writing to userspace

hello,

i got a simple question about writing to a userspace because it is not
100% clear to me.

i got a userspace (which is set to autoextend) and in my rpg program i
am retrieving a pointer to that userspace. through pointer arithmetic i
am filling the userspace with some values.

D usPtr           S               *
D usData          DS                  based(usPtr) qualified
D   header                            likeds(wws_main_us)

D usDataPtr       S               *
D vkst            S              5  0 based(usDataPtr)

dow (not %eof);
  usDataPtr = usPtr + %len(usData.header) +
              (i * %size(vkst));
  vkst = someValue;
  i += 1;
  read file;
enddo;

is this the right way to fill a userspace (it seems to be the easiest) ?
and is the userspace auto extending when i'm not using the os400 api to
write to it?

thanx for any answers

mihael knezevic