing some sharp spikes, it's probably in the unmanaged code. You can't really control anything but your usage there, so it would probably be time to go back to the documentation at that point.

C# application with unmanaged DLL freezes whole system

1 Answers1