DbEnv::set_shm_key()
Prev | Chapter 5. The DbEnv Handle | Next

## DbEnv::set_shm_key()

    #include <db_cxx.h>

    int
    DbEnv::set_shm_key(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 method 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 database environment's base segment ID may also be configured 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 `DbEnv::set_shm_key()` method configures operations performed using the specified DbEnv handle, not all operations performed on the underlying database environment.

The `DbEnv::set_shm_key()` method may not be called after the DbEnv::open() method is called. If the database environment already exists when DbEnv::open() is called, the information specified to `DbEnv::set_shm_key()` must be consistent with the existing environment or corruption can occur.

The `DbEnv::set_shm_key()` method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

### Parameters

#### shm_key

The shm_key parameter is the base segment ID for the database environment.

### Errors

The `DbEnv::set_shm_key()` method may fail and throw a DbException exception, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

#### EINVAL

If the method was called after DbEnv::open() was called; or if an invalid flag value or parameter was specified.

### Class

DbEnv

### See Also

Database Environments and Related Methods

Prev | Up | Next
DbEnv::set_msgfile() | Home | DbEnv::set_thread_count()