| isfd | isam file identifier |
| key | description of secondary index to add, see isam indexes |
SUCCESS, or a negative value on error, in which case the error code is found in 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.