pure / const on function "return"ing an array?

[tom fogal <tfogal at sci dot utah dot edu>]

I have a function with this prototype:

  void to_3d(hsize_t index, const hsize_t dims[3], hsize_t outdex[3]);

The effect of the function is to store appropriate values in the
"outdex" array.  It does not read or write global memory.

I would like to mark this as __attribute__((const)), but it doesn't
quite fit the definition, as it's a void function.  Obviously I can't
just change it to make `outdex' the return value, since outdex is an
array.

Is there anything else I could do in such a situation?

Thanks,

-tom