Struct Engine

pub struct Engine { /* private fields */ }

An Engine which is a global context for compilation and management of wasm modules.

An engine can be safely shared across threads and is a cheap cloneable handle to the actual engine. The engine itself will be deallocated once all references to it have gone away.

Engines store global configuration preferences such as compilation settings, enabled features, etc. You’ll likely only need at most one of these for a program.

Engines and Clone

Using clone on an Engine is a cheap operation. It will not create an entirely new engine, but rather just a new reference to the existing engine. In other words it’s a shallow copy, not a deep copy.

Engines and Default

You can create an engine with default configuration settings using Engine::default(). Be sure to consult the documentation of Config for default settings.

Implementations

impl Engine

pub fn new(config: &Config) -> Result<Engine, Error>

Creates a new Engine with the specified compilation and configuration settings.

Errors

This method can fail if the config is invalid or some configurations are incompatible.

For example, feature reference_types will need to set the compiler setting enable_safepoints and unwind_info to true, but explicitly disable these two compiler settings will cause errors.

pub fn config(&self) -> &Config

Returns the configuration settings that this engine is using.

pub fn weak(&self) -> EngineWeak

Take a weak reference to this engine.

pub fn same(a: &Engine, b: &Engine) -> bool

Returns whether the engine a and b refer to the same configuration.

pub fn is_async(&self) -> bool

Returns whether the engine is configured to support async functions.

pub fn detect_precompiled(&self, bytes: &[u8]) -> Option<Precompiled>

Detects whether the bytes provided are a precompiled object produced by Wasmtime.

This function will inspect the header of bytes to determine if it looks like a precompiled core wasm module or a precompiled component. This does not validate the full structure or guarantee that deserialization will succeed, instead it helps higher-levels of the stack make a decision about what to do next when presented with the bytes as an input module.

If the bytes looks like a precompiled object previously produced by Module::serialize, Component::serialize, Engine::precompile_module, or Engine::precompile_component, then this will return Some(...) indicating so. Otherwise None is returned.

pub fn detect_precompiled_file( &self, path: impl AsRef<Path>, ) -> Result<Option<Precompiled>, Error>

Like Engine::detect_precompiled, but performs the detection on a file.

impl Engine

pub fn precompile_module(&self, bytes: &[u8]) -> Result<Vec<u8>, Error>

Ahead-of-time (AOT) compiles a WebAssembly module.

The bytes provided must be in one of two formats:

• A binary-encoded WebAssembly module. This is always supported.
• A text-encoded instance of the WebAssembly text format. This is only supported when the wat feature of this crate is enabled. If this is supplied then the text format will be parsed before validation. Note that the wat feature is enabled by default.

This method may be used to compile a module for use with a different target host. The output of this method may be used with Module::deserialize on hosts compatible with the Config associated with this Engine.

The output of this method is safe to send to another host machine for later execution. As the output is already a compiled module, translation and code generation will be skipped and this will improve the performance of constructing a Module from the output of this method.

pub fn precompile_component(&self, bytes: &[u8]) -> Result<Vec<u8>, Error>

Same as Engine::precompile_module except for a Component

impl Engine

pub fn tls_eager_initialize()

Eagerly initialize thread-local functionality shared by all Engines.

Wasmtime’s implementation on some platforms may involve per-thread setup that needs to happen whenever WebAssembly is invoked. This setup can take on the order of a few hundred microseconds, whereas the overhead of calling WebAssembly is otherwise on the order of a few nanoseconds. This setup cost is paid once per-OS-thread. If your application is sensitive to the latencies of WebAssembly function calls, even those that happen first on a thread, then this function can be used to improve the consistency of each call into WebAssembly by explicitly frontloading the cost of the one-time setup per-thread.

Note that this function is not required to be called in any embedding. Wasmtime will automatically initialize thread-local-state as necessary on calls into WebAssembly. This is provided for use cases where the latency of WebAssembly calls are extra-important, which is not necessarily true of all embeddings.

pub fn increment_epoch(&self)

Increments the epoch.

When using epoch-based interruption, currently-executing Wasm code within this engine will trap or yield “soon” when the epoch deadline is reached or exceeded. (The configuration, and the deadline, are set on the Store.) The intent of the design is for this method to be called by the embedder at some regular cadence, for example by a thread that wakes up at some interval, or by a signal handler.

See Config::epoch_interruption for an introduction to epoch-based interruption and pointers to the other relevant methods.

When performing increment_epoch in a separate thread, consider using Engine::weak to hold an EngineWeak and performing EngineWeak::upgrade on each tick, so that the epoch ticking thread does not keep an Engine alive longer than any of its consumers.

Signal Safety

This method is signal-safe: it does not make any syscalls, and performs only an atomic increment to the epoch value in memory.

pub fn precompile_compatibility_hash(&self) -> impl Hash

Returns a std::hash::Hash that can be used to check precompiled WebAssembly compatibility.

The outputs of Engine::precompile_module and Engine::precompile_component are compatible with a different Engine instance only if the two engines use compatible Configs. If this Hash matches between two Engines then binaries from one are guaranteed to deserialize in the other.

pub unsafe fn unload_process_handlers(self)

Unload process-related trap/signal handlers and destroy this engine.

This method is not safe and is not widely applicable. It is not required to be called and is intended for use cases such as unloading a dynamic library from a process. It is difficult to invoke this method correctly and it requires careful coordination to do so.

Panics

This method will panic if this Engine handle is not the last remaining engine handle.

Aborts

This method will abort the process on some platforms in some situations where unloading the handler cannot be performed and an unrecoverable state is reached. For example on Unix platforms with signal handling the process will be aborted if the current signal handlers are not Wasmtime’s.

Unsafety

This method is not generally safe to call and has a number of preconditions that must be met to even possibly be safe. Even with these known preconditions met there may be other unknown invariants to uphold as well.

• There must be no other instances of Engine elsewhere in the process. Note that this isn’t just copies of this Engine but it’s any other Engine at all. This unloads global state that is used by all Engines so this instance must be the last.

• On Unix platforms no other signal handlers could have been installed for signals that Wasmtime catches. In this situation Wasmtime won’t know how to restore signal handlers that Wasmtime possibly overwrote when Wasmtime was initially loaded. If possible initialize other libraries first and then initialize Wasmtime last (e.g. defer creating an Engine).

• All existing threads which have used this DLL or copy of Wasmtime may no longer use this copy of Wasmtime. Per-thread state is not iterated and destroyed. Only future threads may use future instances of this Wasmtime itself.

If other crashes are seen from using this method please feel free to file an issue to update the documentation here with more preconditions that must be met.

Trait Implementations

impl Clone for Engine

fn clone(&self) -> Engine

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Default for Engine

fn default() -> Engine

Returns the “default value” for a type.