Serial communication read not working at all time - c++

Question

I am writing a c++ application for half duplex communication to download data from a device. Following is the class i am using for serial communication.
class CSerialCommHelper
{
HANDLE m_pPortHandle; //Handle to the COM port
HANDLE m_hReadThread; //Handle to the Read thread
HANDLE m_hPortMutex; //Handle to Port Mutex
std::wstring m_strPortName; //Portname
COMMTIMEOUTS m_CommTimeouts; //Communication Timeout Structure
_DCB dcb; //Device Control Block
DWORD m_dwThreadID; //Thread ID
string m_strBuffer;
public:
CSerialCommHelper();
HRESULT Open();
HRESULT ConfigPort();
static void * ReadThread(void *);
HRESULT Write(const unsigned char *,DWORD);
string GetFrameFromBuffer();
HRESULT Close();
~CSerialCommHelper(void);
};
ReadThread and Write function is as follows :
void * CSerialCommHelper::ReadThread(void * pObj)
{
CSerialCommHelper *pCSerialCommHelper =(CSerialCommHelper *)pObj;
DWORD dwBytesTransferred =0;
DWORD byte=0;;
while (pCSerialCommHelper->m_pPortHandle != INVALID_HANDLE_VALUE)
{
pCSerialCommHelper->m_strBuffer.clear();
pCSerialCommHelper->m_usBufSize=0;
WaitForSingleObject(pCSerialCommHelper->m_hPortMutex,INFINITE);
do
{
dwBytesTransferred = 0;
ReadFile (pCSerialCommHelper->m_pPortHandle,&byte,1,&dwBytesTransferred,NULL);
if (dwBytesTransferred == 1)
{
pCSerialCommHelper->m_strBuffer.push_back((char)byte);
pCSerialCommHelper->m_usBufSize++;
continue;
}
}
while ((dwBytesTransferred == 1) && (pCSerialCommHelper->m_pPortHandle != INVALID_HANDLE_VALUE));
ReleaseMutex(pCSerialCommHelper->m_hPortMutex);
Sleep(2);
}
ExitThread(0);
return 0;
}
Write function waits for readthread to release mutex and writes to data to port .
GetFrameFromBuffer will be called from application which uses the SerialCommhelper
and it returns the m_strBuffer string .
My problem is whenever i am trying to download huge amount of data.
I am losing some data frames .
I am getting response from device in between .0468 to .1716 secs.
After analysing different error scenarios i came to know that is not problem with time as other frames are getting downloaded at the same time interval.
Function which is calling getframebuffer is continuosly calling it until is gets a filled string.

Answer 1

It seems like these two statements should not be in your outer while loop:
pCSerialCommHelper->m_strBuffer.clear();
pCSerialCommHelper->m_usBufSize=0;
Your inner while loop reads bytes as long as they're immediately available, and the outer loop does a Sleep(2) the moment the inner loop doesn't give you a byte.
If you're waiting until an entire packet is available, it seems like you should keep looping until you get all the bytes, without clearing partway through the process.
I don't really know the ReadFile API, but I'm guessing that ReadFile might return 0 if there's no bytes immediately available, or at least available by whatever timeout you specified when opening the serial device.

Answer 2

ReleaseMutex(pCSerialCommHelper->m_hPortMutex);
Sleep(2);
That Sleep() call is hiding the real problem. It is never correct in threaded code, always a band-aid for a timing bug.
You certainly seem to have one, that m_hPortMutex spells doom as well. If you do in fact have multiple threads trying to read from the serial port then they are going to start fighting over that mutex. The outcome will be very poor, each thread will get a handful of bytes from the port. But clearly you want to read a frame of data. There is zero hope that you can glue the handfuls of bytes that each thread gets back together into a frame, you've lost their sequence. So sleeping for a while seemed like a workaround, it inject delays that can give you a better shot at reading a frame. Usually, not always. You also wrote it in the wrong place.
This code is just broken. Delete the Sleep(). Do not exit the loop until you've read the entire frame.