DbEnv::set_alloc()
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## DbEnv::set_alloc()

    #include <db_cxx.h>

    extern "C" {
        typedef void *(*db_malloc_fcn_type)(size_t);
        typedef void *(*db_realloc_fcn_type)(void *, size_t);
        typedef void *(*db_free_fcn_type)(void *);
    };

    int
    DbEnv::set_alloc(db_malloc_fcn_type app_malloc,
        db_realloc_fcn_type app_realloc,
        db_free_fcn_type app_free);

Set the allocation functions used by the DbEnv and Db methods to allocate or free memory owned by the application.

There are a number of interfaces in Berkeley DB where memory is allocated by the library and then given to the application. For example, the DB_DBT_MALLOC flag, when specified in the Dbt object, will cause the Db methods to allocate and reallocate memory which then becomes the responsibility of the calling application. Other examples are the Berkeley DB interfaces which return statistical information to the application: Db::stat(), DbEnv::lock_stat(), DbEnv::log_archive(), DbEnv::log_stat(), DbEnv::memp_stat(), and DbEnv::txn_stat(). There is one method in Berkeley DB where memory is allocated by the application and then given to the library: the callback specified to Db::associate().

On systems in which there may be multiple library versions of the standard allocation routines (notably Windows NT), transferring memory between the library and the application will fail because the Berkeley DB library allocates memory from a different heap than the application uses to free it. To avoid this problem, the `DbEnv::set_alloc()` and Db::set_alloc() methods can be used to pass Berkeley DB references to the application's allocation routines.

It is not an error to specify only one or two of the possible allocation function parameters to these interfaces; however, in that case the specified interfaces must be compatible with the standard library interfaces, as they will be used together. The functions specified must match the calling conventions of the ANSI C X3.159-1989 (ANSI C) library routines of the same name.

The `DbEnv::set_alloc()` method configures operations performed using the specified DbEnv handle, not all operations performed on the underlying database environment.

The `DbEnv::set_alloc()` method may not be called after the DbEnv::open() method is called.

The `DbEnv::set_alloc()` 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

#### app_malloc

The app_malloc parameter is the application-specified malloc function.

#### app_realloc

The app_realloc parameter is the application-specified realloc function.

#### app_free

The app_free parameter is the application-specified free function.

### Errors

The `DbEnv::set_alloc()` 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

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