Отправка AT команд модему мегафон M21-4

int main()
{
    char szBuffer[80];
    DCB dcb = { 0 };
    DWORD dwRead, dwWritten;
    HANDLE hComm;
    OVERLAPPED ovl_read = { 0 }, ovl_write = { 0 };
    COMMTIMEOUTS cto;
 
    // Create events for overlapped operation
    ovl_read.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    ovl_write.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
 
    // Open the port
    hComm = CreateFileA("\\\\.\\COM4", GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL|FILE_FLAG_OVERLAPPED, NULL);
 
    // Get the state of the device and modify it
 
    dcb.DCBlength = sizeof(dcb);
    GetCommState(hComm, &dcb);
    dcb.BaudRate = CBR_9600;
    SetCommState(hComm, &dcb);
 
    // Set the timeout parameters
    cto.ReadIntervalTimeout = 1000;
    cto.ReadTotalTimeoutConstant = 1000;
    cto.ReadTotalTimeoutMultiplier = 1000;
    cto.WriteTotalTimeoutConstant = 1000;
    cto.WriteTotalTimeoutMultiplier = 1000;
 
    SetCommTimeouts(hComm, &cto);
 
    // Send a command and receieve a response. Note that
    // we post the receive in advance of sending the
    // command in order not to miss anything
 
    printf("\r\nSending: AT\r\n");
 
    ReadFile(hComm, szBuffer, sizeof(szBuffer), &dwRead, &ovl_read);
    WriteFile(hComm, "AT\r", strlen("AT\r"), &dwWritten, &ovl_write);
 
    // Wait for the receive to complete and display the response
 
    if (GetOverlappedResult(hComm, &ovl_read, &dwRead, TRUE)) {
        szBuffer[dwRead] = 0;
        printf("Received: %s\r\n", szBuffer);
    }
 
    // Close the device
    CloseHandle(hComm);
    return 0;
}