Expand description
§Wasmtime’s embedding API
Wasmtime is a WebAssembly engine for JIT-compiled or ahead-of-time compiled
WebAssembly modules and components. More information about the Wasmtime
project as a whole can be found in the documentation
book whereas this documentation mostly focuses
on the API reference of the wasmtime
crate itself.
This crate contains an API used to interact with WebAssembly modules or WebAssembly components. For example you can compile WebAssembly, create instances, call functions, etc. As an embedder of WebAssembly you can also provide guests functionality from the host by creating host-defined functions, memories, globals, etc, which can do things that WebAssembly cannot (such as print to the screen).
The wasmtime
crate is designed to be safe, efficient, and ergonomic.
This enables executing WebAssembly without the embedder needing to use
unsafe
code, meaning that you’re guaranteed there is no undefined behavior
or segfaults in either the WebAssembly guest or the host itself.
The wasmtime
crate can roughly be thought of as being split into two
halves:
-
One half of the crate is similar to the JS WebAssembly API as well as the proposed C API and is intended for working with WebAssembly modules. This API resides in the root of the
wasmtime
crate’s namespace, for examplewasmtime::Module
. -
The second half of the crate is for use with the WebAssembly Component Model. The implementation of the component model is present in
wasmtime::component
and roughly mirrors the structure for core WebAssembly, for examplecomponent::Func
mirrorsFunc
.
An example of using Wasmtime to run a core WebAssembly module looks like:
use wasmtime::*;
fn main() -> wasmtime::Result<()> {
let engine = Engine::default();
// Modules can be compiled through either the text or binary format
let wat = r#"
(module
(import "host" "host_func" (func $host_hello (param i32)))
(func (export "hello")
i32.const 3
call $host_hello)
)
"#;
let module = Module::new(&engine, wat)?;
// Host functionality can be arbitrary Rust functions and is provided
// to guests through a `Linker`.
let mut linker = Linker::new(&engine);
linker.func_wrap("host", "host_func", |caller: Caller<'_, u32>, param: i32| {
println!("Got {} from WebAssembly", param);
println!("my host state is: {}", caller.data());
})?;
// All wasm objects operate within the context of a "store". Each
// `Store` has a type parameter to store host-specific data, which in
// this case we're using `4` for.
let mut store: Store<u32> = Store::new(&engine, 4);
// Instantiation of a module requires specifying its imports and then
// afterwards we can fetch exports by name, as well as asserting the
// type signature of the function with `get_typed_func`.
let instance = linker.instantiate(&mut store, &module)?;
let hello = instance.get_typed_func::<(), ()>(&mut store, "hello")?;
// And finally we can call the wasm!
hello.call(&mut store, ())?;
Ok(())
}
§Core Concepts
There are a number of core types and concepts that are important to be aware
of when using the wasmtime
crate:
-
Engine
- a global compilation and runtime environment for WebAssembly. AnEngine
is an object that can be shared concurrently across threads and is created with aConfig
with many knobs for configuring behavior. Compiling or executing any WebAssembly requires first configuring and creating anEngine
. AllModule
s andComponent
s belong to anEngine
, and typically there’s oneEngine
per process. -
Store
- container for all information related to WebAssembly objects such as functions, instances, memories, etc. AStore<T>
allows customization of theT
to store arbitrary host data within aStore
. This host data can be accessed through host functions via theCaller
function parameter in host-defined functions. AStore
is required for all WebAssembly operations, such as calling a wasm function. TheStore
is passed in as a “context” to methods likeFunc::call
. Dropping aStore
will deallocate all memory associated with WebAssembly objects within theStore
. AStore
is cheap to create and destroy and does not GC objects such as unused instances internally, so it’s intended to be short-lived (or no longer than the instances it contains). -
Linker
(orcomponent::Linker
) - host functions are defined within a linker to provide them a string-based name which can be looked up when instantiating a WebAssembly module or component. Linkers are traditionally populated at startup and then reused for all future instantiations of all instances, assuming the set of host functions does not change over time. Host functions areFn(..) + Send + Sync
and typically do not close over mutable state. Instead it’s recommended to store mutable state in theT
ofStore<T>
which is accessed throughCaller<'_, T>
provided to host functions. -
Module
(orComponent
) - a compiled WebAssembly module or component. These structures contain compiled executable code from a WebAssembly binary which is ready to execute after being instantiated. These are expensive to create as they require compilation of the input WebAssembly. Modules and components are safe to share across threads, however. Modules and components can additionally be serialized into a list of bytes to later be deserialized quickly. This enables JIT-style compilation through constructors such asModule::new
and AOT-style compilation by having the compilation process useModule::serialize
and the execution process useModule::deserialize
. -
Instance
(orcomponent::Instance
) - an instantiated WebAssembly module or component. An instance is where you can actually acquire aFunc
(orcomponent::Func
) from, for example, to call. -
Func
(orcomponent::Func
) - a WebAssembly function. This can be acquired as the export of anInstance
to call WebAssembly functions, or it can be created via functions likeFunc::wrap
to wrap host-defined functionality and give it to WebAssembly. Functions also have typed views asTypedFunc
orcomponent::TypedFunc
for a more efficient calling convention. -
Table
,Global
,Memory
,component::Resource
- other WebAssembly objects which can either be defined on the host or in wasm itself (via instances). These all have various ways of being interacted with likeFunc
.
All “store-connected” types such as Func
, Memory
, etc, require the
store to be passed in as a context to each method. Methods in wasmtime
frequently have their first parameter as either impl AsContext
or impl AsContextMut
. These
traits are implemented for a variety of types, allowing you to, for example,
pass the following types into methods:
&Store<T>
&mut Store<T>
&Caller<'_, T>
&mut Caller<'_, T>
StoreContext<'_, T>
StoreContextMut<'_, T>
A Store
is the sole owner of all WebAssembly internals. Types like
Func
point within the Store
and require the Store
to be provided
to actually access the internals of the WebAssembly function, for instance.
§WASI
The wasmtime
crate does not natively provide support for WASI, but you can
use the wasmtime-wasi
crate for that purpose. With wasmtime-wasi
all
WASI functions can be added to a Linker
and then used to instantiate
WASI-using modules. For more information see the WASI example in the
documentation.
§Crate Features
The wasmtime
crate comes with a number of compile-time features that can
be used to customize what features it supports. Some of these features are
just internal details, but some affect the public API of the wasmtime
crate. Wasmtime APIs gated behind a Cargo feature should be indicated as
such in the documentation.
-
runtime
- Enabled by default, this feature enables executing WebAssembly modules and components. If a compiler is not available (such ascranelift
) thenModule::deserialize
must be used, for example, to provide an ahead-of-time compiled artifact to execute WebAssembly. -
cranelift
- Enabled by default, this features enables using Cranelift at runtime to compile a WebAssembly module to native code. This feature is required to process and compile new WebAssembly modules and components. -
cache
- Enabled by default, this feature adds support for wasmtime to perform internal caching of modules in a global location. This must still be enabled explicitly throughConfig::cache_config_load
orConfig::cache_config_load_default
. -
wat
- Enabled by default, this feature adds support for accepting the text format of WebAssembly inModule::new
andComponent::new
. The text format will be automatically recognized and translated to binary when compiling a module. -
parallel-compilation
- Enabled by default, this feature enables support for compiling functions in parallel withrayon
. -
async
- Enabled by default, this feature enables APIs and runtime support for defining asynchronous host functions and calling WebAssembly asynchronously. For more information seeConfig::async_support
. -
profiling
- Enabled by default, this feature compiles in support for profiling guest code via a number of possible strategies. SeeConfig::profiler
for more information. -
all-arch
- Not enabled by default. This feature compiles in support for all architectures for both the JIT compiler and thewasmtime compile
CLI command. This can be combined withConfig::target
to precompile modules for a different platform than the host. -
pooling-allocator
- Enabled by default, this feature adds support forPoolingAllocationConfig
to pass toConfig::allocation_strategy
. The pooling allocator can enable efficient reuse of resources for high-concurrency and high-instantiation-count scenarios. -
demangle
- Enabled by default, this will affect how backtraces are printed and whether symbol names from WebAssembly are attempted to be demangled. Rust and C++ demanglings are currently supported. -
coredump
- Enabled by default, this will provide support for generating a core dump when a trap happens. This can be configured viaConfig::coredump_on_trap
. -
addr2line
- Enabled by default, this feature configures whether traps will attempt to parse DWARF debug information and convert WebAssembly addresses to source filenames and line numbers. -
debug-builtins
- Enabled by default, this feature includes some built-in debugging utilities and symbols for native debuggers such as GDB and LLDB to attach to the process Wasmtime is used within. The intrinsics provided will enable debugging guest code compiled to WebAssembly. This must also be enabled viaConfig::debug_info
as well for guests. -
component-model
- Enabled by default, this enables support for thewasmtime::component
API for working with components. -
gc
- Enabled by default, this enables support for a number of WebAssembly proposals such asreference-types
,function-references
, andgc
. Note that the implementation of thegc
proposal itself is not yet complete at this time. -
threads
- Enabled by default, this enables compile-time support for the WebAssemblythreads
proposal, notably shared memories. -
call-hook
- Disabled by default, this enables support for the [Store::call_hook
] API. This incurs a small overhead on all entries/exits from WebAssembly and may want to be disabled by some embedders. -
memory-protection-keys
- Disabled by default, this enables support for the [PoolingAllocationConfig::memory_protection_keys
] API. This feature currently only works on x64 Linux and can enable compacting the virtual memory allocation for linear memories in the pooling allocator. This comes with the same overhead as thecall-hook
feature where entries/exits into WebAssembly will have more overhead than before.
More crate features can be found in the manifest of Wasmtime itself for seeing what can be enabled and disabled.
Re-exports§
Modules§
- Embedding API for the Component Model
- Unix-specific extension for the
wasmtime
crate.
Structs§
- An
anyref
GC reference. - A reference to a GC-managed
array
instance. - An allocator for a particular Wasm GC array type.
- The type of a WebAssembly array.
- A structure representing the caller’s context when creating a function via
Func::wrap
. - Builder-style structure used to create a
Module
or pre-compile a module to a serialized list of bytes. - Management of executable memory within a
MmapVec
- A compiled wasm module, ready to be instantiated.
- Global configuration options used to create an
Engine
and customize its behavior. - An
Engine
which is a global context for compilation and management of wasm modules. - A weak reference to an
Engine
. - A reference to a GC-managed object that can be tested for equality.
- An exported WebAssembly value.
- A descriptor for an exported WebAssembly value.
- An opaque, GC-managed reference to some host data that can be passed to WebAssembly.
- The type of a
struct
field or anarray
’s elements. - Description of a frame in a backtrace for a
WasmBacktrace
. - Debug information for a symbol that is attached to a
FrameInfo
. - A WebAssembly function which can be called.
- The type of a WebAssembly function.
- An error returned when attempting to allocate a GC-managed object, but the GC heap is out of memory.
- A WebAssembly
global
value which can be read and written to. - A WebAssembly global descriptor.
- A 31-bit integer.
- A descriptor for an imported value into a wasm module.
- An instantiated WebAssembly module.
- An instance, pre-instantiation, that is ready to be instantiated.
- Structure used to link wasm modules/instances together.
- A rooted reference to a garbage-collected
T
with arbitrary lifetime. - A WebAssembly linear memory.
- Error for out of bounds
Memory
access. - A descriptor for a WebAssembly memory type.
- A builder for
MemoryType
s. - A compiled WebAssembly module, ready to be instantiated.
- Describes the location of an export in a module.
- A reference to the abstract
noextern
heap value. - A reference to the abstract
nofunc
heap value. - An error returned when the pooling allocator cannot allocate a table, memory, etc… because the maximum number of concurrent allocations for that entity has been reached.
- Configuration options used with
InstanceAllocationStrategy::Pooling
to change the behavior of the pooling instance allocator. - Opaque references to data in the Wasm heap or to host data.
- Nested rooting scopes.
- A scoped, rooted reference to a garbage-collected
T
. - A constructor for externally-created shared memory.
- A
Store
is a collection of WebAssembly instances and host-defined state. - A temporary handle to a
&Store<T>
. - A temporary handle to a
&mut Store<T>
. - Provides limits for a
Store
. - Used to build
StoreLimits
. - A reference to a GC-managed
struct
instance. - An allocator for a particular Wasm GC struct type.
- The type of a WebAssembly struct.
- A WebAssembly
table
, or an array of values. - A descriptor for a table in a WebAssembly module.
- A statically typed WebAssembly function.
- Error for an unresolvable import.
- Representation of a 128-bit vector type,
v128
, for WebAssembly. - Representation of a backtrace of function frames in a WebAssembly module for where an error happened.
- Representation of a core dump of a WebAssembly module
Enums§
- Passed to the argument of [
Store::call_hook
] to indicate a state transition in the WebAssembly VM. - Return value of
CodeBuilder::hint
- An external item to a WebAssembly module, or a list of what can possibly be exported from a wasm module.
- A list of all possible types which can be externally referenced from a WebAssembly module.
- Indicator of whether a type is final or not.
- The heap types that can Wasm can have references to.
- Represents the module instance allocation strategy to use.
- Configure the strategy used for versioning in serializing and deserializing
crate::Module
. - Describe the tri-state configuration of memory protection keys (MPK).
- Indicator of whether a global value, struct’s field, or array type’s elements are mutable or not.
- Possible optimization levels for the Cranelift codegen backend.
- Return value from the
Engine::detect_precompiled
API. - Select which profiling technique to support.
- A reference.
- The storage type of a
struct
field orarray
element. - Possible Compilation strategies for a wasm module.
- Representation of a WebAssembly trap and what caused it to occur.
- What to do after returning from a callback when the engine epoch reaches the deadline for a Store during execution of a function using that store.
- Possible runtime values that a WebAssembly module can either consume or produce.
- A list of all possible value types in WebAssembly.
- Result of
Memory::atomic_wait32
andMemory::atomic_wait64
- Select how wasm backtrace detailed information is handled.
Constants§
- Value returned by
ResourceLimiter::instances
default method - Value returned by
ResourceLimiter::memories
default method - Value returned by
ResourceLimiter::tables
default method
Traits§
- A trait used to get shared access to a
Store
in Wasmtime. - A trait used to get exclusive mutable access to a
Store
in Wasmtime. - A common trait implemented by all garbage-collected reference types.
- Internal trait implemented for all arguments that can be passed to
Func::wrap
andLinker::func_wrap
. - A linear memory. This trait provides an interface for raw memory buffers which are used by wasmtime, e.g. inside [‘Memory’]. Such buffers are in principle not thread safe. By implementing this trait together with MemoryCreator, one can supply wasmtime with custom allocated host managed memory.
- A memory creator. Can be used to provide a memory creator to wasmtime which supplies host managed memory.
- Used by hosts to limit resource consumption of instances.
- Used by hosts to limit resource consumption of instances, blocking asynchronously if necessary.
- A trait implemented for GC references that are guaranteed to be rooted:
- A stack creator. Can be used to provide a stack creator to wasmtime which supplies stacks for async support.
- A stack memory. This trait provides an interface for raw memory buffers which are used by wasmtime inside of stacks which wasmtime executes WebAssembly in for async support. By implementing this trait together with StackCreator, one can supply wasmtime with custom allocated host managed stacks.
- A trait used for
Func::typed
and withTypedFunc
to represent the set of parameters for wasm functions. - A trait used for
Func::typed
and withTypedFunc
to represent the set of results for wasm functions. - A trait implemented for types which can be returned from closures passed to
Func::wrap
and friends. - A trait implemented for types which can be arguments and results for closures passed to
Func::wrap
as well as parameters toFunc::typed
. - Trait implemented for various tuples made up of types which implement
WasmTy
that can be passed toFunc::wrap_inner
and [HostContext::from_closure
].
Unions§
- A “raw” and unsafe representation of a WebAssembly value.