Writing to a Serial (Com) Port in Visual C++

Question

I have a device that is connected on port COM4, (115200 baud, 8-N-1). Based on examples I found here I'm opening the port with:

                    Keyboard_Handle=CreateFile("\\\\.\\COM4",GENERIC_WRITE,0,NULL,OPEN_EXISTING,FILE_FLAG_OVERLAPPED,NULL); 
                    if(GetLastError() !=0 || Keyboard_Handle == INVALID_HANDLE_VALUE)
                    {
                        AfxMessageBox("Error opening connection to Keyboard");
                        exit(1);
                    }


                    char buffer[100];
     strcpy(buffer,"baud=115200 parity=N data=8 stop=1");
     BuildCommDCB((char*)&buffer,&dcb)) 

     if(GetCommState(Keyboard_Handle, &dcb))
     {
      dcb.BaudRate = CBR_115200;
      dcb.ByteSize = 8;
      dcb.Parity = 0;
      dcb.StopBits = 1;
      SetCommState(Keyboard_Handle, &dcb);
     }

Later in my code I call WriteFile on the port with:

    LPDWORD bytes_written;
    LPDWORD bytes_read;
    LPOVERLAPPED OVERLAP;
    char write_buf[10];

    write_buf[0] = 's';
    write_buf[1] = '\0';

    if(Keyboard_Handle != NULL) {
    WriteFile(Keyboard_Handle, (LPCVOID)write_buf , strlen(write_buf), bytes_written, OVERLAP); 
    }

And every time I run the code I get the JIT Debugger complaining about an unhandled exception (though the WriteFile is inside a Try/catch block).

Is there something wrong with how I'm doing this?

ChrisW ChrisW · Accepted Answer · 2009-09-22T23:19:34

bytes_written needs to be an address of a variable; the compiler wouldn't compile the statement you posted.

Likewise, "OVERLAP" doesn't make sense.

Did you check whether CreateFile succeeded?

What's in write_buf when you call strlen on it?

Try copy-and-pasting the actual code that you're using.

Also that doesn't seem like a very good/informative sample that you're using. Try Windows Serial Port Programming and http://msdn.microsoft.com/en-us/library/ms810467.aspx

Also start with the sample program from the Microsoft site, test it before you modify it (to check that it's working on your machine) and then modify it.

Writing to a Serial (Com) Port in Visual C++

2 Answers