added some comments

added nice caret

main.c

@@ -87,31 +87,44 @@ int WINAPI WinMain(HINSTANCE hInst, HINSTANCE hprevInstance, LPSTR lspszCmdParam
  NULL
  );
 
+ // Create window storage struct.
  pWData = (PWDATA) malloc(sizeof(PWDATA));
- pWData->state = COMMAND;
- pWData->hCom = INVALID_HANDLE_VALUE;
+
+ // Set default window storage values.
+ pWData->state = COMMAND; // Program state.
+ pWData->hCom = INVALID_HANDLE_VALUE; // Comm port handle.
 
+ // Prepare output storage struct.
  pWData->pOutput = (POUTPUT) malloc(sizeof(OUTPUT));
  pWData->pOutput->size = OUTPUTBUFFSIZE;
  pWData->pOutput->pos = 0;
  pWData->pOutput->out = (TCHAR*) malloc(pWData->pOutput->size * sizeof(TCHAR));
 
+ // Initially white-out the output buffer.
  for (i = 0; i < pWData->pOutput->size; i++) {
  pWData->pOutput->out[i] = ' ';
  }
 
+ // Set up the cursor struct with default values.
  pWData->cursor.xCaret = 0;
  pWData->cursor.yCaret = 0;
  pWData->cursor.cxBuffer = 0;
  pWData->cursor.cyBuffer = 0;
  pWData->cursor.cxChar = 0;
  pWData->cursor.cyChar = 0;
 
+ // Set window struct to the window.
  SetWindowLongPtr(hwnd, GWLP_USERDATA, (LONG_PTR) pWData);
 
  ShowWindow(hwnd, nCmdShow);
  UpdateWindow(hwnd);
 
+ // Message handling uses PeekMessage() instead of GetMessage.
+ // Essentially this will handle all messages when they arrive
+ // on the queue. But whenever the queue is empty, poll the 
+ // serial port.
+ // This provides us with a very clean method of constantly polling the 
+ // open serial port with out jamming the program.
  while (TRUE) {
  if (PeekMessage(&Msg, NULL, 0, 0, PM_REMOVE)) {
  if (Msg.message == WM_QUIT) {
@@ -122,6 +135,7 @@ int WINAPI WinMain(HINSTANCE hInst, HINSTANCE hprevInstance, LPSTR lspszCmdParam
  }
  else {
  if (pWData->state == CONNECT) {
+ // If no messages, poll the open serial port.
  pollPort(hwnd, pWData);
  }
  }

session.c

@@ -66,16 +66,34 @@ LRESULT CALLBACK WndProc (HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
 
  switch (Message) {
 
- case WM_SIZE:
+ case WM_SETFOCUS:
  hdc = GetDC(hwnd);
  hFont = (HFONT) GetStockObject(ANSI_FIXED_FONT);
  SelectObject(hdc, hFont);
  GetTextMetrics(hdc, &tm);
  pWData = (PWDATA) GetWindowLongPtr(hwnd, GWLP_USERDATA);
-
+ 
  pWData->cursor.cxChar = tm.tmMaxCharWidth;
  pWData->cursor.cyChar = tm.tmHeight;
 
+ if (pWData->state == CONNECT) {
+ // Create a solid black caret. 
+ CreateCaret(hwnd, NULL, 2, pWData->cursor.cyChar); 
+ SetCaretPos(pWData->cursor.xCaret * pWData->cursor.cxChar, pWData->cursor.yCaret * pWData->cursor.cyChar);
+ // Display the caret. 
+ ShowCaret(hwnd);
+ }
+ break;
+
+ case WM_KILLFOCUS: 
+ // The window is losing the keyboard focus, so destroy the caret. 
+ DestroyCaret(); 
+ break;
+
+ case WM_SIZE:
+ hdc = GetDC(hwnd);
+ pWData = (PWDATA) GetWindowLongPtr(hwnd, GWLP_USERDATA);
+
  pWData->cursor.cxBuffer = max(1, LOWORD(lParam) / pWData->cursor.cxChar);
  pWData->cursor.cyBuffer = max(1, HIWORD(lParam) / pWData->cursor.cyChar);
 
@@ -126,6 +144,10 @@ LRESULT CALLBACK WndProc (HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
 
  menu = GetMenu(hwnd);
  setMenu(menu, MF_GRAYED);
+
+ CreateCaret(hwnd, NULL, 2, pWData->cursor.cyChar);
+ SetCaretPos(pWData->cursor.xCaret * pWData->cursor.cxChar, pWData->cursor.yCaret * pWData->cursor.cyChar);
+ ShowCaret(hwnd);
  break;
  }
  break;
@@ -141,6 +163,7 @@ LRESULT CALLBACK WndProc (HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
  menu = GetMenu(hwnd);
  setMenu(menu, MF_ENABLED);
  InvalidateRect(hwnd, NULL, FALSE);
+ DestroyCaret();
  }
  break;
 
@@ -169,20 +192,28 @@ LRESULT CALLBACK WndProc (HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
  // v
 
  default:
+ HideCaret(hwnd);
  Transmit(pWData->hCom, wParam);
  if (!outputAddChar((TCHAR) wParam, pWData->pOutput)) {
  MessageBox(hwnd, TEXT("MASSIVE ERROR."), NULL, MB_ICONERROR);
  CloseHandle(pWData->hCom);
  PostQuitMessage(0);
  }
- //InvalidateRect(hwnd, NULL, FALSE);
+
+ // Prepare for printing.
  hdc = GetDC(hwnd);
  hFont = (HFONT) GetStockObject(ANSI_FIXED_FONT);
  SelectObject(hdc, hFont);
+
+ // Print character.
  printChar(hwnd, &pWData->cursor, pWData->pOutput, hdc);
  ReleaseDC(hwnd, hdc);
+
+ // Increment caret position.
  pWData->cursor.xCaret = ++pWData->cursor.xCaret % pWData->cursor.cxBuffer;
  pWData->cursor.yCaret = max(0, pWData->pOutput->pos / pWData->cursor.cxBuffer);
+ SetCaretPos(pWData->cursor.xCaret * pWData->cursor.cxChar, pWData->cursor.yCaret * pWData->cursor.cyChar);
+ ShowCaret(hwnd);
  }
  }
  break;
@@ -191,6 +222,7 @@ LRESULT CALLBACK WndProc (HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
  pWData = (PWDATA) GetWindowLongPtr(hwnd, GWLP_USERDATA);
  hdc = BeginPaint(hwnd, &paintstruct); // Acquire DC 
 
+ // Prepare for painting output.
  hFont = (HFONT) GetStockObject(ANSI_FIXED_FONT);
  SelectObject(hdc, hFont); 
  printOut(hwnd, &pWData->cursor, pWData->pOutput, hdc);
@@ -316,12 +348,16 @@ void pollPort(HWND hwnd, PWDATA pWData) {
  PostQuitMessage(0);
  }
 
- //InvalidateRect(hwnd, NULL, FALSE);
+ // Prepare to print character.
  hdc = GetDC(hwnd);
  hFont = (HFONT) GetStockObject(ANSI_FIXED_FONT);
  SelectObject(hdc, hFont);
+
+ // Print character.
  printChar(hwnd, &pWData->cursor, pWData->pOutput, hdc);
  ReleaseDC(hwnd, hdc);
+
+ // Repostion caret.
  pWData->cursor.xCaret = ++pWData->cursor.xCaret % pWData->cursor.cxBuffer;
  pWData->cursor.yCaret = max(0, pWData->pOutput->pos / pWData->cursor.cxBuffer);
 }