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  1. .gitignore
  3. Cargo.toml
  7. wasmer.hh

Wasmer Runtime C API

Wasmer is a standalone JIT WebAssembly runtime, aiming to be fully compatible with Emscripten, Rust and Go. Learn more.

This crate exposes a C and a C++ API for the Wasmer runtime.


The C and C++ header files can be found in the source tree of this crate, respectively wasmer.h and wasmer.hh. They are automatically generated, and always up-to-date in this repository.

Here is a simple example to use the C API:

#include <stdio.h>
#include "../wasmer.h"
#include <assert.h>
#include <stdint.h>

int main()
    // Read the Wasm file bytes.
    FILE *file = fopen("sum.wasm", "r");
    fseek(file, 0, SEEK_END);
    long len = ftell(file);
    uint8_t *bytes = malloc(len);
    fseek(file, 0, SEEK_SET);
    fread(bytes, 1, len, file);

    // Prepare the imports.
    wasmer_import_t imports[] = {};

    // Instantiate!
    wasmer_instance_t *instance = NULL;
    wasmer_result_t instantiation_result = wasmer_instantiate(&instance, bytes, len, imports, 0);

    assert(instantiation_result == WASMER_OK);

    // Let's call a function.
    // Start by preparing the arguments.

    // Value of argument #1 is `7i32`.
    wasmer_value_t argument_one;
    argument_one.tag = WASM_I32;
    argument_one.value.I32 = 7;

    // Value of argument #2 is `8i32`.
    wasmer_value_t argument_two;
    argument_two.tag = WASM_I32;
    argument_two.value.I32 = 8;

    // Prepare the arguments.
    wasmer_value_t arguments[] = {argument_one, argument_two};

    // Prepare the return value.
    wasmer_value_t result_one;
    wasmer_value_t results[] = {result_one};

    // Call the `sum` function with the prepared arguments and the return value.
    wasmer_result_t call_result = wasmer_instance_call(instance, "sum", arguments, 2, results, 1);

    // Let's display the result.
    printf("Call result:  %d\n", call_result);
    printf("Result: %d\n", results[0].value.I32);

    // `sum(7, 8) == 15`.
    assert(results[0].value.I32 == 15);
    assert(call_result == WASMER_OK);


    return 0;


The tests can be run via cargo test, such as:

$ cargo test -- --nocapture

To run tests manually, enter the lib/runtime-c-api/tests directory and run the following commands:

$ cmake .
$ make
$ make test


Wasmer is primarily distributed under the terms of the MIT license (LICENSE).