# DbEnv::set_shm_key |

`

###

    #include <db_cxx.h>

    int
    DbEnv::set_shm_key(long shm_key);

# Description

Specify a base segment ID for Berkeley DB environment shared memory regions created in system memory on systems supporting X/Open-style shared memory interfaces, e.g., 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 a segment ID between 36 and 40 or so. A Berkeley DB environment always creates a master shared memory region, plus 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 DbEnv::set_shm_key interface may only be used to configure Berkeley DB before the DbEnv::open interface is called.

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.

The 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. Configuration in the DB_CONFIG file silently overrules configuration done using the DbEnv::set_shm_key method.

# Errors

EINVAL
  An invalid flag value or parameter was specified.

  Called after DbEnv::open was called.

### Class

DbEnv

# See Also

DbEnv::close, DbEnv::open, DbEnv::remove, DbEnv::err, DbEnv::strerror, DbEnv::version, DbEnv::set_cachesize, DbEnv::set_errcall, DbEnv::set_errfile, DbEnv::set_error_stream, DbEnv::set_errpfx, DbEnv::set_paniccall, and DbEnv::set_verbose.`

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