Crate wasmi

wasmi

This library allows WebAssembly modules to be loaded in binary format and their functions invoked.

Introduction

WebAssembly (wasm) is a safe, portable and compact format that is designed for efficient execution.

Wasm code is distributed in the form of modules that contains definitions of:

• functions,
• global variables,
• linear memory instances and
• tables.

Each of these definitions can be imported and exported.

In addition to these definitions, modules can define initialization data for their memory or tables. This initialization data can take the form of segments, copied to given offsets. They can also define a start function that is automatically executed when the module is loaded.

Loading and Validation

Before execution, a module must be validated. This process checks that the module is well-formed and makes only allowed operations.

A valid module can’t access memory outside its sandbox, can’t cause stack underflows and can only call functions with correct signatures.

Instantiation

In order to execute code from a wasm module, it must be instantiated. Instantiation includes the following steps:

1. Creating an empty module instance.
2. Resolving the definition instances for each declared import in the module.
3. Instantiating definitions declared in the module (e.g. allocate global variables, allocate linear memory, etc.).
4. Initializing memory and table contents by copying segments into them.
5. Executing the start function, if any.

After these steps, the module instance is ready to execute functions.

Execution

It only is allowed to call functions which are exported by the module. Functions can either return a result or trap (e.g. there can’t be linking error in the middle of the function execution). This property is ensured by the validation process.

Examples

extern crate wasmi;
extern crate wabt;

use wasmi::{ModuleInstance, ImportsBuilder, NopExternals, RuntimeValue};

fn main() {
    // Parse WAT (WebAssembly Text format) into wasm bytecode.
    let wasm_binary: Vec<u8> =
        wabt::wat2wasm(
            r#"
            (module
                (func (export "test") (result i32)
                    i32.const 1337
                )
            )
            "#,
        )
        .expect("failed to parse wat");

    // Load wasm binary and prepare it for instantiation.
    let module = wasmi::Module::from_buffer(&wasm_binary)
        .expect("failed to load wasm");

    // Instantiate a module with empty imports and
    // assert that there is no `start` function.
    let instance =
        ModuleInstance::new(
            &module,
            &ImportsBuilder::default()
        )
        .expect("failed to instantiate wasm module")
        .assert_no_start();

    // Finally, invoke the exported function "test" with no parameters
    // and empty external function executor.
    assert_eq!(
        instance.invoke_export(
            "test",
            &[],
            &mut NopExternals,
        ).expect("failed to execute export"),
        Some(RuntimeValue::I32(1337)),
    );
}

Modules

memory_units	WebAssembly-specific sizes and units.
nan_preserving_float

Structs

FuncInstance	Runtime representation of a function.
FuncInvocation	A resumable invocation handle. This struct is returned by FuncInstance::invoke_resumable.
FuncRef	Reference to a function (See FuncInstance for details).
GlobalDescriptor	Description of a global variable.
GlobalInstance	Runtime representation of a global variable (or global for short).
GlobalRef	Reference to a global variable (See GlobalInstance for details).
ImportsBuilder	Convenience builder of ImportResolver.
MemoryDescriptor	Description of a linear memory.
MemoryInstance	Runtime representation of a linear memory (or memory for short).
MemoryRef	Reference to a memory (See MemoryInstance for details).
Module	Deserialized module prepared for instantiation.
ModuleInstance	A module instance is the runtime representation of a module.
ModuleRef	Reference to a ModuleInstance.
NopExternals	Implementation of Externals that just traps on invoke_index.
NotStartedModuleRef	Mostly instantiated ModuleRef.
RuntimeArgs	Wrapper around slice of RuntimeValue for using it as an argument list conveniently.
Signature	Signature of a function.
StackRecycler	Used to recycle stacks instead of allocating them repeatedly.
TableDescriptor	Description of a table.
TableInstance	Runtime representation of a table.
TableRef	Reference to a table (See TableInstance for details).
Trap	Error type which can be thrown by wasm code or by host environment.

Enums

Error	Internal interpreter error.
ExternVal	An external value is the runtime representation of an entity that can be imported or exported.
ResumableError	A resumable invocation error.
RuntimeValue	Runtime representation of a value.
TrapKind	Error type which can be thrown by wasm code or by host environment.
ValueError	Error for LittleEndianConvert
ValueType	Type of a value.

Constants

DEFAULT_CALL_STACK_LIMIT	Maximum number of levels on the call stack.
DEFAULT_VALUE_STACK_LIMIT	Maximum number of bytes on the value stack.
LINEAR_MEMORY_PAGE_SIZE	Size of a page of linear memory - 64KiB.

Traits

Externals	Trait that allows to implement host functions.
FromRuntimeValue	Trait for creating value from a RuntimeValue.
HostError	Trait that allows the host to return custom error.
ImportResolver	Resolver of a module’s dependencies.
LittleEndianConvert	Convert from and to little endian.
ModuleImportResolver	Version of ImportResolver specialized for a single module.