calling-guest-functions.md

description
A Wasm module can export entities, like functions, memories, globals and tables. This example illustrates how to call exported functions.

↪️ Calling guest (exported) functions

In this example we'll see how to use exported functions.

Exported function are the entities you will probably use the most: they will be your entrypoint to calling Wasm module logic.

Exported function come in two flavors:

• Dynamic functions;
• Typed functions.

We'll cover both flavors 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:

{% tabs %} {% tab title="Rust" %} {% hint style="info" %} The final Rust code for this example can be found on Github: exports_function.rs.

Please take a look at the setup steps for Rust. {% endhint %}

cargo new exports-function
cd exports-function

We have to modify Cargo.toml to add the Wasmer dependencies as shown below:

[dependencies]
# The Wasmer API
wasmer = "3.0"

{% endtab %}

{% tab title="Go" %} {% hint style="info" %} The final Go code for this example can be found on Github: exports_function.go.

Please take a look at the setup steps for Go. {% endhint %}

mkdir wasmer-example-exports-function
cd wasmer-example-exports-function
go mod init github.com/$USER/wasmer-example-exports-function

{% endtab %}

{% tab title="Python" %} {% hint style="info" %} The final Python code for this example can be found on Github: exports_function.py.

Please take a look at the setup steps for Python. {% endhint %}

mkdir wasmer-example-exports-function
cd wasmer-example-exports-function
pip install wasmer wasmer_compiler_cranelift

{% endtab %}

{% tab title="PHP" %} {% hint style="info" %} The final PHP code for this example can be found on Github: exports-function.php.

Please take a look at the setup steps for PHP. {% endhint %}

mkdir wasmer-example-exports-function
cd wasmer-example-exports-function
composer init --name=wasmer-example-exports-function
composer require wasm/wasm

{% endtab %}

{% tab title="C/C++" %} {% hint style="info" %} The final C code for this example can be found on Github: instance.c.

Please take a look at the setup steps for C/C++. {% endhint %}

mkdir wasmer-example-exports-function
cd wasmer-example-exports-function
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-function exports-function.o
exports-function: exports-function.o

.PHONY: clean
.SILENT: clean
clean:
    rm -f exports-function.o exports-function

{% endtab %}

{% tab title="Ruby" %} {% hint style="info" %} The final Ruby code for this example can be found on Github: exports_function.rb.

Please take a look at the setup steps for Ruby. {% endhint %}

gem install wasmer

{% endtab %} {% endtabs %}

Now that we have everything set up, let's go ahead and try it out!

Using the dynamic flavor

We'll start by fetching the guest function and see how to call it using the dynamic flavor. Our Wasm module exports a sum function, let's get and call it:

{% tabs %} {% tab title="Rust" %}

let sum = instance.exports.get_function("sum")?;
let args = [Value::I32(1), Value::I32(2)];
let result: Box<[Val]> = sum.call(&mut store, &args)?;

{% endtab %}

{% tab title="Go" %}

sum, err := instance.Exports.GetRawFunction("sum")

if err != nil {
  panic(fmt.Sprintf("Failed to get the `%s` function: %s\n", name, err))
}

result, err := sum.Call(1, 2)

{% endtab %}

{% tab title="Python" %} (not possible) {% endtab %}

{% tab title="PHP" %}

$firstArg = Wasm\Module\Val::newI32(1);
$secondArg = Wasm\Module\Val::newI32(2);
$args = new Wasm\Vec\Val([$firstArg->inner(), $secondArg->inner()]);

$result = $sum($args);

{% endtab %}

{% tab title="C/C++" %}

wasm_val_t args_val[2] = { WASM_I32_VAL(3), WASM_I32_VAL(4) };
wasm_val_t results_val[1] = { WASM_INIT_VAL };
wasm_val_vec_t args = WASM_ARRAY_VEC(args_val);
wasm_val_vec_t results = WASM_ARRAY_VEC(results_val);

if (wasm_func_call(sum_func, &args, &results)) {
    printf("> Error calling the `sum` function!\n");

    return 1;
}

{% endtab %}

{% tab title="Ruby" %}

sum = instance.exports.sum
results = sum.call(1, 2)

{% endtab %} {% endtabs %}

Easy right?

Both example look nice but it does not seem like we are using standard functions. In fact, we are calling an external entity. With the native flavor we can get something that feels more like we are using functions as if they were provided by the host directly.

Let's have a look at this.

Using the native flavor

Let's continue with our previous sum function and see how we can make interacting with it better. To do so, we'll be using the native flavor. With this flavor, passing arguments and getting result will feel more natural.

To use this flavor, we have the choice of fetching the function again or transforming the one we already have into a native function:

{% tabs %} {% tab title="Rust" %}

let sum: TypedFunction<(i32, i32), i32> = sum.typed(&store)?;
let result: i32 = sum.call(3, 4)?;

{% hint style="info" %} Here we reused the previously fetched function and turned it into a typed one. We could have directly fetched it as a typed function:

let sum: TypedFunction<(i32, i32), i32> = instance
    .exports
    .get_typed_function::<(i32, i32), i32>("sum")?;

{% endhint %} {% endtab %}

{% tab title="Go" %}

sumNative := sum.Native()
result, err = sumNative(3, 4)

{% hint style="info" %} Here we reused the previously fetched function and turned it into a native one. We could have directly fetched it as a native function:

sum, err := instance.Exports.GetFunction("sum")

if err != nil {
  panic(fmt.Sprintf(
    "Failed to get the `%s` function: %s\n", 
    name, 
    err
  ))
}

{% endhint %} {% endtab %}

{% tab title="Python" %}

sum = instance.exports.sum
result = sum(3, 4)

{% endtab %}

{% tab title="PHP" %} (not possible) {% endtab %}

{% tab title="C/C++" %} (not possible) {% endtab %}

{% tab title="Ruby" %} (not possible) {% endtab %} {% endtabs %}

Running

We now have everything we need to run the Wasm module, let's do it!

{% tabs %} {% tab title="Rust" %} You should be able to run it using the cargo run command. The output should look like this:

Compiling module...
Instantiating module...
Calling `sum` function...
Results: [I32(3)]
Calling `sum` function (natively)...
Results: 7

{% hint style="info" %} If you want to run the examples from the Wasmer repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer.git
cd wasmer
cargo run --example exported-function --release --features "cranelift"

{% endhint %} {% endtab %}

{% tab title="Go" %} You should be able to run it using the go run main.go command. The output should look like this:

Compiling module...
Instantiating module...
Calling `sum` function...
Result of the `sum` function: 3
Calling `sum` function (natively)...
Result of the `sum` function: 7

{% hint style="info" %} 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.git
cd wasmer-go
go test examples/example_exports_function_test.go

{% endhint %} {% endtab %}

{% tab title="Python" %} You should be able to run it using the python exports_function.py command.

{% hint style="info" %} 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.git
cd wasmer-python
python examples/exports_function.py

{% endhint %} {% endtab %}

{% tab title="PHP" %} You should be able to run it using the php exports-function.php command.

{% hint style="info" %} 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.git
cd wasmer-php
make EXAMPLE=exports-function test-doc-examples

{% endhint %} {% endtab %}

{% tab title="C/C++" %} You should be able to run it using the make clean exports-function && ./exports-function command. The output should look like this:

Creating the store...
Compiling module...
Creating imports...
Instantiating module...
Retrieving exports...
Retrieving the `sum` function...
Calling `sum` function...
Results of `sum`: 7

{% hint style="info" %} If you want to run the examples from the Wasmer repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer.git
cd wasmer/lib/c-api/examples/exports-function.c
make clean exports-function
./exports-function

{% endhint %} {% endtab %}

{% tab title="Ruby" %} You should be able to run it using the ruby exports_function.rb command.

{% hint style="info" %} If you want to run the examples from the Wasmer Ruby repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer-ruby.git
cd wasmer-ruby
ruby examples/exports_function.rb

{% endhint %} {% endtab %} {% endtabs %}