Question

I have a dll which includes a function called ReadPort that reads data from serial COM port, written in c/c++. This function is called within an extra thread from another WINAPI function using the _beginthreadex. When COM port has data to be read, the worker thread returns the data, ends normaly, the calling thread closes the worker’s thread handle and the dll works fine.

However, if ReadPort is called without data pending on the COM port, when timeout occurs then WaitForSingleObject returns WAIT_TIMEOUT but the worker thread never ends. As a result, virtual memory grows at about 1 MB every time, physical memory grows some KBs and the application that calls the dll becomes unstable. I also tryied to use TerminateThread() but i got the same results.

I have to admit that although i have enough developing experience, i am not familiar with c/c++. I did a lot of research before posting but unfortunately i didn’t manage to solve my problem.

Does anyone have a clue on how could i solve this problem? However, I really want to stick to this kind of solution. Also, i want to mention that i think i can’t use any global variables to use some kind of extra events, because each dll’s functions may be called many times for every COM port.

I post some parts of my code below:

The Worker Thread:

unsigned int __stdcall ReadPort(void* readstr){

DWORD  dwError; int   rres;DWORD  dwCommModemStatus, dwBytesTransferred;
int ret;
char szBuff[64] = "";

ReadParams* params = (ReadParams*)readstr;

ret = SetCommMask(params->param2, EV_RXCHAR | EV_CTS | EV_DSR | EV_RLSD | EV_RING);
if (ret == 0)
{
    _endthreadex(0);
    return -1;
}
ret = WaitCommEvent(params->param2, &dwCommModemStatus, 0);
if (ret == 0)
{
    _endthreadex(0);
    return -2;
}
ret = SetCommMask(params->param2, EV_RXCHAR | EV_CTS | EV_DSR | EV_RLSD| EV_RING);
if (ret == 0)
{
    _endthreadex(0);
    return -3;
}

if (dwCommModemStatus & EV_RXCHAR||dwCommModemStatus & EV_RLSD)
{
    rres = ReadFile(params->param2, szBuff, 64, &dwBytesTransferred,NULL);
    if (rres == 0)
    {
        switch (dwError = GetLastError())
        {
            case ERROR_HANDLE_EOF:
            _endthreadex(0);
            return -4;
        }

        _endthreadex(0);
        return -5;
    }
    else
    {
        strcpy(params->param1,szBuff);
        _endthreadex(0);
        return 0;
    }
}
else
{
    _endthreadex(0);
    return 0;
}
_endthreadex(0);
return 0;}

The Calling Thread:

int WINAPI StartReadThread(HANDLE porthandle, HWND windowhandle){

HANDLE hThread;
unsigned threadID;
ReadParams readstr;
DWORD ret, ret2;

readstr.param2 = porthandle;

hThread = (HANDLE)_beginthreadex( NULL, 0, ReadPort, &readstr, 0, &threadID );
ret = WaitForSingleObject(hThread, 500);

if (ret == WAIT_OBJECT_0)
{
    CloseHandle(hThread);  
    if (readstr.param1 != NULL)
        // Send message to GUI
    return 0;
}
else if (ret == WAIT_TIMEOUT)
{
    ret2 = CloseHandle(hThread);
    return -1;
}
else
{
    ret2 = CloseHandle(hThread);
    if (ret2 == 0)
    return -2;
}}

Thank you in advance,

Sna.

Answer 1

Don’t use WaitCommEvent. You can call ReadFile even when there is no data waiting.

Use SetCommTimeouts to make ReadFile itself timeout, instead of building a timeout on the inter-thread communications.