thread related issue

hi all,
i am having one problem with threading

i created one dialog based application whose work is to read the data
from serial port and display it.so i created one thred for this work.

means now i got two threads one is application thread and other is
serial communication thread which is created in OnInitDialog.

now everything is working i mean the thread got created it read some
bytes
from port and now it wants to show the data so need to send this data
to main
thread. so i am doing this with ::SendMessage() but it is giving access
violation. i don't know why this is happening and currently i am not
sending the data also i.e. buffer so please help me
below is code (stepwise)

1)In Oninitdialog
THREADPARAM *ptp = new THREADPARAM;
        if(ptp)
        {
            memset(ptp->array,0,30);
            strcpy(ptp->array,"Harshal");
            ptp->appHandle = this->m_hWnd;
            hSerialThread = CreateThread( NULL,
                                    0,
                                    (LPTHREAD_START_ROUTINE)
SerialCommunication,
                                    (LPVOID) NULL,
                                    0,
                                    &dwSerialThreadID);
        }

2)Thread function
UINT SerialCommunication(LPVOID pParam)
{
    int write_req=0;
    THREADPARAM *ptp = (THREADPARAM *)pParam;
    while(1)
    {
        if(write_req == 2)// send write request and read it
        {
            DWORD length=0;
            DWORD dwRead=0;
            if (WriteFile(hCom,
                  SEND_CMD, // pointer to data to write to
file
                  NOOFBYTE, // number of bytes to write
                  &length, // pointer to number of bytes
written
                  NULL) == 0)
            {// write failed
                write_req = 0;
                continue;
            }
            // writefile succeeded issue readfile
            char buffer[MAX_MESSAGE];
                memset(buffer,0,MAX_MESSAGE);
                if (!ReadFile(hCom, buffer, MAX_MESSAGE, &dwRead,
NULL))
                {
                    write_req = 0;
                    continue;
                }
                if(dwRead > 0)
                {
                    //MessageBox(NULL,buffer,"Data with packet",MB_OK);

                    ::SendMessage(ptp->appHandle,MY_SHOW_DATA,0,0);//
error is here
                }
                write_req = 0;
        }
        else // only read from port and increment counter
        {
            DWORD dwRead=0;
            char buffer[MAX_MESSAGE];
                memset(buffer,0,MAX_MESSAGE);
                if (!ReadFile(hCom, buffer, MAX_MESSAGE, &dwRead,
NULL))
                {
                    write_req++;
                    continue;
                }
                if(dwRead > 0)
                {// you are in thread you need to send message to
                 // main application
                    //char a;
                    HWND hwnd = (HWND)AfxGetApp()->m_pMainWindow;
                    ::SendMessage(ptp->appHandle,MY_SHOW_DATA,0,0);
                    //MessageBox(NULL,buffer,"Data without
packet",MB_OK);
                }
                write_req++;

        }
    }

    return 0;
}

3) Sent message function

LRESULT CReadDataDlg::OnShowDataMessage(UINT wParam, LONG lParam)
{
    return 0;
}

Thanks and regards
Harshal