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?

cp.engr · Accepted Answer · 2015-06-30 16:59:34Z

8

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.

edited Jun 30, 2015 at 16:59

cp.engr

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answered Sep 22, 2009 at 23:19

ChrisW

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4 Comments

thepocketwade Over a year ago

First of all, I mention that the code runs, which means that it compiled. Secondly, I added some code that was trimmed off the initial post. Thirdly, thanks for the links.

ChrisW Over a year ago

Now that you've added the declaration of bytes_written, I see the problem: it's a pointer, and you haven't initialized it before you use it. Ditto the OVERLAP pointer (you haven't initialized it before you used it either). Also, a failure to open the COM port would mean that the handle contained a value of INVALID_HANDLE_VALUE (not 0).

ChrisW Over a year ago

It isn't normal to use a pointer for the "number of bytes written" parameter. A more usual way would be to declare it as type DWORD (and not as type LPDWORD), and to pass it by pointer using a & e.g. as &dwBytesWritten: so that you're passing a value by pointer, instead of passing a pointer by value (and in your case, an uninitialized pointer).

thepocketwade Over a year ago

Thanks for that, it's little things like that that keep tripping me up working on this code. Making the changes you suggested allowed me to see I was getting an error code of 997, which I'm working on resolving now. Thanks.

Joshua · Accepted Answer · 2009-09-22 23:25:25Z

0

When you call SetCommState is the return value 0? It could be erroring out which could be causing more problems.

Also, have you stepped through line by line to make sure that it is the WriteFile call that crashes it?

Lastly, you could have some antivirus or other software hooking into your app causing these problems (look for unknown dll's loaded in your module list).

answered Sep 22, 2009 at 23:25

Joshua

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3 Comments

thepocketwade Over a year ago

The return value is 0. On a second look, I replaced the WriteFile line with a ReadFile call, and it also crashed, but without either the code runs fine.

thepocketwade Over a year ago

I'm new to Visual Studio, and I still haven't figured out where the module list is.

Joshua Over a year ago

When you're debugging, click on the Debug menu, Windows -> Modules (Ctrl + D, M if you're using default shortcuts)

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