lib wrapped into dll imported function access memory error

Well, one problem I can see is that the long type in C/C++ is a 4 byte integer  whereas the long type in C# is an 8 byte integer. So you want to replace long with int on the C# side.

The short type is 2 bytes in C/C++/C# so you have the right type there. However, as it's a pointer to a buffer I'd have thought that it would be expecting an array of shorts rather than a scalar short (albeit as an 'out' parameter).

I'm also a bit worried about the dev parameter. If you pass that as an int array how will the unmanaged function know what its length is?

One other possible problem is the calling convention. I see the unmanaged function is using WINAPI (typically StdCall on Windows) whereas you're using Cdecl. For consistency, I'd change to Winapi.

So, taking all that into account and hoping for the best with the int[], I'd try:

[DllImport(@dllname, CallingConvention = CallingConvention.Winapi)]
  static extern int MmdRandREx(int path, int netno, int stno, int[] dev, [In, Out] short[] buf, int bufsize);

and call with:

short[] buf = new short[bufsize/2]; // assuming bufsize is in bytes not elements
int  retval = MmdRandREx(path, 1, stanoPLC, dev1, buf, bufsize);