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Use in Rust {#flatbuffers_guide_use_rust}
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==========
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## Before you get started
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Before diving into the FlatBuffers usage in Rust, it should be noted that
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the [Tutorial](@ref flatbuffers_guide_tutorial) page has a complete guide
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to general FlatBuffers usage in all of the supported languages (including Rust).
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This page is designed to cover the nuances of FlatBuffers usage, specific to
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Rust.
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#### Prerequisites
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This page assumes you have written a FlatBuffers schema and compiled it
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with the Schema Compiler. If you have not, please see
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[Using the schema compiler](@ref flatbuffers_guide_using_schema_compiler)
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and [Writing a schema](@ref flatbuffers_guide_writing_schema).
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Assuming you wrote a schema, say `mygame.fbs` (though the extension doesn't
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matter), you've generated a Rust file called `mygame_generated.rs` using the
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compiler (e.g. `flatc --rust mygame.fbs` or via helpers listed in "Useful
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tools created by others" section bellow), you can now start using this in
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your program by including the file. As noted, this header relies on the crate
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`flatbuffers`, which should be in your include `Cargo.toml`.
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## FlatBuffers Rust library code location
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The code for the FlatBuffers Rust library can be found at
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`flatbuffers/rust`. You can browse the library code on the
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[FlatBuffers GitHub page](https://github.com/google/flatbuffers/tree/master/rust).
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## Testing the FlatBuffers Rust library
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The code to test the Rust library can be found at `flatbuffers/tests/rust_usage_test`.
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The test code itself is located in
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[integration_test.rs](https://github.com/google/flatbuffers/blob/master/tests/rust_usage_test/tests/integration_test.rs)
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This test file requires `flatc` to be present. To review how to build the project,
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please read the [Building](@ref flatbuffers_guide_building) documentation.
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To run the tests, execute `RustTest.sh` from the `flatbuffers/tests` directory.
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For example, on [Linux](https://en.wikipedia.org/wiki/Linux), you would simply
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run: `cd tests && ./RustTest.sh`.
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*Note: The shell script requires [Rust](https://www.rust-lang.org) to
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be installed.*
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## Using the FlatBuffers Rust library
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*Note: See [Tutorial](@ref flatbuffers_guide_tutorial) for a more in-depth
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example of how to use FlatBuffers in Rust.*
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FlatBuffers supports both reading and writing FlatBuffers in Rust.
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To use FlatBuffers in your code, first generate the Rust modules from your
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schema with the `--rust` option to `flatc`. Then you can import both FlatBuffers
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and the generated code to read or write FlatBuffers.
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For example, here is how you would read a FlatBuffer binary file in Rust:
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First, include the library and generated code. Then read the file into
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a `u8` vector, which you pass, as a byte slice, to `root_as_monster()`.
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This full example program is available in the Rust test suite:
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[monster_example.rs](https://github.com/google/flatbuffers/blob/master/tests/rust_usage_test/bin/monster_example.rs)
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It can be run by `cd`ing to the `rust_usage_test` directory and executing: `cargo run monster_example`.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.rs}
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extern crate flatbuffers;
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#[allow(dead_code, unused_imports)]
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#[path = "../../monster_test_generated.rs"]
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mod monster_test_generated;
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pub use monster_test_generated::my_game;
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use std::io::Read;
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fn main() {
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let mut f = std::fs::File::open("../monsterdata_test.mon").unwrap();
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let mut buf = Vec::new();
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f.read_to_end(&mut buf).expect("file reading failed");
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let monster = my_game::example::root_as_monster(&buf[..]);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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`monster` is of type `Monster`, and points to somewhere *inside* your
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buffer (root object pointers are not the same as `buffer_pointer` !).
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If you look in your generated header, you'll see it has
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convenient accessors for all fields, e.g. `hp()`, `mana()`, etc:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.rs}
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println!("{}", monster.hp()); // `80`
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println!("{}", monster.mana()); // default value of `150`
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println!("{:?}", monster.name()); // Some("MyMonster")
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*Note: That we never stored a `mana` value, so it will return the default.*
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## Direct memory access
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As you can see from the above examples, all elements in a buffer are
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accessed through generated accessors. This is because everything is
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stored in little endian format on all platforms (the accessor
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performs a swap operation on big endian machines), and also because
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the layout of things is generally not known to the user.
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For structs, layout is deterministic and guaranteed to be the same
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across platforms (scalars are aligned to their
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own size, and structs themselves to their largest member), and you
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are allowed to access this memory directly by using `safe_slice`
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on the reference to a struct, or even an array of structs.
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To compute offsets to sub-elements of a struct, make sure they
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are structs themselves, as then you can use the pointers to
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figure out the offset without having to hardcode it. This is
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handy for use of arrays of structs with calls like `glVertexAttribPointer`
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in OpenGL or similar APIs.
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It is important to note is that structs are still little endian on all
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machines, so the functions to enable tricks like this are only exposed on little
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endian machines. If you also ship on big endian machines, using an
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`#[cfg(target_endian = "little")]` attribute would be wise or your code will not
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compile.
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The special function `safe_slice` is implemented on Vector objects that are
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represented in memory the same way as they are represented on the wire. This
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function is always available on vectors of struct, bool, u8, and i8. It is
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conditionally-compiled on little-endian systems for all the remaining scalar
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types.
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The FlatBufferBuilder function `create_vector_direct` is implemented for all
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types that are endian-safe to write with a `memcpy`. It is the write-equivalent
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of `safe_slice`.
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## Access of untrusted buffers
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The safe Rust functions to interpret a slice as a table (`root`,
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`size_prefixed_root`, `root_with_opts`, and `size_prefixed_root_with_opts`)
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verify the data first. This has some performance cost, but is intended to be
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safe for use on flatbuffers from untrusted sources. There are corresponding
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`unsafe` versions with names ending in `_unchecked` which skip this
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verification, and may access arbitrary memory.
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The generated accessor functions access fields over offsets, which is
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very quick. The current implementation uses these to access memory without any
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further bounds checking. All of the safe Rust APIs ensure the verifier is run
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over these flatbuffers before accessing them.
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When you're processing large amounts of data from a source you know (e.g.
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your own generated data on disk), the `_unchecked` versions are acceptable, but
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when reading data from the network that can potentially have been modified by an
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attacker, it is desirable to use the safe versions which use the verifier.
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## Threading
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Reading a FlatBuffer does not touch any memory outside the original buffer,
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and is entirely read-only (all immutable), so is safe to access from multiple
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threads even without synchronisation primitives.
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Creating a FlatBuffer is not thread safe. All state related to building
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a FlatBuffer is contained in a FlatBufferBuilder instance, and no memory
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outside of it is touched. To make this thread safe, either do not
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share instances of FlatBufferBuilder between threads (recommended), or
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manually wrap it in synchronisation primitives. There's no automatic way to
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accomplish this, by design, as we feel multithreaded construction
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of a single buffer will be rare, and synchronisation overhead would be costly.
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Unlike most other languages, in Rust these properties are exposed to and
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enforced by the type system. `flatbuffers::Table` and the generated table types
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are `Send + Sync`, indicating they may be freely shared across threads and data
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may be accessed from any thread which receives a const (aka shared) reference.
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There are no functions which require a mutable (aka exclusive) reference, which
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means all the available functions may be called like this.
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`flatbuffers::FlatBufferBuilder` is also `Send + Sync`, but all of the mutating
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functions require a mutable (aka exclusive) reference which can only be created
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when no other references to the `FlatBufferBuilder` exist, and may not be copied
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within the same thread, let alone to a second thread.
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## Useful tools created by others
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* [flatc-rust](https://github.com/frol/flatc-rust) - FlatBuffers compiler
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(flatc) as API for transparent `.fbs` to `.rs` code-generation via Cargo
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build scripts integration.
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<br>
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