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

BDB works in mode TDS. You want to iterate over a database using a DBcursor from beginning to the end. set the flag DB_DBT_USERMEM on the structure of the DBT with data pointing to a block of memory allocated fixed size to organize content in a single record reading. Currently the cursor get fails with DB_BUFFER_SMALL. I guess that's because the cursor-> get retrieves several records.

It is possible to iterate over the DB using the cursor said while allocating memory for only a 1 user database? Each call to cursor-> get with DB_NEXT / DB_PREV / DB_FIRST DB_LAST etc. would be updated of the unique registration entry.

Hi Kedar,

No, DBcursor-> get() picked up several key/data elements if you are 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 want only to retrieve a single record per call, so do not use the above indicators. In this case, the DB_BUFFER_SMALL error indicates that the length of the item requested/retrieved is greater than that specified for the DBT via his "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 through all the records in the database (including duplicates, if the database is configured to support), you must use the DB_NEXT flag.
Note that when the DB_BUFFER_SMALL error returned the field 'Size' of the DBT is on the length needed for the item requested; You can check this value to determine how the size of your buffer provided (or you can know in advance the size of the items in the database).
Here is an excerpt of the example code to "Retrieve records with a cursor" with the necessary adjustments for the TCD data:
[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:
     // ...

Kind regards
Andrei

Tags: Database

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