# DB_ENV->set_mp_mmapsize

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    #include <db.h>

    int
    DB_ENV->set_mp_mmapsize(DB_ENV *dbenv, size_t mp_mmapsize);

# Description

Files that are opened read-only in the pool (and that satisfy a few other criteria) are, by default, mapped into the process address space instead of being copied into the local cache. This can result in better-than-usual performance because available virtual memory is normally much larger than the local cache, and page faults are faster than page copying on many systems. However, it can cause resource starvation in the presence of limited virtual memory, and it can result in immense process sizes in the presence of large databases.

Set the maximum file size, in bytes, for a file to be mapped into the process address space. If no value is specified, it defaults to 10MB.

The DB_ENV->set_mp_mmapsize interface may be used only to configure Berkeley DB before the DB_ENV->open interface is called.

The DB_ENV->set_mp_mmapsize function returns a non-zero error value on failure and 0 on success.

The database environment's maximum mapped file size 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_mp_mmapsize", one or more whitespace characters, and the size in bytes. Because the DB_CONFIG file is read when the database environment is opened, it will silently overrule configuration done before that time.

# Errors

The DB_ENV->set_mp_mmapsize function may fail and return a non-zero error for the following conditions:

EINVAL
  An invalid flag value or parameter was specified.

  Called after DB_ENV->open was called.

The DB_ENV->set_mp_mmapsize function 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_mp_mmapsize function may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

# See Also

DB_ENV->set_cachesize, DB_ENV->set_mp_mmapsize, memp_fclose, memp_fget, memp_fopen, memp_fput, memp_fset, memp_fsync, memp_register, memp_stat, memp_sync, and memp_trickle.`

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