Das Problem ist, dass ich immer "FEHLER #06" bekomme ( => WriteFile gibt FALSE zurück )
Kann das daran liegen, dass kein gerät am com port angeschlossen ist, oder wie oder was???

hier mal mein quelltext:

#include <windows.h>
#include <iostream.h>

WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
  HANDLE hCom = CreateFile("COM1", GENERIC_WRITE|GENERIC_READ, 0, NULL,
              OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, NULL);
  if(!SetupComm(hCom, 512, 512))
  {
    MessageBoxA(NULL, "FEHLER #01", "comPort", NULL);
    return 0;
  }

  DCB dcb;
  ZeroMemory(&dcb, sizeof(DCB));

  char buffer[100];
  strcpy(buffer,"baud=1200 parity=N data=8 stop=1");
  if (BuildCommDCB((char*)&buffer, &dcb))
  {
    if (dcb.BaudRate != CBR_1200)
    {
      MessageBoxA(NULL, "FEHLER #02", "comPort", NULL);
      return 0;
    }
  }
  else
  {
    MessageBoxA(NULL, "FEHLER #03", "comPort", NULL);   
    return 0;
  }

  if(GetCommState(hCom, &dcb))
  {
   dcb.BaudRate = CBR_19200;
   dcb.ByteSize = 8;
   dcb.Parity = 0;
   dcb.StopBits = 0;
   SetCommState(hCom, &dcb);
  }
  else
  {
    MessageBoxA(NULL, "FEHLER #04", "comPort", NULL);
    return 0;
  }

  if (hCom == INVALID_HANDLE_VALUE)
  {
    MessageBoxA(NULL, "FEHLER #05", "comPort", NULL);
    return 0;
  }

  DWORD temp, numbytes_ok;
  COMSTAT ComState;
  OVERLAPPED Overlap;
  char buf_out[1] = "a";

  ClearCommError(hCom, &temp, &ComState);
  ZeroMemory(&Overlap, sizeof(OVERLAPPED));
  if(!temp)
  {
   if(!WriteFile(hCom, &buf_out, 1, &numbytes_ok, &Overlap))
   {
    MessageBoxA(NULL, "FEHLER #06", "comPort", NULL);
    return 0;
   }
  }

  DWORD error;
  error = GetLastError();
  if(error)
   MessageBoxA(NULL, (char *)error, "comPort", NULL);

  CloseHandle(hCom);

  MessageBoxA(NULL, "Alles im grünen Bereich!", "comPort", NULL);

  return 0;

LPVOID lpMsgBuf;
  FormatMessage (FORMAT_MESSAGE_ALLOCATE_BUFFER | 
                   FORMAT_MESSAGE_FROM_SYSTEM | 
                   FORMAT_MESSAGE_IGNORE_INSERTS,
                 NULL,
                 GetLastError(),
                 MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
                 (LPTSTR) &lpMsgBuf,
                 0,
                 NULL);
  // Display the string.
  MessageBox (NULL, (LPCTSTR)lpMsgBuf, TEXT("Error WriteFile"), 
              MB_OK | MB_ICONINFORMATION);
  // Free the buffer.
  LocalFree(lpMsgBuf);

Auch solltest Du im Fehlerfalle nicht return 0; ohne CloseHandle (hCom); machen.
Wozu brauchst Du OVERLAPPED Overlap; ?
Setze es auf NULL, wenn Du es nicht benutzt. Bist Du sicher, was Du mit der Abfrage if (!tmp) bezweckst? MSDN: "This parameter can be one or more of the following values. "

Blackbird

char buf_out[1] = "a";
// ...
if(!WriteFile(hCom, &buf_out, 1, &numbytes_ok, &Overlap)) <= Ist das "&" vor buf_out da nicht zuviel ?