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<title>Berkeley DB: DB_ENV->set_shm_key</title>
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<h1>DB_ENV->set_shm_key</h1>
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DB_ENV->set_shm_key(DB_ENV *dbenv, long shm_key);
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<p>Specify a base segment ID for Berkeley DB environment shared memory regions
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created in system memory on VxWorks or systems supporting X/Open-style
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shared memory interfaces; for example, UNIX systems supporting
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<b>shmget</b>(2) and related System V IPC interfaces.
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<p>This base segment ID will be used when Berkeley DB shared memory regions are
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first created. It will be incremented a small integer value each time
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a new shared memory region is created; that is, if the base ID is 35,
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the first shared memory region created will have a segment ID of 35,
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and the next one will have a segment ID between 36 and 40 or so. A
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Berkeley DB environment always creates a master shared memory region; an
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additional shared memory region for each of the subsystems supported by
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the environment (Locking, Logging, Memory Pool and Transaction); plus
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an additional shared memory region for each additional memory pool cache
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that is supported. Already existing regions with the same segment IDs
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will be removed. See <a href="../ref/env/region.html">Shared Memory
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Regions</a> for more information.
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<p>The intent behind this interface is two-fold: without it, applications
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have no way to ensure that two Berkeley DB applications don't attempt to use
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the same segment IDs when creating different Berkeley DB environments. In
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addition, by using the same segment IDs each time the environment is
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created, previously created segments will be removed, and the set of
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segments on the system will not grow without bound.
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<p>The DB_ENV->set_shm_key method configures operations performed using the specified
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<a href="../api_c/env_class.html">DB_ENV</a> handle, not all operations performed on the underlying
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<p>The DB_ENV->set_shm_key interface may not be called after the <a href="../api_c/env_open.html">DB_ENV->open</a>
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If the database environment already exists when
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<a href="../api_c/env_open.html">DB_ENV->open</a> is called, the information specified to DB_ENV->set_shm_key
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must be consistent with the existing environment or corruption can
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<p>The DB_ENV->set_shm_key method returns a non-zero error value on failure and 0 on success.
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<p>The database environment's base segment ID may also be set using the environment's
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<b>DB_CONFIG</b> file. The syntax of the entry in that file is a
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single line with the string "set_shm_key", one or more whitespace characters,
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and the ID. Because the <b>DB_CONFIG</b> file is read when the database
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environment is opened, it will silently overrule configuration done
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<p>The DB_ENV->set_shm_key method may fail and return a non-zero error for the following conditions:
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
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<p>Called after <a href="../api_c/env_open.html">DB_ENV->open</a> was called.
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<p>The DB_ENV->set_shm_key method may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
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If a catastrophic error has occurred, the DB_ENV->set_shm_key method may fail and
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return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>,
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in which case all subsequent Berkeley DB calls will fail in the same way.
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<a href="../api_c/env_class.html">DB_ENV</a>
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<a href="../api_c/env_list.html">Database Environments and Related Methods</a>
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