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<title>Berkeley DB Reference Guide: Shared Memory Regions</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Environment</dl></h3></td>
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<h1 align=center>Shared Memory Regions</h1>
<p>Each of the Berkeley DB subsystems within an environment is described by one or
more regions. The regions contain all of the per-process and per-thread
shared information, including mutexes, that comprise a Berkeley DB environment.
These regions are created in one of three areas, depending on the flags
specified to the <a href="../../api_c/env_open.html">DBENV->open</a> function:
<p><ol>
<p><li>If the <a href="../../api_c/env_open.html#DB_PRIVATE">DB_PRIVATE</a> flag is specified to <a href="../../api_c/env_open.html">DBENV->open</a>, regions
are created in per-process heap memory, i.e., memory returned by
<b>malloc</b>(3). In this case, the Berkeley DB environment may only be
accessed by a single process, although that process may be
multi-threaded.
<p><li>If the <a href="../../api_c/env_open.html#DB_SYSTEM_MEM">DB_SYSTEM_MEM</a> flag is specified to <a href="../../api_c/env_open.html">DBENV->open</a>,
regions are created in system memory. When regions are created in system
memory, the Berkeley DB environment may be accessed by both multiple processes
and multiple threads within processes.
<p>The system memory used by Berkeley DB is potentially useful past the lifetime
of any particular process. Therefore, additional cleanup may be necessary
after an application fails, as there may be no way for Berkeley DB to ensure
that system resources backing the shared memory regions are returned to
the system.
<p>The system memory that is used is architecture-dependent. For example,
on systems supporting X/Open-style shared memory interfaces, e.g., UNIX
systems, the <b>shmget</b>(2) and related System V IPC interfaces are
used. Additionally, VxWorks systems use system memory.
In these cases, an initial segment ID must be specified by the
application to ensure that applications do not overwrite each other's
database environments, and so that the number of segments created does
not grow without bound. See the <a href="../../api_c/env_set_shm_key.html">DBENV->set_shm_key</a> function for more
information.
<p><li>If no memory-related flags are specified to <a href="../../api_c/env_open.html">DBENV->open</a>, then
memory backed by the filesystem is used to store the regions. On UNIX
systems, the Berkeley DB library will use the POSIX mmap interface. If mmap is
not available, the UNIX shmget interfaces will be used, assuming they are
available.
</ol>
<p>Any files created in the filesystem to back the regions are created in
the environment home directory specified to the <a href="../../api_c/env_open.html">DBENV->open</a> call.
These files are named __db.###, e.g., __db.001, __db.002 and so on.
When region files are backed by the filesystem, one file per region is
created. When region files are backed by system memory, a single file
will still be created, as there must be a well-known name in the
filesystem so that multiple processes can locate the system shared memory
that is being used by the environment.
<p>Statistics about the shared memory regions in the environment can be
displayed using the <b>-e</b> option to the <a href="../../utility/db_stat.html">db_stat</a> utility.
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