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WebAssembly for Proxies (C++ SDK)

Build Status Apache 2.0 License

The SDK has dependencies on specific versions of the C++ WebAssembly toolchain Emscripten ( and the protobuf library, therefor use of a Docker image is recommended.


A Dockerfile for the C++ SDK is provided in Dockerfile-sdk.

It can built in this directory by:

docker build -t wasmsdk:v2 -f Dockerfile-sdk .

The docker image can be used for compiling wasm files.

Creating a project for use with the Docker build image

Create a directory with your source files and a Makefile:


all: myproject.wasm

include ${PROXY_WASM_CPP_SDK}/Makefile.base_lite

Source file (

#include <string>
#include <unordered_map>

#include "proxy_wasm_intrinsics.h"

class ExampleContext : public Context {
  explicit ExampleContext(uint32_t id, RootContext* root) : Context(id, root) {}

  FilterHeadersStatus onRequestHeaders(uint32_t headers, bool end_of_stream) override;
  void onDone() override;
static RegisterContextFactory register_ExampleContext(CONTEXT_FACTORY(ExampleContext));

FilterHeadersStatus ExampleContext::onRequestHeaders(uint32_t headers, bool end_of_stream) {
  logInfo(std::string("onRequestHeaders ") + std::to_string(id()));
  auto path = getRequestHeader(":path");
  logInfo(std::string("header path ") + std::string(path->view()));
  return FilterHeadersStatus::Continue;

void ExampleContext::onDone() { logInfo("onDone " + std::to_string(id())); }

Compiling with the Docker build image

Run docker:

docker run -v $PWD:/work -w /work  wasmsdk:v2 /

Caching the standard libraries

The first time that emscripten runs it will generate the standard libraries. To cache these in the docker image, after the first successful compilation (e.g above), commit the image with the standard libraries:

docker commit `docker ps -l | grep wasmsdk:v2 | awk '{print $1}'` wasmsdk:v2

This will save time on subsequent compiles.

Using the SDK from a newer/specific version of Envoy

To use a newer/specific version of the SDK (e.g. from the version of Enovy you are going to deploy the WebAssembly module to) bind that volume and use it in the Makefile.

Here is an example Makefile referencing the SDK at ../envoy/api/wasm/cpp and mounted as 'sdk' in the /work directory:


all: myproject.wasm

include ${PROXY_WASM_CPP_SDK}/Makefile.base_lite

Run docker pointing to Envoy sources in a directory parallel (at the same level) as your project directory:

docker run -v $PWD:/work -v $PWD/../envoy/api/wasm/cpp:/work/sdk -w /work  wasmsdk:v2 bash /

Using abseil form the image

Abseil (optionally) is built in /root/abseil and can be used. Note that the abseil containers (e.g. absl::flat_hash_set) exercise many syscalls which are not supported. Consequantally individual files should be pulled in which are relatively self contained (e.g. strings). Example customized Makefile:

ABSL = /root/abseil-cpp
ABSL_CPP = ${ABSL}/absl/strings/ ${ABSL}/absl/strings/ ${ABSL}/absl/strings/ ${ABSL}/absl/strings/

all: plugin.wasm

%.wasm %.wat: ${CPP_API}/proxy_wasm_intrinsics.h ${CPP_API}/proxy_wasm_enums.h ${CPP_API}/proxy_wasm_externs.h ${CPP_API}/proxy_wasm_api.h ${CPP_API}/proxy_wasm_intrinsics.js ${CPP_CONTEXT_LIB}
        ls /root
                em++ --no-entry -s EXPORTED_FUNCTIONS=['_malloc'] --std=c++17 -O3 -flto -I${CPP_API} -I${CPP_API}/google/protobuf -I/usr/local/include -I${ABSL} --js-library ${CPP_API}/proxy_wasm_intrinsics.js ${ABSL_CPP} $*.cc ${CPP_API}/ ${CPP_CONTEXT_LIB} ${CPP_API}/libprotobuf.a -o $*.wasm

Precompiled abseil libraries are also available, so the above can also be done as:

ABSL = /root/abseil-cpp
ABSL_LIBS = ${ABSL}/absl/strings/libabsl_strings.a ${ABSL}/absl/strings/libabsl_strings_internal.a  ${ABSL}/absl/strings/libabsl_str_format_internal.a

all: plugin.wasm

%.wasm %.wat: ${CPP_API}/proxy_wasm_intrinsics.h ${CPP_API}/proxy_wasm_enums.h ${CPP_API}/proxy_wasm_externs.h ${CPP_API}/proxy_wasm_api.h ${CPP_API}/proxy_wasm_intrinsics.js ${CPP_CONTEXT_LIB}
        ls /root
                em++ --no-entry -s EXPORTED_FUNCTIONS=['_malloc'] --std=c++17 -O3 -flto -I${CPP_API} -I${CPP_API}/google/protobuf -I/usr/local/include -I${ABSL} --js-library ${CPP_API}/proxy_wasm_intrinsics.js  $*.cc ${CPP_API}/ ${CPP_CONTEXT_LIB} ${CPP_API}/libprotobuf.a ${ABSL_LIBS} -o $*.wasm

Ownership of the resulting .wasm files

The compiled files may be owned by root. To chown them add the follow lines to the Makefile and docker invocation:


all: myproject.wasm
  chown ${uid}.${gid} $^

include ${PROXY_WASM_CPP_SDK}/Makefile.base_lite

Invocation file (e.g.

docker run -e uid="$(id -u)" -e gid="$(id -g)" -v $PWD:/work -w /work wasmsdk:v2 /

Dependencies for building WASM modules:

If you do not wish to use the Docker file, the dependencies can be installed by script (, or by hand.

protobuf v3.9.1

You must install the version of protobuf on your build system that matches the libprotobuf.a files (without any patches) so that the generated code matches the .a library. Currently this is based on tag v3.9.1 of

git clone
cd protobuf
git checkout v3.9.1
git submodule update --init --recursive
make check
sudo make install


git clone
cd emsdk
./emsdk update-tags
./emsdk install 3.1.7
./emsdk activate 3.1.7

source ./emsdk\

It is possible later versions will work, e.g.

./emsdk update-tags
./emsdk install latest
./emsdk activate latest

However 3.1.7 is known to work.

Rebuilding the libprotobuf.a files

If want to rebuild the libprotobuf.a files or use a different version see the instructions at Commit 4bba8b2f38b5004f87489642b6ca4525ae72fe7f works for protobuf v3.9.x.

git clone protobuf-wasm
cd protobuf-wasm
git checkout v3.9.1
git clone wasm-patches
cd wasm-patches && git checkout 4bba8b2f38b5004f87489642b6ca4525ae72fe7f && cd ..
git apply wasm-patches/*.patch
emconfigure ./configure --disable-shared CXXFLAGS="-O3 -flto"
emmake make
cd ..
cp protobuf-wasm/src/.libs/libprotobuf-lite.a ${CPP_API}/libprotobuf-lite.a
cp protobuf-wasm/src/.libs/libprotobuf.a ${CPP_API}/libprotobuf.a

WAVM binaries

git clone
cmake "."
sudo make install

Note: ensure /usr/local/bin is in your path