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mlua

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Guided Tour

mlua is bindings to Lua programming language for Rust with a goal to provide safe (as far as it's possible), high level, easy to use, practical and flexible API.

Started as rlua fork, mlua supports Lua 5.4, 5.3, 5.2 and 5.1 including LuaJIT (2.0.5 and 2.1 beta) and allows to write native Lua modules in Rust as well as use Lua in a standalone mode.

mlua tested on Windows/macOS/Linux including module mode in GitHub Actions on x86_64 platform and cross-compilation to aarch64 (other targets are also supported).

Usage

Feature flags

mlua uses feature flags to reduce the amount of dependencies, compiled code and allow to choose only required set of features. Below is a list of the available feature flags. By default mlua does not enable any features.

  • lua54: activate Lua 5.4 support
  • lua53: activate Lua 5.3 support
  • lua52: activate Lua 5.2 support
  • lua51: activate Lua 5.1 support
  • luajit: activate LuaJIT support
  • vendored: build static Lua(JIT) library from sources during mlua compilation using lua-src or luajit-src crates
  • module: enable module mode (building loadable cdylib library for Lua)
  • async: enable async/await support (any executor can be used, eg. tokio or async-std)
  • send: make mlua::Lua transferable across thread boundaries (adds Send requirement to mlua::Function and mlua::UserData)
  • serialize: add serialization and deserialization support to mlua types using serde framework

Async/await support

mlua supports async/await for all Lua versions. This works using Lua coroutines and require running Thread along with enabling feature = "async" in Cargo.toml.

Examples:

Serialization (serde) support

With serialize feature flag enabled, mlua allows you to serialize/deserialize any type that implements serde::Serialize and serde::Deserialize into/from mlua::Value. In addition mlua provides serde::Serialize trait implementation for it (including UserData support).

Example

Compiling

You have to enable one of the features lua54, lua53, lua52, lua51 or luajit, according to the chosen Lua version.

By default mlua uses pkg-config tool to find lua includes and libraries for the chosen Lua version. In most cases it works as desired, although sometimes could be more preferable to use a custom lua library. To achieve this, mlua supports LUA_INC, LUA_LIB, LUA_LIB_NAME and LUA_LINK environment variables. LUA_LINK is optional and may be dylib (a dynamic library) or static (a static library, .a archive).

An example how to use them:

my_project $ LUA_INC=$HOME/tmp/lua-5.2.4/src LUA_LIB=$HOME/tmp/lua-5.2.4/src LUA_LIB_NAME=lua LUA_LINK=static cargo build

mlua also supports vendored lua/luajit using the auxiliary crates lua-src and luajit-src. Just enable the vendored feature and cargo will automatically build and link specified lua/luajit version. This is the easiest way to get started with mlua.

Standalone mode

In a standalone mode mlua allows to add to your application scripting support with a gently configured Lua runtime to ensure safety and soundness.

Add to Cargo.toml :

[dependencies]
mlua = { version = "0.5", features = ["lua53", "vendored"] }

main.rs

use mlua::prelude::*;

fn main() -> LuaResult<()> {
    let lua = Lua::new();

    let map_table = lua.create_table()?;
    map_table.set(1, "one")?;
    map_table.set("two", 2)?;

    lua.globals().set("map_table", map_table)?;

    lua.load("for k,v in pairs(map_table) do print(k,v) end").exec()?;

    Ok(())
}

Module mode

In a module mode mlua allows to create a compiled Lua module that can be loaded from Lua code using require. In this case mlua uses an external Lua runtime which could lead to potential unsafety due to unpredictability of the Lua environment and usage of libraries such as debug.

Example

Add to Cargo.toml :

[lib]
crate-type = ["cdylib"]

[dependencies]
mlua = { version = "0.5", features = ["lua53", "vendored", "module"] }

lib.rs :

use mlua::prelude::*;

fn hello(_: &Lua, name: String) -> LuaResult<()> {
    println!("hello, {}!", name);
    Ok(())
}

#[mlua::lua_module]
fn my_module(lua: &Lua) -> LuaResult<LuaTable> {
    let exports = lua.create_table()?;
    exports.set("hello", lua.create_function(hello)?)?;
    Ok(exports)
}

And then (macOS example):

$ cargo rustc -- -C link-arg=-undefined -C link-arg=dynamic_lookup
$ ln -s ./target/debug/libmy_module.dylib ./my_module.so
$ lua5.3 -e 'require("my_module").hello("world")'
hello, world!

On macOS, you need to set additional linker arguments. One option is to compile with cargo rustc --release -- -C link-arg=-undefined -C link-arg=dynamic_lookup, the other is to create a .cargo/config with the following content:

[target.x86_64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

[target.aarch64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

On Linux you can build modules normally with cargo build --release. Vendored and non-vendored builds are supported for these OS.

On Windows vendored mode for modules is not supported since you need to link to a Lua dll. Easiest way is to use either MinGW64 (as part of MSYS2 package) with pkg-config or MSVC with LUA_INC / LUA_LIB / LUA_LIB_NAME environment variables.

More details about compiling and linking Lua modules can be found on the Building Modules page.

Safety

One of the mlua goals is to provide safe API between Rust and Lua. Every place where the Lua C API may trigger an error longjmp in any way is protected by lua_pcall, and the user of the library is protected from directly interacting with unsafe things like the Lua stack, and there is overhead associated with this safety.

Unfortunately, mlua does not provide absolute safety even without using unsafe . This library contains a huge amount of unsafe code. There are almost certainly bugs still lurking in this library! It is surprisingly, fiendishly difficult to use the Lua C API without the potential for unsafety.

Panic handling

mlua wraps panics that are generated inside Rust callbacks in a regular Lua error. Panics could be resumed then by returning or propagating the Lua error to Rust code.

For example:

let lua = Lua::new();
let f = lua.create_function(|_, ()| -> LuaResult<()> {
    panic!("test panic");
})?;
lua.globals().set("rust_func", f)?;

let _ = lua.load(r#"
    local status, err = pcall(rust_func)
    print(err) -- prints: test panic
    error(err) -- propagate panic
"#).exec();

unreachable!()

Optionally mlua can disable Rust panics catching in Lua via pcall/xpcall and automatically resume them across the Lua API boundary. This is controlled via LuaOptions and done by wrapping the Lua pcall/xpcall functions on a way to prevent catching errors that are wrapped Rust panics.

mlua should also be panic safe in another way as well, which is that any Lua instances or handles remains usable after a user generated panic, and such panics should not break internal invariants or leak Lua stack space. This is mostly important to safely use mlua types in Drop impls, as you should not be using panics for general error handling.

Below is a list of mlua behaviors that should be considered a bug. If you encounter them, a bug report would be very welcome:

  • If you can cause UB with mlua without typing the word "unsafe", this is a bug.

  • If your program panics with a message that contains the string "mlua internal error", this is a bug.

  • Lua C API errors are handled by longjmp. All instances where the Lua C API would otherwise longjmp over calling stack frames should be guarded against, except in internal callbacks where this is intentional. If you detect that mlua is triggering a longjmp over your Rust stack frames, this is a bug!

  • If you detect that, after catching a panic or during a Drop triggered from a panic, a Lua or handle method is triggering other bugs or there is a Lua stack space leak, this is a bug. mlua instances are supposed to remain fully usable in the face of user generated panics. This guarantee does not extend to panics marked with "mlua internal error" simply because that is already indicative of a separate bug.

License

This project is licensed under the MIT license


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