[HELP] Call Library Function Node...

[ChristopherPovey]

I cannot get the LStrPrintf version to work at all it just crashes LabView.

I do have a BlockMove version (see attachment), but it only seems to get the first 10 characters of the string if there are spaces in it.  If I change my test DLL string "ABCDEFGHIJKLMNOPQRSTUVWXYZ" I get the lot out????

[ChristopherPovey]

Fixed it.  I managed to get LStrPrintf to work...

Thanks all.

[rolfk]

---

ChristopherPovey wrote:

And parameter 3 is:

Type: Numeric

Data Type: Signed 8-bit Integer

Pass: Pointer to Value

???

Also the "problem" DLL function.  Should that be set up as per "this little trick"?

Thanks.

---

Well LabVIEW 8.5 does not have pointer sized integers. So you will want to use a 32 Bit integer and set Pass: Value.

For your own DLL you will want to set it to Pass: Pointer to Value

[BPerlman]

Rolf,

Thank you for your answer to the original question.  It just helped me too -- I came here with the same question, searching for a solution.

Just one note.  You wrote: "As such the function will allocate a pointer and return it (a highly non-standard C practice at least for strings). This automatically begs the question when and how will this pointer be deallocated?".

This is not completely true.  The same prototype is often used in a different, quite standard C practice.  Often for things like retrieving an error string by code, or some such.  The strings are constant strings, and they are returned either via a "char *" return value or via a "char **" parameter, where nothing is allocated, and nothing needs to be deallocated.  The user's pointer is simply pointed to the constant string.

eg:

void GetErrorString(int errorCode, char **ppErrorString) {

    switch (errorCode)  {
          case error1:
               *ppErrorString = "Error 1 occured");

                return;
          case error2:
              *ppErrorString = "Error 2 occured");
                return;
          default:
               "Unknown error";

    }

}

or, more common:

char * GetErrorString(int errorCode) {

    switch (errorCode) {
         case error1:
             return "error 1 occured";
         case error2:
             return "error 2 occured";
         default:
             return "Unknown error";
    }

}

Batya

[rolfk]

You are of course right with this. However the fact that the returned pointer is really a static memory location should be just as much documented as if it would be dynamic. C simply isn't verbose enough in itself to allow this indication, and assuming that no documentation means static memory is really a dangerous one.

[BPerlman]

No, you are right, you can't assume it. 

It is much more standard/common usage in C, but you can't assume it.