Getting a string from the UART/COM port

I cannot get the string that's sent to the com port. I know that it's sent there because I checked using a Serial Port Monitor.

Here is the function that I use for reading the string from the COM port.

Code:

BOOL ReadString(void *outstring, int *length)
{
    BYTE data;
    BYTE dataout[4096]={0};
    int index = 0;
    while(ReadByte(data)== TRUE)
    {
        dataout[index++] = data;
    }
    memcpy(outstring, dataout, index);
    *length = index;
    return TRUE;
}

Here is its in instance in the main function.

Code:

    ReadString(str,&i);

Can you please help me on this?
When I try to display the contents of str, what I get is an empty string.

Would it be possible to post your code you are using to open the COM port for reading? Perhaps you are making a mistake at that point in your program. Also, how does ReadByte() work?

some questions before jumping into this - BYTE is a typedef for what? what is data initialized to? in a nutshell explain the code in more detail.

Here is ReadByte.

Code:

BOOL ReadByte(BYTE  *res)
{
    BOOL bReturn = TRUE;
    BYTE rx;
    DWORD dwBytesTransferred=0;

    if (ReadFile (hPort, &rx, 1, &dwBytesTransferred, 0)> 0)
    {
        if (dwBytesTransferred == 1)
        {
            res=rx;
            bReturn  = TRUE;
        }
        else bReturn = FALSE;
    }
    else    bReturn = FALSE;
    return bReturn;
}

Here is an overview of my setup. A ZigBee module (equipped with an Atmel microcontroller) sends data to my PC, specifically to a GUI I made using Glade/GTK.

My experimentation has led me to the conclusion that although the module is able to send the string (the data), the GUI program is unable to save the string in the variable str.

I use a serial port monitor to see what is sent and received at the COM port.

Code:

void
on_BT_COLLECT_clicked                  (GtkButton       *button,
                                        gpointer         user_data)
{
                                        
    gchar x;
    gchar str[100];
    gchar str2[100];
    //gchar *p_str;
    int i,j=0;
    
    GtkWidget *dialog, *label, *okay_button; // These will be used for the dialog   
   
    
    hPort = ConfigureSerialPort("COM5");
    if(hPort == NULL)
    {
        printf("Com port configuration failed\n");
       // return -1;
    }
    // Call your ReadString and WriteString functions here

    WriteString("C\n",1);
    sleep(50);
    ReadByte(&x);
    sleep(50);
    
    WriteString("~",1);
    sleep(50);
    //ReadByte(&x);
    //sleep(50); 
    
    //sprintf(str,"tsktsk\n");
    //sleep(50);
    ReadString(str,&i);
    sprintf(str2,"%c%c\n",x,x);
    //sleep(50);
    /*
    while (1)
    {
        ReadByte(x);
        //sleep(5);
        //if (x=='~')
         //  break;
        str[j]=x;
    }
    */
    //sleep(2000);
    
    
    dialog = gtk_dialog_new();
    label = gtk_label_new(str2); // Create the widgets of the dialog box
    okay_button = gtk_button_new_with_label("OK");
    gtk_signal_connect_object(GTK_OBJECT (okay_button), "clicked", gtk_widget_destroy, GTK_OBJECT(dialog));
    gtk_container_add (GTK_CONTAINER (GTK_DIALOG(dialog)->action_area),
    okay_button);
    // Add the Okay Button to the dialog
    gtk_container_add (GTK_CONTAINER (GTK_DIALOG(dialog)->vbox), label); // Add the label
    gtk_widget_show_all (dialog); // Show everything the dialog.
    
    //gtk_entry_set_text(GTK_OBJECT("entry1"),str);  
    ClosePort();

}

Originally Posted by thatchergrey

Here is ReadByte.

Code:

BOOL ReadByte(BYTE  *res)
{
    BOOL bReturn = TRUE;
    BYTE rx;
    DWORD dwBytesTransferred=0;

    if (ReadFile (hPort, &rx, 1, &dwBytesTransferred, 0)> 0)
    {
        if (dwBytesTransferred == 1)
        {
            res=rx;  /* oops */
            bReturn  = TRUE;
        }
        else bReturn = FALSE;
    }
    else    bReturn = FALSE;
    return bReturn;
}

I would have expected a warning on the red line there.

> while(ReadByte(data)== TRUE)
So this doesn't generate an error message?
It should, it's expecting a pointer.
Without a pointer, it's not going to change.

> ReadString(str,&i);
You should also pass the size of this buffer, to prevent buffer overflow.

Yes, there's a warning. Is this the reason why I'm unable to read the complete string? So how can I address this?

Originally Posted by tabstop

I would have expected a warning on the red line there.

Originally Posted by thatchergrey

Yes, there's a warning. Is this the reason why I'm unable to read the complete string? So how can I address this?

If you want to assign to *res, then do so. Note also Salem's note that the function call is wrong too.

My experimentation has led me to the conclusion that although the module is able to send the string (the data), the GUI program is unable to save the string in the variable str.

My conclusion is that you set out to write the complete program -- including GUI -- without first having written something simple to make sure that you could actually read a string from the com port into a variable (which does not require a GUI). Of course, if you were already competent at gtk, communicating with comports, and C generally, that might be okay, but since you are obviously new to all three of them, why don't you try what anyone but a complete fool would try -- make sure your idea works before you add a graphical front end to it, or you will have more potential red herring to chase than anyone but a complete fool could possibly want.

Thank you everyone for all your insights! Thank you tabstop!

Code:

BOOL ReadByte(BYTE  *res)
{
    BOOL bReturn = TRUE;
    BYTE rx;
    DWORD dwBytesTransferred=0;

    if (ReadFile (hPort, &rx, 1, &dwBytesTransferred, 0)> 0)
    {
        if (dwBytesTransferred == 1)
        {
            *res=(char)rx;
            bReturn  = TRUE;
        }
        else bReturn = FALSE;
    }
    else    bReturn = FALSE;
    return bReturn;
}

BOOL ReadString(char *outstring, int *length)
{
    char data;
    char dataout[4096]={0};
    int index = 0;
    while(ReadByte(&data)== TRUE)
    {
        dataout[index++] = data;
    }
    memcpy(outstring, dataout, index);
    *length = index;
    return TRUE;
}

I was finally able to retrieve the string and display it in a dialog box. Thanks again!

Code:

            *res=(char)rx;

Why the cast - res is a pointer to BYTE, rx is a BYTE - no point in casting this, is there?

You should also pay attention to iMalc's comment about passing the max length to the ReadString function, as you may find yourself overwriting the end of the input buffer. And you should at the very least check that index never reaches sizeof(dataout).

--
Mats