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