| isfd | isam file descriptor |
| key | description of secondary index to add, see isam indexes |
ISTRUE, or ISFALSE, in which case the error code is returned by isErrno, or found in isfd->iserrno
when adding an index to a populated file, isAddIndex will load the new index with keys for all the existing data records.
in the case of larger files this could take a while.
if the file is not exclusively locked, an automatic ISEXCLLOCK will be placed around the operation, and if this is not possible, an ENOTEXCL error will be returned.