# lmdb-zero
[](https://travis-ci.org/AltSysrq/lmdb-zero)
[](https://crates.io/crates/lmdb-zero)
lmdb-zero is a near-zero-cost wrapper around [LMDB](http://lmdb.tech/) designed
to allow using the full range of features offered by LMDB while keeping it
reasonably easy to write safe programs.
`lmdb-zero` is as much as possible a 1:1 mapping of the raw API, mainly
providing RAII constructs and integration into Rust's borrow checker to ensure
safety.
[Documentation](https://docs.rs/lmdb-zero)
## Features
- Zero-copy API. Reads return references into the memory-mapped file. Using
`MDB_RESERVE` to allocate space in the file and directly write to it is
supported.
- Cursors directly map to the same operations provided by LMDB, but in a
typesafe manner.
- Nested transactions.
- Full integration with the borrow checker. Read references are checked to not
outlive their transaction or overlap with a write in the same transaction.
- Cursors and read transactions can be reset and reused.
## Status
The API is complete and reasonably stable and is believed to be sound insofar
as Rust's unsafety rules are actually defined.
This crate has not been thoroughly tested on architectures with strong
alignment constraints, though the tests pass on ARM7. While the conversion API
checks for correct alignment by default, issues such as
[#27060](https://github.com/rust-lang/rust/issues/27060) could come up, and it
is of course possible there are bugs in handling alignment here.
## Changelog
**0.4.4**: New `Environment` and `Database` methods to enable interoperating
with native C/C++ code. `Database::dbi()` renamed to `as_raw()`. The old name
is still available but deprecated.
**0.4.3**: Fix panic on `Cursor::get_multiple()` if the current key has exactly
one item. `LmdbResultExt` is now reexported from the crate root for better
visibility.
**0.4.2**: Fix being unable to open databases in read-only environments. Fix
future-incompatibility warning arising from `Unaligned`.
**0.4.1**: Tests updated to work on Rust 1.20. `bitflags` dependency updated.
Neither of these are expected to have any impact on external code.
**0.4.0**: Minor breaking changes. `ConstAccessor` and `WriteAccessor` can now
be dropped and re-obtained. Most types now support additional
ownership/borrowing modes, which allows for dynamic lifetime management and
other possibilities. Upgrade to `liblmdb-sys` 0.2.2 and `bitflags` 0.8.0.
Fixes to documentation.
**0.3.1**: Metadata updates to reflect change of crate ownership. No software
changes were made in this version.
**0.3.0**: **Breaking Changes** to the API, see section below. Migration is
expected to be easy for most use-cases. Slight performance improvement due to
additions of `#[inline]`.
**0.2.2**: `ResetTransaction` is now actually public, making that part of the
API more accessible. Add documentation for lifetimes.
**0.2.1**: Fix use-after-free when passing database name to `mdb_dbi_open`. Fix
calling `mdb_txn_abort` after transaction commit fails.
[#1](https://github.com/AltSysrq/lmdb-zero/pull/1).
**0.2.0**: Switch from `lmdb-sys` to newer `liblmdb-sys`.
**0.1.0**: Initial release.
### Breaking Changes in 0.4.0
A number of functions which formerly took an `&SomeType` parameter now take an
`Into<Supercow<SomeType>>`. For the vast majority of existing code, this has no
effect, but it could cause issues if older code was relying on an implicit
`Deref` call (for example, via `lazy_static!`) to produce the correct reference
type. If this causes issues, explicitly writing the dereferencing is required.
For example, if your code originally had `lmdb::Database::open(&ENV, ...)` where
`ENV` was declared via `lazy_static!`, it would need to be changed to
`lmdb::Database::open(&*ENV, ...)`.
`ConstAccessor` and `WriteAccessor` must now be _strictly_ outlived by their
transactions. No practical cases where this would be an issue are apparent, but
if it comes up, code must be rearranged to ensure the accessor is dropped
before the transaction. Note that now one can drop the accessor and later
re-obtain it.
### Breaking Changes in 0.3.0
`lmdb::Error` has been completely reworked. It is now an enum with the
lmdb-zero errors cleanly separated from native LMDB errors. `ValRejected` now
includes an error message.
`FromLmdbBytes.from_lmdb_bytes()` now returns a `Result<&Self, String>` instead
of an `Option`. This is mainly to make alignment issues less subtle and point
people directly to advice on how to fix the problem, but should be able to make
other things clearer as well.
The mostly untested and somewhat questionable `lax_alignment` feature has been
dropped. `LmdbRaw` now always enforces alignment requirements. Client code
which wishes to operate on misaligned values which cannot use the `Unaligned`
or `#[repr(packed)]` solutions will need to provide its own `FromLmdbBytes`
implementations.
The primitive types which have alignment requirements (eg, `i32`, `u64`) are no
longer `LmdbRaw`, as this made it too easy to write code depending on
happenstance to align the values correctly. Client code now _must_ wrap them in
`Unaligned` to read them directly, or else provide its own unit structs if it
has other needs. Note that these types and their arrays are still
`AsLmdbBytes`.
Unfortunately, as a side-effect of the above, `Wrapping<u8>` and `Wrapping<i8>`
are no longer `LmdbRaw` or `LmdbOrdKey`, but instead only
`LmdbRawIfUnaligned` and `LmdbOrdKeyIfUnaligned`. Wrapping these in `Unalinged`
will work in most cases without overhead.
## Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.