Berkeley DB Reference Guide: Error returns to applications

Berkeley DB Reference Guide:
Programmer Notes

Error returns to applications

Except for the historic dbm, ndbm, and hsearch interfaces, Berkeley DB does not use the global variable errno to return error values. The return values for all Berkeley DB functions are grouped into the following three categories:

0: A return value of 0 indicates that the operation was successful.
> 0: A return value that is greater than 0 indicates that there was a system error. The errno value returned by the system is returned by the function; for example, when a Berkeley DB function is unable to allocate memory, the return value from the function will be ENOMEM.
< 0: A return value that is less than 0 indicates a condition that was not a system failure, but was not an unqualified success, either. For example, a routine to retrieve a key/data pair from the database may return DB_NOTFOUND when the key/data pair does not appear in the database; as opposed to the value of 0, which would be returned if the key/data pair were found in the database.
All values returned by Berkeley DB functions are less than 0 in order to avoid conflict with possible values of errno. Specifically, Berkeley DB reserves all values from -30,800 to -30,999 to itself as possible error values. There are a few Berkeley DB interfaces where it is possible for an application function to be called by a Berkeley DB function and subsequently fail with an application-specific return. Such failure returns will be passed back to the function that originally called a Berkeley DB interface. To avoid ambiguity about the cause of the error, error values separate from the Berkeley DB error name space should be used.

Although possible error returns are specified by each individual function's manual page, there are a few error returns that deserve general mention:

DB_NOTFOUND and DB_KEYEMPTY

There are two special return values that are similar in meaning and that are returned in similar situations, and therefore might be confused: DB_NOTFOUND and DB_KEYEMPTY.

The DB_NOTFOUND error return indicates that the requested key/data pair did not exist in the database or that start-of- or end-of-file has been reached by a cursor.

The DB_KEYEMPTY error return indicates that the requested key/data pair logically exists but was never explicitly created by the application (the Recno and Queue access methods will automatically create key/data pairs under some circumstances; see DB->open for more information), or that the requested key/data pair was deleted and never re-created. In addition, the Queue access method will return DB_KEYEMPTY for records that were created as part of a transaction that was later aborted and never re-created.

DB_KEYEXIST

The DB_KEYEXIST error return indicates the DB_NOOVERWRITE option was specified to the DB->put method and the key already exists in the database.

DB_LOCK_DEADLOCK

When multiple threads of control are modifying the database, there is normally the potential for deadlock. In Berkeley DB, deadlock is signified by an error return from the Berkeley DB function of the value DB_LOCK_DEADLOCK. Whenever a Berkeley DB function returns DB_LOCK_DEADLOCK, the enclosing transaction should be aborted.

Any Berkeley DB function that attempts to acquire locks can potentially return DB_LOCK_DEADLOCK. Practically speaking, the safest way to deal with applications that can deadlock is to handle a DB_LOCK_DEADLOCK return from any Berkeley DB access method call.

DB_LOCK_NOTGRANTED

If a lock is requested from the DB_ENV->lock_get or DB_ENV->lock_vec methods with the DB_LOCK_NOWAIT flag specified, the method will return DB_LOCK_NOTGRANTED if the lock is not immediately available.

If the DB_TIME_NOTGRANTED flag is specified to the DB_ENV->set_flags method, database calls timing out based on lock or transaction timeout values will return DB_LOCK_NOTGRANTED instead of DB_LOCK_DEADLOCK.

DB_RUNRECOVERY

There exists a class of errors that Berkeley DB considers fatal to an entire Berkeley DB environment. An example of this type of error is a corrupted database page. The only way to recover from these failures is to have all threads of control exit the Berkeley DB environment, run recovery of the environment, and re-enter Berkeley DB. (It is not strictly necessary that the processes exit, although that is the only way to recover system resources, such as file descriptors and memory, allocated by Berkeley DB.)

When this type of error is encountered, the error value DB_RUNRECOVERY is returned. This error can be returned by any Berkeley DB interface. Once DB_RUNRECOVERY is returned by any interface, it will be returned from all subsequent Berkeley DB calls made by any threads of control participating in the environment.

Optionally, applications may also specify a fatal-error callback function using the DB_ENV->set_paniccall method. This callback function will be called with two arguments: a reference to the DB_ENV structure associated with the environment and the errno value associated with the underlying error that caused the problem.

Applications can handle such fatal errors in one of two ways: by checking for DB_RUNRECOVERY as part of their normal Berkeley DB error return checking, similarly to DB_LOCK_DEADLOCK or any other error, or by simply exiting the application when the callback function is called in applications that have no cleanup processing of their own.