Imports & exports

A Wasm module can import and export entities, like functions, memories, globals and tables. This example illustrates the basics of using these entities.

In this example we'll be using a sample Wasm module which exports some entities and requires us to also import some of them.

The goal here is to give you an idea of how to work with imports and exports. We won't go into the details of each entities, they'll be covered in more details in the other examples.

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

The final code for this example can be found on GitHub.

Please take a look at the setup steps for Rust.

cargo new imports-exports
cd imports-exports

This should generate two important files for us, Cargo.toml and src/main.rs. The Cargo.toml is a file that describes your project and its dependencies. The src/main.rs is the entry point for your project, and contains the fn main() { .. } that is run when the project is executed.

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

[package]
name = "imports-exports"
version = "0.1.0"
authors = ["The Wasmer Engineering Team <[e-mail]>"]
edition = "2018"

[dependencies]
# The Wasmer API
wasmer = "1.0.0-alpha5"

Go

The final code for this example can be found on GitHub.

Please take a look at the setup steps for Go.

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

C/C++

The final code for this example can be found on GitHub.

Please take a look at the setup steps for C/C++.

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

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

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

Declaring imports

When a Wasm modules declares imports you will have to make them available before you can instantiate the module. Our first task will be to create the required entities.

The module we are using needs two imports:

• A function named host_function in a namespace with an empty name;

• A global named host_global in the env namespace.

Let's create the import object:

Rust

let import_object = imports! {
    "" => {
        "host_function" => host_function,
    },
    "env" => {
        "host_global" => host_global,
    }
}

Go

importObject := wasmer.NewImportObject()

importObject.Register(
    "",
    map[string]wasmer.IntoExtern{
        "host_function": hostFunction,
    },
)

importObject.Register(
    "env",
    map[string]wasmer.IntoExtern{
        "host_global":   hostGlobal,
    },
)

C/C++

wasm_functype_t* host_func_type = wasm_functype_new_0_1(wasm_valtype_new_i32());
wasm_func_t* host_func = wasm_func_new(store, host_func_type, host_func_callback);
wasm_functype_delete(host_func_type);
      
wasm_globaltype_t* host_global_type = wasm_globaltype_new(wasm_valtype_new(WASM_F32), WASM_CONST);
wasm_val_t host_global_val = WASM_I32_VAL(42);
wasm_global_t* host_global = wasm_global_new(store, host_global_type, &host_global_val);
wasm_globaltype_delete(host_global_type);

wasm_extern_t* externs[] = {
  wasm_func_as_extern(host_func),
  wasm_global_as_extern(host_global)
};

wasm_extern_vec_t import_object = WASM_ARRAY_VEC(externs);

Now that we have our import object ready, we'll need to use it when instantiating the module:

Rust

let instance = Instance::new(&module, &import_object)?;

Go

instance, err := wasmer.NewInstance(module, importObject)

C/C++

wasm_instance_t* instance = wasm_instance_new(store, module, &import_object, NULL);

That's it! Easy right?

We did not go into the details of how to create the imported entities, we encourage you to read other examples to know more about this:

Exposing host functions

Fetching exports

Let's have a look at our module again: it exports some entities for us in our host program:

• A function named guest_function;

• A global name guest_global;

• A table name guest_table;

• A memory named guest_memory.

To get these entities we'll use the exports API:

Rust

TODO: Write this section

Go

function, err := instance.Exports.GetFunction("guest_function")
if err != nil {
  panic(fmt.Sprintln("Failed to get the exported function:", err))
}

global, err := instance.Exports.GetGlobal("guest_global")
if err != nil {
    panic(fmt.Sprintln("Failed to get the exported global:", err))
}

table, err := instance.Exports.GetTable("guest_table")
if err != nil {
    panic(fmt.Sprintln("Failed to get the exported table:", err))
}

memory, err := instance.Exports.GetMemory("guest_memory")
if err != nil {
    panic(fmt.Sprintln("Failed to get the exported memory:", err))
}

C/C++

wasm_func_t* func = wasm_extern_as_func(exports.data[0]);
if (func == NULL) {
    printf("> Failed to get the exported function!\n");

    return 1;
}

wasm_global_t* global = wasm_extern_as_global(exports.data[1]);
if (global == NULL) {
    printf("> Failed to get the exported global!\n");

    return 1;
}

wasm_table_t* table = wasm_extern_as_table(exports.data[2]);
if (table == NULL) {
    printf("> Failed to get the exported table!\n");

    return 1;
}

wasm_memory_t* memory = wasm_extern_as_memory(exports.data[3]);
if (memory == NULL) {
    printf("> Failed to get the exported memory!\n");

    return 1;
}

Again, we'll not cover how to use these entities here as this is the topic of other, more detailed, examples:

Calling guest functionsExposing host functionsUsing exported globalsInteracting with memory

Running

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

Rust

You should be able to run it using the cargo run command. The output should look like this:

TODO: Write this section

Go

You should be able to run it using the go run main.go command. The output should look like this:

Compiling module...
Creating the imported function...
Creating the imported global...
Instantiating module...
Getting the exported function...
Got the exported function: func(...interface {}) (interface {}, error)
Getting the exported global...
Got the exported global: *wasmer.Global
Getting the exported memory...
Got the exported memory: *wasmer.Memory
Getting the exported table...
Got the exported table: *wasmer.Table

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_imports_exports_test.go

C/C++

You should be able to run it using the make clean imports-exports && ./imports-exports command. The output should look like this:

Creating the store...
Compiling module...
Creating the imported function...
Creating the imported global...
Instantiating module...
Retrieving exports...
Retrieving the exported function...
Got the exported function: 0x7f9317e05e00
Retrieving the exported global...
Got the exported global: 0x7f9317e05e90
Retrieving the exported table...
Got the exported table: 0x7f9317e05ec0
Retrieving the exported memory...
Got the exported memory: 0x7f9317e05ef0

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/imports-exports.c
make clean imports-exports
./imports-exports