> [...] There are only one task: to convince the library's users do not
> forget add 0 in the end of string.

   You could write a wrapper function which would do this.  The wrapper
function could be written in BASIC or C.  Suitable BASIC code should now
be obvious.  I assume that BASIC normally passes a string using a
descriptor, so the C code for such a wrapper function would be more
complicated, but not impossible.  (Declare the srgument as a pointer to
a suitable descriptor, allocate storage for the NUL-terminated string,
copy the non-NUL-terminated string into the new storage, add the NUL,
invoke the real C function, free the allocated storage, ...)

   VMS is generally pretty good for multi-language compatibility.  You
need to know how each language passes arguments (or can be made to pass
them the way you want), and/or how to declare in one language the method
used by another.  BASIC has those "BY DESC|REF|VALUE" things; Fortran
has %LOC, %REF, %VAL, %DESCR; C has nothing or everything, depending on
how you look at it; and so on.  I suspect that, with enough effort, you
could write a wrapper in Fortran or Pascal to do this job, too, but
dragging in another language (and its run-time stuff) might be less
efficient than sticking with one of the languages which you're already
using.