✏️ Using guest (exported) globals
A Wasm module can export entities, like functions, memories, globals and tables. This example illustrates how to use exported globals.
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:
Rust
Go
Python
PHP
C/C++
cargo new exports-global
cd exports-global
We have to modify
Cargo.toml
to add the Wasmer dependencies as shown below:[dependencies]
# The Wasmer API
wasmer = "3.0"
mkdir wasmer-example-imports-exports
cd wasmer-example-imports-exports
go mod init github.com/$USER/wasmer-example-imports-exports
mkdir wasmer-example-imports-exports
cd wasmer-example-imports-exports
pip install wasmer wasmer_compiler_cranelift
mkdir wasmer-example-exports-globals
cd wasmer-example-exports-globals
composer init --name=wasmer-example-exports-globals
composer require wasm/wasm
mkdir wasmer-example-exports-global
cd wasmer-example-exports-global
vim Makefile
Let's create a simple
Makefile
:CFLAGS = -g -I$(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.o
exports-global: exports-global.o
.PHONY: clean
.SILENT: clean
clean:
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?Rust
Go
Python
PHP
C/C++
let one = instance.exports.get_global("one")?;
let some = instance.exports.get_global("some")?;
let one_type = one.ty(&store);
let some_type = some.ty(&store);
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.one
some = instance.exports.some
one_type = one.type
assert one_type.type == Type.F32
assert one_type.mutable == False
some_type = some.type
assert some_type.type == Type.F32
assert 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:
Rust
Go
Python
PHP
C/C++
let some_value = some.get(&mut store);
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.value
assert 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:
Rust
Go
Python
PHP
C/C++
let result = one.set(&mut store, 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 = 42.0
except 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:
Rust
Go
Python
PHP
C/C++
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!
Rust
Go
Python
PHP
C/C++
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 F32
Getting 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)
git clone https://github.com/wasmerio/wasmer.git
cd wasmer
cargo 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 f32
Getting global values...
`one` value: 1.0
`some` value: 0.0
Setting global values...
`one` value: 1.0
`some` value after `set_some`: 21.0
`some` value after `set`: 42.0
git clone https://github.com/wasmerio/wasmer-go.git
cd wasmer-go
go test examples/example_exports_global_test.go
You should be able to run it using the
python exports_global.py
command.git clone https://github.com/wasmerio/wasmer-python.git
cd wasmer-python
python examples/exports_global.py
You should be able to run it using the
php exports-global.php
command.git clone https://github.com/wasmerio/wasmer-php.git
cd wasmer-php
make 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: 2
Getting global values...`one` value: 1.0
`some` value: 0.0
Setting global values...
Attempted to set an immutable global: `RuntimeError: Attempted to set an immutable global`
`some` value: 0.0
git clone https://github.com/wasmerio/wasmer.git
cd wasmer/lib/c-api/examples/exports-global.c
make clean exports-global
./exports-global
Last modified 7mo ago