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152 строки
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152 строки
7.7 KiB
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<TITLE>db_thread</TITLE>
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<BODY BGCOLOR=white>
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<H1>db_thread</H1>
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<HR SIZE=1 NOSHADE>
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<PRE>
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<!-- Manpage converted by man2html 3.0.1 -->
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The DB library is a family of groups of functions that
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provides a modular programming interface to transactions
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and record-oriented file access. The library includes
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support for transactions, locking, logging and file page
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caching, as well as various indexed access methods. Many
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of the functional groups (e.g., the file page caching
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functions) are useful independent of the other DB
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functions, although some functional groups are explicitly
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based on other functional groups (e.g., transactions and
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logging). For a general description of the DB package,
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see <B><A HREF="db_intro.html">db_intro(3)</A></B>.
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This manual pages describes the specific details of using
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DB from within threaded programs.
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The DB library is not itself multi-threaded. The library
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was deliberately architected to not use threads internally
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because of the portability problems that using threads
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within the library would introduce. Object handles
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returned from DB library functions are free-threaded,
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i.e., threads may use handles concurrently, by specifying
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the DB_THREAD flag to <B><A HREF="db_appinit.html">db_appinit(3)</A></B> (or, in the C++ API,
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<B><A HREF="DbEnv.html">DbEnv::appinit(3)</A></B>) and the other subsystem open functions.
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Threading is assumed in the Java API, so no special flags
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are required, and DB functions will always behave as if
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the DB_THREAD flag was specified.
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DB supports multi-threaded applications with the caveat
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that it loads and calls functions that are commonly
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available in C language environments and which may not
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themselves be thread-safe. Other than this usage, DB has
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no static data and maintains no local context between
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calls to DB functions. To ensure that applications can
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safely use threads in the context of DB, porters to new
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operating systems and/or C libraries must confirm that the
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system and C library functions used by the DB library are
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thread-safe.
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There are some additional caveats about using threads to
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access the DB library:
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<B>o</B> The DB_THREAD flag must be specified for all subsystems
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either explicitly or via the db_appinit (DbEnv::appinit)
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function in the C and C++ APIs. Threading is assumed in
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the Java API, so no special flags are required, and DB
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functions will always behave as if the DB_THREAD flag
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was specified.
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Setting the DB_THREAD flag inconsistently may result in
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database corruption.
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<B>o</B> Only a single thread may call the close function for a
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returned database or subsystem handle. See <B><A HREF="db_open.html">db_open(3)</A></B>
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(<B><A HREF="Db.html">Db::open(3)</A></B>) and the appropriate subsystem manual pages
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for more information.
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<B>o</B> Either the DB_DBT_MALLOC or DB_DBT_USERMEM flags must be
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set in a DBT used for key or data retrieval. See
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<B><A HREF="db_dbt.html">db_dbt(3)</A></B> (<B><A HREF="Dbt.html">Dbt(3)</A></B>) for more information.
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<B>o</B> The DB_CURRENT, DB_NEXT and DB_PREV flags to the log_get
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(DbLog::get) function may not be used by a free-threaded
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handle. If such calls are necessary, a thread should
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explicitly create a unique DB_LOG handle by calling
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<B><A HREF="db_log.html">log_open(3)</A></B> (DbLog::open). See <B><A HREF="db_log.html">db_log(3)</A></B> (<B><A HREF="DbLog.html">DbLog(3)</A></B>) for
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more information.
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<B>o</B> Each database operation (i.e., any call to a function
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underlying the handles returned by <B><A HREF="db_open.html">db_open(3)</A></B> and
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<B><A HREF="db_cursor.html">db_cursor(3)</A></B>) (<B><A HREF="Db.html">Db(3)</A></B> and <B><A HREF="Dbc.html">Dbc(3)</A></B>) is normally performed
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on behalf of a unique locker. If, within a single
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thread of control, multiple calls on behalf of the same
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locker are desired, then transactions must be used. For
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example, consider the case where a cursor scan locates a
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record, and then based on that record, accesses some
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other item in the database. If these are done using the
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default lockers for the handle, there is no guarantee
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that these two operations will not conflict. If the
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application wishes to guarantee that the operations do
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not conflict, locks must be obtained on behalf of a
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transaction, instead of the default locker id, and a
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transaction must be specified to the cursor creation and
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the subsequent db call.
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<B>o</B> Transactions may not span threads, i.e., each
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transaction must begin and end in the same thread, and
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each transaction may only be used by a single thread.
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<B>o</B> Spinlocks must have been implemented for the
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compiler/architecture combination. Attempting to
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specify the DB_THREAD flag will fail if spinlocks are
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not available.
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<B>o</B> The DB library makes a system call to pause for some
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number of microseconds when it is necessary to wait on a
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lock. This may not be optimal, especially in a thread-
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only environment where it will be more efficient to
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explicitly yield the processor to another thread. It is
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possible to specify a yield function on an per-
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application basis, see <B><A HREF="db_internal.html">db_jump_set(3)</A></B> for more
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information.
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</PRE>
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<H2>COMPILING THREADED APPLICATIONS</H2><PRE>
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Special compile-time flags are required when compiling
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threaded applications with the UNIX include files on some
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architectures.
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On IRIX, if you are compiling a threaded application, you
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must compile with the -D_SGI_MP_SOURCE flag:
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cc -D_SGI_MP_SOURCE ...
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On OSF/1, if you are compiling a threaded application, you
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must compile with the -D_REENTRANT flag:
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cc -D_REENTRANT ...
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On Solaris, if you are compiling a threaded application,
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you must compile with the -D_REENTRANT flag and link with
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the -lthread library:
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cc -D_REENTRANT ... -lthread
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</PRE>
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<H2>SEE ALSO</H2><PRE>
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<B><A HREF="db_archive.html">db_archive(1)</A></B>, <B><A HREF="db_checkpoint.html">db_checkpoint(1)</A></B>, <B><A HREF="db_deadlock.html">db_deadlock(1)</A></B>, <B><A HREF="db_dump.html">db_dump(1)</A></B>,
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<B><A HREF="db_load.html">db_load(1)</A></B>, <B><A HREF="db_recover.html">db_recover(1)</A></B>, <B><A HREF="db_stat.html">db_stat(1)</A></B>, <B><A HREF="db_intro.html">db_intro(3)</A></B>,
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<B><A HREF="db_appinit.html">db_appinit(3)</A></B>, <B><A HREF="db_cursor.html">db_cursor(3)</A></B>, <B><A HREF="db_dbm.html">db_dbm(3)</A></B>, <B><A HREF="db_internal.html">db_internal(3)</A></B>,
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<B><A HREF="db_lock.html">db_lock(3)</A></B>, <B><A HREF="db_log.html">db_log(3)</A></B>, <B><A HREF="db_mpool.html">db_mpool(3)</A></B>, <B><A HREF="db_open.html">db_open(3)</A></B>, <B><A HREF="db_thread.html">db_thread(3)</A></B>,
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<B><A HREF="db_txn.html">db_txn(3)</A></B>
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</PRE>
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<HR SIZE=1 NOSHADE>
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<ADDRESS>
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Man(1) output converted with
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<a href="http://www.oac.uci.edu/indiv/ehood/man2html.html">man2html</a>
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</ADDRESS>
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