Array values changed to 0

Question

I am passing an array through a message queue:

double data[20];
if(i!=20)
{
    data[i] = getInstance()->real_time_data.f_ChannelData;
    i++;
}
else
{
    i = 0;
    MsgQueueCommunicator::getInstance()->Write_Array_To_Queue(s_hMsgQueue_Communication, data);
}

Write_Array_To_Queue() looks like this:

BOOL MsgQueueCommunicator::Write_Array_To_Queue(HANDLE hq,double data[20])
{
    return WriteMsgQueue(hq,(LPVOID)data, 160,INFINITE,0);
}

Reading from the queue is pretty much the same as writing:

BOOL MsgQueueCommunicator::Read_Buffer_From_Queue(HANDLE hq,double data[20])
{
    DWORD dwBytesRead;
    DWORD dwFlags;
    return ReadMsgQueue(hq, (LPVOID)data, 160, &dwBytesRead, INFINITE, &dwFlags);
}

To initialize the queue on both ends, I use the following function:

HANDLE MsgQueueCommunicator::InitMessageQueue(bool IsRead,wchar16_t* wQueueName)
{
  MSGQUEUEOPTIONS msgopts;
  msgopts.dwSize        = sizeof(MSGQUEUEOPTIONS);   
  msgopts.dwFlags       = MSGQUEUE_ALLOW_BROKEN;//0;
  msgopts.dwMaxMessages = 0;
  msgopts.cbMaxMessage  = 160;   
  msgopts.bReadAccess   = IsRead;   
  HANDLE hq = CreateMsgQueue(wQueueName, &msgopts);
  return hq;
}

The issue i am getting is that the last 3 values of the array will occasionally be 0. I checked by printing the values in the array before and after writing to the queue and noticed the discrepancy. I am not sure where the issue is. I've tried changing the number of bytes for the destination buffer, i printed out the number of bytes read to make sure it was correct, and I have not yet been able to find the root cause.

Here is a snippet of what i see when i print out the values:

6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088488 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088490 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088490 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088490 PID:7bb0046 TID:6a50052 Channel Data (USB): 0
6088490 PID:7bb0046 TID:6a50052 Channel Data (USB): 0
6088492 PID:7bb0046 TID:6a50052 Channel Data (USB): 0
6088528 PID:7bb0046 TID:6a50052 Bytes Read: 160
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088528 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088530 PID:7bb0046 TID:6a50052 Channel Data (USB): 0
6088530 PID:7bb0046 TID:6a50052 Channel Data (USB): 0
6088530 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Bytes Read: 160
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088568 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088569 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088569 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088570 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088570 PID:7bb0046 TID:6a50052 Channel Data (USB): 0
6088570 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100
6088570 PID:7bb0046 TID:6a50052 Channel Data (USB): 1100

The 0's are not intended, but the 1100's are. Im not sure what I am doing wrong (if any), but is it possible to send arrays over message queues without possible memory issues?

Notes: This is being developed for Platform Builder (WEC7) and VS2008.

Answer 1

That looks like a timing problem in inter-process communication. A read succeeds but with fewer characters than were sent, because of the relative timing.
You need to check for a short read and retry to complete the desired amount.