In this example we'll be using a simple Wasm module which exports some globals.
Globals are probably the simplest entity we'll encounter in Wasm modules but there is still some interesting things to talk about. For example, globals come in two flavors:
Immutable globals (const
)
Mutable globals (var
)
We will cover both in this example.
First we are going to want to initialize a new project. To do this we can navigate to our project folder, or create one. In this example, we will create a new project. Lets create it and navigate to it:
The final Rust code for this example can be found on Github: exports_global.rs.
Please take a look at the setup steps for Rust.
cargo new exports-globalcd exports-global
We have to modify Cargo.toml
to add the Wasmer dependencies as shown below:
[dependencies]# The Wasmer APIwasmer = "1.0"
The final Go code for this example can be found on Github: exports_global.go.
Please take a look at the setup steps for Go.
mkdir wasmer-example-imports-exportscd wasmer-example-imports-exportsgo mod init github.com/$USER/wasmer-example-imports-exports
The final Python code for this example can be found on Github: exports_global.py.
Please take a look at the setup steps for Python.
mkdir wasmer-example-imports-exportscd wasmer-example-imports-exportspip install wasmer wasmer_compiler_cranelift
The final PHP code for this example can be found on Github: exports-global.php.
Please take a look at the setup steps for PHP.
mkdir wasmer-example-exports-globalscd wasmer-example-exports-globalscomposer init --name=wasmer-example-exports-globalscomposer require wasm/wasm
The final C code for this example can be found on Github: exports-global.c.
Please take a look at the setup steps for C/C++.
mkdir wasmer-example-exports-globalcd wasmer-example-exports-globalvim Makefile
Let's create a simple Makefile
:
CFLAGS = -g -I$(shell $(WASMER_DIR)/bin/wasmer config --includedir)LDFLAGS = -Wl,-rpath,$(shell $(WASMER_DIR)/bin/wasmer config --libdir)LDLIBS = $(shell $(WASMER_DIR)/bin/wasmer config --libs).SILENT: exports-global exports-global.oexports-global: exports-global.o.PHONY: clean.SILENT: cleanclean:rm -f exports-global.o exports-global
Now that we have everything set up, let's go ahead and try it out!
The first interesting thing to do is to query their type information in order to know if they are mutable or not. Our module exports two globals, one
and some
. Which one is mutable and which one is not?
let one = instance.exports.get_global("one")?;let some = instance.exports.get_global("some")?;let one_type = one.ty();let some_type = some.ty();println!("one type: {:?} {:?}", one_type.mutability, one_type.ty);println!("some type: {:?} {:?}", some_type.mutability, some_type.ty);
one, err := instance.Exports.GetGlobal("one")if err != nil {panic(fmt.Sprintln("Failed to retrieve the `one` global:", err))}some, err := instance.Exports.GetGlobal("some")if err != nil {panic(fmt.Sprintln("Failed to retrieve the `some` global:", err))}oneType := one.Type()someType := some.Type()fmt.Printf("`one` type: %s %s\n",oneType.Mutability(),oneType.ValueType().Kind().String())fmt.Printf("`some` type: %s %s\n",someType.Mutability(),someType.ValueType().Kind().String())
one = instance.exports.onesome = instance.exports.someone_type = one.typeassert one_type.type == Type.F32assert one_type.mutable == Falsesome_type = some.typeassert some_type.type == Type.F32assert some_type.mutable == True
$exports = $instance->exports();$one = (new Wasm\Extern($exports[0]))->asGlobal();$some = (new Wasm\Extern($exports[1]))->asGlobal();$oneType = $one->type();assert($oneType->mutability() === GlobalType::MUTABILITY_CONST);$someType = $some->type();assert($oneType->mutability() === GlobalType::MUTABILITY_VAR);
wasm_mutability_t one_mutability = wasm_globaltype_mutability(one_type);const wasm_valtype_t* one_content = wasm_globaltype_content(one_type);wasm_valkind_t one_kind = wasm_valtype_kind(one_content);wasm_mutability_t some_mutability = wasm_globaltype_mutability(some_type);const wasm_valtype_t* some_content = wasm_globaltype_content(some_type);wasm_valkind_t some_kind = wasm_valtype_kind(some_content);printf("`one` type: %s %hhu\n",one_mutability == WASM_CONST ? "const" : "",one_kind);printf("`some` type: %s %hhu\n",some_mutability == WASM_CONST ? "const" : "",some_kind);
The global API is straightforward: it provides a dedicated method to get the value of a given global. Look how easy it is:
let some_value = some.get();println!("`some` value: {:?}", some_value);
someValue, err := some.Get()if err != nil {panic(fmt.Sprintln("Failed to get the `some` global value:", err))}fmt.Printf("`some` value: %.1f\n", someValue)
some_value = some.valueassert some_value == 0.0
$someValue = $some->get()->value();assert($someValue === 0);
wasm_val_t some_value;wasm_global_get(some, &some_value);printf("`some` value: %.1f\n", some_value.of.f32);
As we said before, globals come in two flavor. Immutable globals, for which we can only set a value once and mutable ones.
First we'll try to set the value of a immutable global and see what happens:
let result = one.set(Value::F32(42.0));assert_eq!(result.expect_err("Expected an error").message(),"Attempted to set an immutable global");
err = one.Set(float32(42.0), wasmer.F32)if err == nil {panic(fmt.Sprintln("Setting value to `one` did not error"))}
try:one.value = 42.0except RuntimeError as err:assert str(err) == 'The global variable is not mutable, cannot set a new value'else:assert False
try {$one->set(42.0);} catch(\Wasm\Exception\RuntimeException $exception) {assert($exception->getMessage() === 'RuntimeError: Attempted to set an immutable global');}
wasm_val_t one_set_value = WASM_F32_VAL(42);wasm_global_set(one, &one_set_value);int error_length = wasmer_last_error_length();if (error_length > 0) {char *error_message = malloc(error_length);wasmer_last_error_message(error_message, error_length);printf("Attempted to set an immutable global: `%s`\n", error_message);}
As you can see here, trying to set a value on a immutable global will always lead to an error.
Now let's see how to correctly set a value on a mutable global:
some.set(Value::F32(21.0))?;
err = some.Set(float32(42.0), wasmer.F32)if err != nil {panic(fmt.Sprintln("Failed to set the `some` global value:", err))}
some.value = 21.0
$some->set(42.0);
wasm_val_t some_set_value = WASM_F32_VAL(21);wasm_global_set(some, &some_set_value);
We now have everything we need to run the Wasm module, let's do it!
You should be able to run it using the cargo run
command. The output should look like this:
Compiling module...Instantiating module...Getting globals types information...`one` type: Const F32`some` type: Var F32Getting global values...`one` value: 1.0`some` value: F32(0.0)Setting global values...`one` value after `set`: F32(1.0)`some` value after `set_some`: F32(21.0)`some` value after `set`: F32(42.0)
If you want to run the examples from the Wasmer repository codebase directly, you can also do:
git clone https://github.com/wasmerio/wasmer.gitcd wasmercargo run --example exported-function --release --features "cranelift"
You should be able to run it using the go run main.go
command. The output should look like this:
Compiling module...Instantiating module...Getting globals types information...`one` type: const f32`some` type: var f32Getting global values...`one` value: 1.0`some` value: 0.0Setting global values...`one` value: 1.0`some` value after `set_some`: 21.0`some` value after `set`: 42.0
If you want to run the examples from the Wasmer repository codebase directly, you can also do:
git clone https://github.com/wasmerio/wasmer-go.gitcd wasmer-gogo test examples/example_exports_global_test.go
You should be able to run it using the python exports_global.py
command.
If you want to run the examples from the Wasmer repository codebase directly, you can also do:
git clone https://github.com/wasmerio/wasmer-python.gitcd wasmer-pythonpython examples/exports_global.py
You should be able to run it using the php exports-global.php
command.
If you want to run the examples from the Wasmer PHP repository codebase directly, you can also do:
git clone https://github.com/wasmerio/wasmer-php.gitcd wasmer-phpmake EXAMPLE=exports-global test-doc-examples
You should be able to run it using the make clean exports-global && ./exports-global
command. The output should look like this:
Creating the store...Compiling module...Creating imports...Instantiating module...Retrieving exports...Getting globals types information...`one` type: const 2`some` type: 2Getting global values...`one` value: 1.0`some` value: 0.0Setting global values...Attempted to set an immutable global: `RuntimeError: Attempted to set an immutable global``some` value: 0.0
If you want to run the examples from the Wasmer repository codebase directly, you can also do:
git clone https://github.com/wasmerio/wasmer.gitcd wasmer/lib/c-api/examples/exports-global.cmake clean exports-global./exports-global