How does one pass a string back to labview using a call Library function node

Question

I want to use LabVIEW's Call Library Function Node to access a DLL function, and have this function return a string to displayed on my VI. How would I go about doing this? I am quite happy returning numbers from my DLL, but am really struggling to find any examples of how to return a string.

Answer 1

I assume from your question that you already have a DLL that can return numbers to Labview. To return a string from the DLL, I have created a DLL with the following C++ function

void returnString(char myString[])
{
  const char *aString = "test string";
  memcpy(myString, aString, 12);
}

In Labview I then use the Call Library Function Node and configure it as follows

    Library Name or Path: c:\path\to\my\custom.dll
           Function Name: returnString
           Calling Convention: C

    Parameters: 
           Parameter: return type
                type: void

           Parameter: arg1
                type: String
                string format: C String Pointer

    Function prototype:
        void returnString(CStr arg1);

After connect the arg1 output in the block diagram to a string indicator and run. The string "test string" should appear in the front panel.

I tried to have the returnString function be of type CStr as in

CStr returnString()
{ ...
 }

but I got build errors when compiling the DLL project.

Update

Thanks to @bk1e comment don't forget to pre-allocate space in Labview for the string.