lmdb++: a C++11 wrapper for LMDB

This is a comprehensive C++ wrapper for the LMDB embedded database library, offering both an error-checked procedural interface and an object-oriented resource interface with RAII semantics.

Example

Here follows a simple motivating example demonstrating basic use of the object-oriented resource interface:

#include <cstdio>
#include <cstdlib>
#include <lmdb++.h>

int main() {
  /* Create and open the LMDB environment: */
  auto env = lmdb::env::create();
  env.set_mapsize(1UL * 1024UL * 1024UL * 1024UL); /* 1 GiB */
  env.open("./example.mdb", 0, 0664);

  /* Insert some key/value pairs in a write transaction: */
  auto wtxn = lmdb::txn::begin(env);
  auto dbi = lmdb::dbi::open(wtxn, nullptr);
  dbi.put(wtxn, "username", "jhacker");
  dbi.put(wtxn, "email", "[email protected]");
  dbi.put(wtxn, "fullname", "J. Random Hacker");
  wtxn.commit();

  /* Fetch key/value pairs in a read-only transaction: */
  auto rtxn = lmdb::txn::begin(env, nullptr, MDB_RDONLY);
  auto cursor = lmdb::cursor::open(rtxn, dbi);
  std::string key, value;
  while (cursor.get(key, value, MDB_NEXT)) {
    std::printf("key: '%s', value: '%s'\n", key.c_str(), value.c_str());
  }
  cursor.close();
  rtxn.abort();

  /* The enviroment is closed automatically. */

  return EXIT_SUCCESS;
}

Should any operation in the above fail, an lmdb::error exception will be thrown and terminate the program since we don't specify an exception handler. All resources will regardless get automatically cleaned up due to RAII semantics.

Note

In order to run this example, you must first manually create the ./example.mdb directory. This is a basic characteristic of LMDB: the given environment path must already exist, as LMDB will not attempt to automatically create it.

Features

• Designed to be entirely self-contained as a single <lmdb++.h> header file that can be dropped into a project.
• Implements a straightforward mapping from C to C++, with consistent naming.
• Provides both a procedural interface and an object-oriented RAII interface.
• Simplifies error handling by translating error codes into C++ exceptions.
• Carefully differentiates logic errors, runtime errors, and fatal errors.
• Exception strings include the name of the LMDB function that failed.
• Plays nice with others: all symbols are placed into the lmdb namespace.
• 100% free and unencumbered public domain software, usable in any context and for any purpose.

Requirements

The <lmdb++.h> header file requires a C++11 compiler and standard library. Recent releases of Clang or GCC will work fine.

In addition, for your application to build and run, the underlying <lmdb.h> header file shipped with LMDB must be available in the preprocessor's include path, and you must link with the liblmdb native library. On Ubuntu Linux 14.04 and newer, these prerequisites can be satisfied by installing the liblmdb-dev package.

Overview

This wrapper offers both an error-checked procedural interface and an object-oriented resource interface with RAII semantics. The former will be useful for easily retrofitting existing projects that currently use the raw C interface, but we recommend the latter for all new projects due to the exception safety afforded by RAII semantics.

Resource Interface

The high-level resource interface wraps LMDB handles in a loving RAII embrace. This way, you can ensure e.g. that a transaction will get automatically aborted when exiting a lexical scope, regardless of whether the escape happened normally or by throwing an exception.

C handle	C++ wrapper class
MDB_env*	lmdb::env
MDB_txn*	lmdb::txn
MDB_dbi	lmdb::dbi
MDB_cursor*	lmdb::cursor
MDB_val	lmdb::val

The methods available on these C++ classes are named consistently with the procedural interface, below, with the obvious difference of omitting the handle type prefix which is already implied by the class in question.

Procedural Interface

The low-level procedural interface wraps LMDB functions with error-checking code that will throw an instance of a corresponding C++ exception class in case of failure. This interface doesn't offer any convenience overloads as does the resource interface; the parameter types are exactly the same as for the raw C interface offered by LMDB itself. The return type is generally void for these functions since the wrapper eats the error code returned by the underlying C function, throwing an exception in case of failure and otherwise returning values in the same output parameters as the C interface.

This interface is implemented entirely using static inline functions, so there are no hidden extra costs to using these wrapper functions so long as you have a decent compiler capable of basic inlining optimization.

C function	C++ wrapper function
mdb_version()	N/A
mdb_strerror()	N/A
mdb_env_create()	lmdb::env_create()
mdb_env_open()	lmdb::env_open()
mdb_env_copy()	lmdb::env_copy() [1]
mdb_env_copyfd()	lmdb::env_copy_fd() [1]
mdb_env_copy2()	lmdb::env_copy() [1]
mdb_env_copyfd2()	lmdb::env_copy_fd() [1]
mdb_env_stat()	lmdb::env_stat()
mdb_env_info()	lmdb::env_info()
mdb_env_sync()	lmdb::env_sync()
mdb_env_close()	lmdb::env_close()
mdb_env_set_flags()	lmdb::env_set_flags()
mdb_env_get_flags()	lmdb::env_get_flags()
mdb_env_get_path()	lmdb::env_get_path()
mdb_env_get_fd()	lmdb::env_get_fd()
mdb_env_set_mapsize()	lmdb::env_set_mapsize()
mdb_env_set_maxreaders()	lmdb::env_set_max_readers()
mdb_env_get_maxreaders()	lmdb::env_get_max_readers()
mdb_env_set_maxdbs()	lmdb::env_set_max_dbs()
mdb_env_get_maxkeysize()	lmdb::env_get_max_keysize()
mdb_env_set_userctx()	lmdb::env_set_userctx() [2]
mdb_env_get_userctx()	lmdb::env_get_userctx() [2]
mdb_env_set_assert()	N/A
mdb_txn_begin()	lmdb::txn_begin()
mdb_txn_env()	lmdb::txn_env()
mdb_txn_id()	lmdb::txn_id() [3]
mdb_txn_commit()	lmdb::txn_commit()
mdb_txn_abort()	lmdb::txn_abort()
mdb_txn_reset()	lmdb::txn_reset()
mdb_txn_renew()	lmdb::txn_renew()
mdb_dbi_open()	lmdb::dbi_open()
mdb_stat()	lmdb::dbi_stat() [4]_
mdb_dbi_flags()	lmdb::dbi_flags()
mdb_dbi_close()	lmdb::dbi_close()
mdb_drop()	lmdb::dbi_drop() [4]_
mdb_set_compare()	lmdb::dbi_set_compare() [4]_
mdb_set_dupsort()	lmdb::dbi_set_dupsort() [4]_
mdb_set_relfunc()	lmdb::dbi_set_relfunc() [4]_
mdb_set_relctx()	lmdb::dbi_set_relctx() [4]_
mdb_get()	lmdb::dbi_get() [4]_
mdb_put()	lmdb::dbi_put() [4]_
mdb_del()	lmdb::dbi_del() [4]_
mdb_cursor_open()	lmdb::cursor_open()
mdb_cursor_close()	lmdb::cursor_close()
mdb_cursor_renew()	lmdb::cursor_renew()
mdb_cursor_txn()	lmdb::cursor_txn()
mdb_cursor_dbi()	lmdb::cursor_dbi()
mdb_cursor_get()	lmdb::cursor_get()
mdb_cursor_put()	lmdb::cursor_put()
mdb_cursor_del()	lmdb::cursor_del()
mdb_cursor_count()	lmdb::cursor_count()
mdb_cmp()	N/A
mdb_dcmp()	N/A
mdb_reader_list()	TODO
mdb_reader_check()	TODO

Footnotes

[1]	(1, 2, 3, 4) Three-parameter signature available since LMDB 0.9.14 (2014/09/20).

[2]	(1, 2) Only available since LMDB 0.9.11 (2014/01/15).

[3]	Only available in LMDB HEAD, not yet in any 0.9.x release (as of 0.9.16). Define the LMDBXX_TXN_ID preprocessor symbol to unhide this.

[4]	Note the difference in naming. (See below.)

Caveats

• The C++ procedural interface is more strictly and consistently grouped by handle type than is the LMDB native interface. For instance, mdb_put() is wrapped as the C++ function lmdb::dbi_put(), not lmdb::put(). These differences--a handful in number--all concern operations on database handles.
• The C++ interface takes some care to be const-correct for input-only parameters, something the original C interface largely ignores. Hence occasional use of const_cast in the wrapper code base.
• lmdb::dbi_put() does not throw an exception if LMDB returns the MDB_KEYEXIST error code; it instead just returns false. This is intended to simplify common usage patterns.
• lmdb::dbi_get(), lmdb::dbi_del(), and lmdb::cursor_get() do not throw an exception if LMDB returns the MDB_NOTFOUND error code; they instead just return false. This is intended to simplify common usage patterns.
• lmdb::env_get_max_keysize() returns an unsigned integer, instead of a signed integer as the underlying mdb_env_get_maxkeysize() function does. This conversion is done since the return value cannot in fact be negative.

Error Handling

This wrapper draws a careful distinction between three different classes of possible LMDB error conditions:

• Logic errors, represented by lmdb::logic_error. Errors of this class are thrown due to programming errors where the function interfaces are used in violation of documented preconditions. A common strategy for handling this class of error conditions is to abort the program with a core dump, facilitating introspection to locate and remedy the bug.
• Fatal errors, represented by lmdb::fatal_error. Errors of this class are thrown due to the exhaustion of critical system resources, in particular available memory (ENOMEM), or due to attempts to exceed applicable system resource limits. A typical strategy for handling this class of error conditions is to terminate the program with a descriptive error message. More robust programs and shared libraries may wish to implement another strategy, such as retrying the operation after first letting most of the call stack unwind in order to free up scarce resources.
• Runtime errors, represented by lmdb::runtime_error. Errors of this class are thrown as a matter of course to indicate various exceptional conditions. These conditions are generally recoverable, and robust programs will take care to correctly handle them.

Note

The distinction between logic errors and runtime errors mirrors that found in the C++11 standard library, where the <stdexcept> header defines the standard exception base classes std::logic_error and std::runtime_error. The standard exception class std::bad_alloc, on the other hand, is a representative example of a fatal error.

Error code	Exception class	Exception type
MDB_KEYEXIST	lmdb::key_exist_error	runtime
MDB_NOTFOUND	lmdb::not_found_error	runtime
MDB_CORRUPTED	lmdb::corrupted_error	fatal
MDB_PANIC	lmdb::panic_error	fatal
MDB_VERSION_MISMATCH	lmdb::version_mismatch_error	fatal
MDB_MAP_FULL	lmdb::map_full_error	runtime
MDB_BAD_DBI	lmdb::bad_dbi_error	runtime [4]_
(others)	lmdb::runtime_error	runtime

Footnotes

[4]	Available since LMDB 0.9.14 (2014/09/20).

Note

MDB_KEYEXIST and MDB_NOTFOUND are handled specially by some functions.

Versioning Policy

The lmdb++ version tracks the upstream LMDB release (x.y.z) that it is compatible with, and appends a sub-patch-level version (x.y.z.N) to indicate changes to the wrapper itself.

For example, an lmdb++ release of 0.9.14.2 would indicate that it is designed for compatibility with LMDB 0.9.14, and is the third wrapper release (the first being .0, and the second .1) for that upstream target.

Note

To the extent that LMDB will preserve API and ABI compatibility going forward, older versions of the wrapper should work with newer versions of LMDB; and newer versions of the wrapper will generally work with older versions of LMDB by using the preprocessor to conditionalize the visibility of newer symbols--see, for example, the preprocessor guards around the definition of lmdb::env_set_userctx().

Installation

lmdb++ is currently available as a package/port in the following operating system distributions and package management systems:

Distribution	Package Name	Installation Hint
Arch Linux AUR	liblmdb++	yaourt -Sa liblmdb++
Fink [5]	lmdb++	sudo fink install lmdb++
MacPorts	lmdbxx	sudo port install lmdbxx
Portage [6]	lmdb++	sudo emerge --ask lmdb++

Footnotes

[5]	Still pending review.

[6]	Compatible with Gentoo Linux, Funtoo Linux, and Sabayon Linux.

Support

To report a bug or submit a patch for lmdb++, please file an issue in the issue tracker on GitHub.

Questions and discussions about LMDB itself should be directed to the OpenLDAP mailing lists.

Elsewhere

Find this project at: GitHub, Bitbucket, Open Hub, SourceForge, Travis CI, and Coverity Scan.

The API documentation is published at: http://lmdbxx.sourceforge.net/

Author

Arto Bendiken - http://ar.to/

License

This is free and unencumbered public domain software. For more information, see http://unlicense.org/ or the accompanying UNLICENSE file.