int (*dup_compare_fcn)(DB *, const DBT *, const DBT *));
Set the duplicate data item comparison function. The comparison function
is called when it is necessary to compare a data item specified by the
application with a data item currently stored in the tree. The first
argument to the comparison function is the DBT representing the
application's data item; the second is the current tree's data item.
Calling DB->set_dup_compare implies calling DB->set_flags
with the DB_DUPSORT flag.
The comparison function must return an integer value less than, equal
to, or greater than zero if the first data item argument is considered
to be respectively less than, equal to, or greater than the second data
item argument. In addition, the comparison function must cause the data
items in the set to be well-ordered. The comparison function
must correctly handle any data item values used by the application
(possibly including zero-length data items). The data and
size fields of the DBT are the only fields that may be
used for the purposes of this comparison, and no particular alignment
of the memory to which the data field refers may be assumed.
If no comparison function is specified, the data items are compared
lexically, with shorter data items collating before longer data items.
The DB->set_dup_compare interface may not be called after the DB->open
interface is called.
If the database already exists when
DB->open is called, the information specified to DB->set_dup_compare must
be the same as that historically used to create the database or
corruption can occur.
The DB->set_dup_compare method returns a non-zero error value on failure and 0 on success.
The DB->set_dup_compare method may fail and return a non-zero error for the following conditions:
- An invalid flag value or parameter was specified.
The DB->set_dup_compare method may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
If a catastrophic error has occurred, the DB->set_dup_compare method may fail and
in which case all subsequent Berkeley DB calls will fail in the same way.
Databases and Related Methods
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