Help! DLL's replying with strings give errors

Question

Hi ,
I use the Call Library node to call a DLL function I wrote in C
void test (char Text[50])
strcpy(Text, "This is a test\0\n");
Next, I configured the Call Library Function as follows:
Parameter RETURN TYPE: Type = Void;
Parameter ARG1: Type = String; String format = C String Pointer;
(Which leads to Function prototype: void test(CStr arg1) )
The function executes just fine, but when I close the VI, I get an
APPLCATION ERROR! ("The instruction at '0x123456' referenced memory at
'0x123456'. The memory could not be read"). I guess this is when the DLL
is unloaded, but what am I doing wrong?
I am Using LabVIEW 5.1.1 & Win NT 4.0
Please help!
Patrick Pol
The Netherlands

Answer 1

Thanks Greg,
I wired a string constant containing at least 50 characters to the input of the
Library node, and that got rid of my crashing-problems.
Patrick Pol
Greg McKaskle wrote:
> I believe that your problem here is that you told the C compiler that there
> were 50 characters reserved for the function to write in. A caller of this
> function will need to allocate storage for the DLL function to scribble into,
> or this will be a possible crash/corruption.
>
> The same thing is true of LV. The caller, in this case, LV has no idea
> how much space to allocate for the DLL call. You will need to do this
> on the diagram using a constant, a control, or some other function that
> produces a string large enough to store the data into.
>
> Greg M
cKaskle