Bunused — Return the number bytes free in buffer
On success, Bunused() return the number of free bytes; on error, -1 is returned, with Berror set to indicate the error.
Bstrerror(3) can give more description for error (for last call).
BALIGNERR Corrupted buffer or pointing to not aligned memory area.
BNOTFLD Buffer not fielded, not correctly allocated or corrupted. p_ub is NULL.