int DB->set_dup_compare(DB *db, 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:
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 return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.
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