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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 entity, 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
Go
Python
C/C++
cargo new imports-exports
cd imports-exports
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/$USER/wasmer-example-imports-exports
mkdir wasmer-example-imports-exports
cd wasmer-example-imports-exports
pip install wasmer wasmer_compiler_cranelift
mkdir wasmer-example-imports-exports
cd wasmer-example-imports-exports
vim Makefile
Let's create a simple
Makefile
:CFLAGS = -g -I$(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!
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 theenv
namespace.
Let's create the import object:
Rust
Go
Python
C/C++
let imports = imports! {
"" => {
"host_function" => host_function,
},
"env" => {
"host_global" => host_global,
},
};
importObject := wasmer.NewImportObject()
importObject.Register(
"",
map[string]wasmer.IntoExtern{
"host_function": hostFunction,
},
)
importObject.Register(
"env",
map[string]wasmer.IntoExtern{
"host_global": hostGlobal,
},
)
import_object = ImportObject()
import_object.register(
"",
{
"host_function": host_function,
}
)
import_object.register(
"env",
{
"host_global": host_global,
}
)
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
Go
Python
C/C++
let instance = Instance::new(&mut store, &module, &imports)?;
instance, err := wasmer.NewInstance(module, importObject)
instance = Instance(module, import_object)
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:
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
Go
Python
C/C++
let function = instance.exports.get::<Function>("guest_function")?;
let global = instance.exports.get::<Global>("guest_global")?;
let memory = instance.exports.get::<Memory>("guest_memory")?;
let table = instance.exports.get::<Table>("guest_table")?;
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))
}
function = instance.exports.guest_function
global = instance.exports.guest_global
memory = instance.exports.guest_memory
table = instance.exports.guest_table
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:
We now have everything we need to run the Wasm module, let's do it!
Rust
Go
Python
C/C++
You should be able to run it using the
cargo run
command. The output should look like this:Compiling module...
Creating the imported function...
Creating the imported global...
Instantiating module...
Getting the exported function...
Got exported function of type: FunctionType { params: [], results: [I32] }
Getting the exported global...
Got exported global of type: GlobalType { ty: I32, mutability: Const }
Getting the exported memory...
Got exported memory of type: MemoryType { minimum: 1 pages, maximum: None, shared: false }
Getting the exported table...
Got exported table of type: TableType { ty: FuncRef, minimum: 1, maximum: Some(1) }
git clone https://github.com/wasmerio/wasmer.git
cd wasmer
cargo run --example imports-exports --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...
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
git clone https://github.com/wasmerio/wasmer-go.git
cd wasmer-go
go test examples/example_imports_exports_test.go
You should be able to run it using the
python imports_exports.py
command.git clone https://github.com/wasmerio/wasmer-python.git
cd wasmer-python
python examples/imports_exports.py
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
git clone https://github.com/wasmerio/wasmer.git
cd wasmer/lib/c-api/examples/imports-exports.c
make clean imports-exports
./imports-exports
Last modified 7mo ago