C++ application send data on serial only when closed - c++

Question

Using this guide/class (http://playground.arduino.cc/Interfacing/CPPWindows) I wrote a little application to send data on the serial port to an Arduino. (This will be added on a larger project later on)
int _tmain(int argc, _TCHAR* argv[]) {
Serial* SP = new Serial(argv[1]);
char outcomingData[256];
int dataLength = 255;
int sendData;
while (SP->IsConnected()) {
strcpy_s(outcomingData, argv[2]);
sendData = SP->WriteData(outcomingData, dataLength);
printf("%s", outcomingData);
}
return 0;
}
The problem is that the data is sent only when I close the application or delete SP through the destructor. How can I fix this? Is there a workaround or another method that I can add to the class?

Answer 1

It's likely that your data is being cached such that it isn't actually written to your serial device until CloseHandle is called in ~Serial. You can avoid this caching behavior by specifying FILE_FLAG_WRITE_THROUGH in your call to CreateFile, so that the file creation would look something like:
//Try to connect to the given port throuh CreateFile
this->hSerial = CreateFile(portName,
GENERIC_READ | GENERIC_WRITE,
0,
NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL | FILE_FLAG_WRITE_THROUGH,
NULL);
If that doesn't work, there's also FILE_FLAG_NO_BUFFERING, but that may impose limitations on the alignment of the data you write to the device.
If you would like to retain the normal buffering behavior, you could add a Serial::Flush method that calls FlushFileBuffers to force Windows to write any data it has cached out to the device:
void Serial::Flush()
{
FileFlushBuffers(this->hSerial);
}
You would then call SP->Flush() after SP->WriteData(...) to ensure that Windows' buffers are flushed to the device. This would allow you to make multiple small writes without having to flush each one, possibly improving performance.
As a side note, I would question your use of new here. It doesn't seem like it's really needed, and SP could just be constructed locally:
Serial SP(argv[1]);