Berkeley DB Reference Guide: Programmer Notes
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Berkeley DB Reference Guide: Programmer Notes

Building multi-threaded applications

The Berkeley DB library is not itself multi-threaded. The library was deliberately architected to not use threads internally because of the portability problems that using threads within the library would introduce. Object handles returned from Berkeley DB library functions are free-threaded, i.e., threads may use handles concurrently, by specifying the DB_THREAD flag to db_appinit and the other subsystem open functions. Threading is assumed in the Java API, so no special flags are required, and Berkeley DB functions will always behave as if the DB_THREAD flag was specified.

Berkeley DB supports multi-threaded applications with the caveat that it loads and calls functions that are commonly available in C language environments and which may not themselves be thread-safe. Other than this usage, Berkeley DB has no static data and maintains no local context between calls to Berkeley DB functions. To ensure that applications can safely use threads in the context of Berkeley DB, porters to new operating systems and/or C libraries must confirm that the system and C library functions used by the Berkeley DB library are thread-safe.

There are some additional caveats about using threads to access the Berkeley DB library:

  1. The DB_THREAD flag must be specified for all subsystems either explicitly or via the db_appinit function. Threading is assumed in the Java API, so no special flags are required, and Berkeley DB functions will always behave as if the DB_THREAD flag was specified.

    Setting the DB_THREAD flag inconsistently may result in database corruption.

  2. Only a single thread may call the DB->close function for a returned database or subsystem handle.

  3. When using the non-cursor Berkeley DB calls to retrieve key/data items (e.g., DB->get), the memory referenced by the pointer stored into the Dbt is only valid until the next call to using the DB handle returned by db_open. (This includes any use of the returned DB handle, including by another thread of control within the process. For this reason, when multiple threads are using the returned DB handle concurrently, either the DB_DBT_MALLOC or DB_DBT_USERMEM flag must be specified for any non-cursor DBT used for key or data retrieval.) When using the cursor Berkeley DB calls to retrieve key/data items (e.g., dbc_get), the memory referenced by the pointer into the DBT is only valid until the next call to Berkeley DB using the DBC handle returned by DB->cursor.

  4. The DB_CURRENT, DB_NEXT and DB_PREV flags to the log_get function may not be used by a free-threaded handle. If such calls are necessary, a thread should explicitly create a unique DB_LOG handle by calling log_open.

  5. Each database operation (i.e., any call to a function underlying the handles returned by db_open and DB->cursor is normally performed on behalf of a unique locker. If, within a single thread of control, multiple calls on behalf of the same locker are desired, then transactions must be used. For example, consider the case where a cursor scan locates a record, and then based on that record, accesses some other item in the database. If these are done using the default lockers for the handle, there is no guarantee that these two operations will not conflict. If the application wishes to guarantee that the operations do not conflict, locks must be obtained on behalf of a transaction, instead of the default locker id, and a transaction must be specified to the cursor creation and the subsequent Berkeley DB calls.

  6. Transactions may not span threads, i.e., each transaction must begin and end in the same thread, and each transaction may only be used by a single thread.

  7. Spinlocks must have been implemented for the compiler/architecture combination. Attempting to specify the DB_THREAD flag will fail if spinlocks are not available.

  8. The Berkeley DB library makes a system call to pause for some number of microseconds when it is necessary to wait on a lock. This may not be optimal, especially in a thread-only environment where it will be more efficient to explicitly yield the processor to another thread. It is possible to specify a yield function on an per-application basis.

Compiling Threaded Applications

Special compile-time flags are required when compiling threaded applications with the UNIX include files on some architectures.

On FreeBSD, if you are compiling a threaded application, you must compile with the _THREAD_SAFE flag and link with the libc_r.a library:

On IRIX, if you are compiling a threaded application, you must compile with the _SGI_MP_SOURCE flag:

On OSF/1, if you are compiling a threaded application, you must compile with the _REENTRANT flag:

On Solaris, if you are compiling a threaded application, you must compile with the _REENTRANT flag and link with the libthread.a library: