Wasmer runtime emscripten implementation library

8 unstable releases (3 breaking)

✓ Uses Rust 2018 edition

new 0.5.6 Jul 16, 2019
0.5.5 Jul 10, 2019
0.4.2 May 16, 2019
0.4.0 Apr 29, 2019
0.2.1 Feb 28, 2019

#40 in WebAssembly

Download history 2/week @ 2019-03-30 7/week @ 2019-04-06 13/week @ 2019-04-13 6/week @ 2019-04-20 14/week @ 2019-04-27 19/week @ 2019-05-04 26/week @ 2019-05-11 26/week @ 2019-05-18 7/week @ 2019-05-25 8/week @ 2019-06-01 19/week @ 2019-06-08 16/week @ 2019-06-15 17/week @ 2019-06-22 12/week @ 2019-06-29 45/week @ 2019-07-06

73 downloads per month

MIT license

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Wasmer is a standalone JIT WebAssembly runtime, aiming to be fully compatible with WASI and Emscripten.

Install Wasmer with:

curl https://get.wasmer.io -sSfL | sh

Wasmer runtime can also be embedded in different languages, so you can use WebAssembly anywhere ✨:


Wasmer can execute both the standard binary format (.wasm) and the text format defined by the WebAssembly reference interpreter (.wat).

Once installed, you will be able to run any WebAssembly files (including Lua, PHP, SQLite and nginx!):

# Run Lua
wasmer run examples/lua.wasm

# Run PHP
wasmer run examples/php.wasm

# Run SQLite
wasmer run examples/sqlite.wasm

# Run nginx
wasmer run examples/nginx/nginx.wasm -- -p examples/nginx -c nginx.conf


Installing Wasmer through wasmer.io includes wapm, the WebAssembly package manager.

Wapm allows you to easily download, run, and distribute WebAssembly binaries.

# Install cowsay globally
wapm install -g cowsay

# Run cowsay
wapm run cowsay "Hello, world!"

For more information about wapm, check out the website and this example program.

Code Structure

Wasmer is structured into different directories:

  • src: code related to the Wasmer executable itself
  • lib: modularized libraries that Wasmer uses under the hood
  • examples: some useful examples to getting started with Wasmer


Building Wasmer requires rustup.

To build on Windows, download and run rustup-init.exe then follow the onscreen instructions.

To build on other systems, run:

curl https://sh.rustup.rs -sSf | sh

Other dependencies

Please select your operating system:


If you have Homebrew installed:

brew install cmake

Or, in case you have MacPorts:

sudo port install cmake

Debian-based Linuxes

sudo apt install cmake pkg-config libssl-dev


pkg install cmake

Windows (MSVC)

Windows support is highly experimental. Only simple Wasm programs may be run, and no syscalls are allowed. This means nginx and Lua do not work on Windows. See this issue regarding Emscripten syscall polyfills for Windows.

  1. Install Visual Studio

  2. Install Rust for Windows

  3. Install Python for Windows. The Windows x86-64 MSI installer is fine. Make sure to enable "Add python.exe to Path" during installation.

  4. Install Git for Windows. Allow it to add git.exe to your PATH (default settings for the installer are fine).

  5. Install CMake. Ensure CMake is in your PATH.

  6. Install LLVM 7.0


Wasmer is built with Cargo, the Rust package manager.

Set Rust Nightly:

rustup default nightly

And install Wasmer

# checkout code
git clone https://github.com/wasmerio/wasmer.git
cd wasmer

# install tools
# make sure that `python` is accessible.
make install


Thanks to spec tests we can ensure 100% compatibility with the WebAssembly spec test suite.

You can run all the tests with:

rustup default nightly
make test

Testing backends

Each backend can be tested separately:

  • Singlepass: make singlepass
  • Cranelift: make cranelift
  • LLVM: make llvm

Testing integrations

Each integration can be tested separately:

  • Spec tests: make spectests
  • Emscripten: make emtests
  • WASI: make wasi
  • Middleware: make middleware
  • C API: make capi


Benchmarks can be run with:

make bench


Wasmer is an open project guided by strong principles, aiming to be modular, flexible and fast. It is open to the community to help set its direction.

Below are some of the goals of this project (in order of priority):

  • It should be 100% compatible with the WebAssembly spec tests
  • It should be fast (partially achieved)
  • Support WASI - released in 0.3.0
  • Support Emscripten calls (in the works)
  • Support Rust ABI calls
  • Support Go ABI calls


If you would like to know how Wasmer works under the hood, please see ARCHITECTURE.md.


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



~113K SLoC