Iterate DB using DBcursor- get with DB_DBT_USERMEM flag set for DBT

Question

Have BDB running in TDS mode. Want to iterate over a complete database using a DBcursor from start to end. Set the DB_DBT_USERMEM flag on the DBT structure with data pointing to a fixed sized user allocated memory block to hold the contents of a single record read. Currently cursor-get fails with DB_BUFFER_SMALL. I assume that this is because cursor->get retrieves more than one record.
Is it possible to iterate over the DB using the said cursor while allocating user-memory for only one (1) database record? Each call to cursor->get with DB_NEXT / DB_PREV / DB_FIRST /DB_LAST etc would update the single record entry.

Answer 1

Hi Kedar,
No, DBcursor->get() retrieves multiple key/data items if you're using the DB_MULTIPLE or DB_MULTIPLE_KEY flags. See "Bulk Retrieval":
[http://www.oracle.com/technology/documentation/berkeley-db/db/programmer_reference/am_misc_bulk.html#am_misc_bulk_get]
You only want to retrieve a single record per call, hence are not using the aforementioned flags. In this case the DB_BUFFER_SMALL error indicates that the length of the requested/retrieved item is larger than that specified for the DBT via its "ulen" field.
[http://www.oracle.com/technology/documentation/berkeley-db/db/api_reference/C/dbt.html#dbt_DB_DBT_USERMEM]
If you want to iterate over all the records in the database (including duplicates, if the database is configured to support them) you should use the DB_NEXT flag.
Note than when the DB_BUFFER_SMALL error is returned the "size" field of the DBT is set to the the length needed for the requested item; you can inspect that value to decide how to size your supplied buffer (or you may know in advance the size of the data items in the database).
Here is an excerpt from the example code in "Retrieving records with a cursor" with the necessary adjustments for the data DBT:
[http://www.oracle.com/technology/documentation/berkeley-db/db/programmer_reference/am_cursor.html#am_curget]
     DB *dbp;
     DBC *dbcp;
     DBT key, data;
     int close_db, close_dbc, ret;
     /* Acquire a cursor for the database. */
     if ((ret = dbp->cursor(dbp, NULL, &dbcp, 0)) != 0) {
          dbp->err(dbp, ret, "DB->cursor");
          goto err;
     close_dbc = 1;
     /* Initialize the key/data return pair. */
     memset(&key, 0, sizeof(key));
     memset(&data, 0, sizeof(data));
     /* Retrieve data item in user suplied buffer. */
#define BUFFER_LENGTH 1024
     if ((data.data = malloc(BUFFER_LENGTH)) == NULL)
          return (errno);
     data.ulen = BUFFER_LENGTH;
     data.flags = DB_DBT_USERMEM;
     /* You can supply your own buffer for the key as well. */
     /* Iterate through the database. */
     while ((ret = dbcp->c_get(dbcp, &key, &data, DB_NEXT)) == 0)
          /* Operate on the retrieved items. */
     if (ret != DB_NOTFOUND) {
          dbp->err(dbp, ret, "DBcursor->get");
          goto err;
err:
     // ...Regards,
Andrei