Extism cpp-sdk

The C++ SDK for integrating with the Extism runtime. Add this library in your host C++ applications to run Extism plugins.

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Building and Installation

Install Dependencies

libextism
cmake and jsoncpp
- Debian: sudo apt install cmake jsoncpp
- macOS: brew install cmake jsoncpp

If you wish to do link jsoncpp statically, or do an in-tree build, See Alternative Dependency Strategies.

Build and Install cpp-sdk

cmake -B build
cmake --build build -j
sudo cmake --install build

Getting Started

To add the cpp-sdk to a CMake project:

find_package(extism-cpp)
target_link_libraries(getting-started extism-cpp)

Loading a Plug-in

The primary concept in Extism is the plug-in. A plug-in is a code module stored in a .wasm file.

Plug-in code can come from a file on disk, object storage or any number of places. Since you may not have one handy, let's load a demo plug-in from the web. Let's start by creating a main function that loads a plug-in:

#include <extism.hpp>

int main(void) {
  const auto manifest =
      extism::Manifest::wasmURL("https://github.com/extism/plugins/releases/"
                                "latest/download/count_vowels.wasm");
  extism::Plugin plugin(manifest, true);
}

Calling A Plug-in's Exports

This plug-in was written in Rust and it does one thing, it counts vowels in a string. It exposes one "export" function: count_vowels. We can call exports using Plugin::call. Let's add code to call count_vowels to our main func:

#include <extism.hpp>
#include <iostream>
#include <string>

int main(void) {
  // ...

  const std::string hello("Hello, World!");
  auto out = plugin.call("count_vowels", hello);
  std::string response(out.string());
  std::cout << response << std::endl;
  // => {"count":3,"total":3,"vowels":"aeiouAEIOU"}
}

Build it:

cmake -B build && cmake --build build

Running this should print out the JSON vowel count report:

./build/getting-started
{"count":3,"total":3,"vowels":"aeiouAEIOU"}

All exports have the same interface, optional bytes in and optional bytes out. This plug-in happens to take a string and return a JSON encoded string with a report of results.

Plug-in State

Plug-ins may be stateful or stateless. Plug-ins can maintain state between calls using variables. Our count vowels plug-in remembers the total number of counted vowels in the "total" key in the result. You can see this by making subsequent calls to the export:

  const std::string hello("Hello, World!");
  auto out = plugin.call("count_vowels", hello);
  std::string response(out.string());
  std::cout << response << std::endl;
  // => {"count":3,"total":3,"vowels":"aeiouAEIOU"}

  out = plugin.call("count_vowels", hello);
  response = out.string();
  std::cout << response << std::endl;
  // => {"count":3,"total":6,"vowels":"aeiouAEIOU"}

The state variables will persist until the plug-in is freed or reinitialized.

Configuration

Plug-ins may optionally take a configuration object. This is a static way to configure the plug-in. Our count-vowels plugin takes an optional configuration to change out which characters are considered vowels. Example:

#include <extism.hpp>

int main(void) {
  auto manifest =
      extism::Manifest::wasmURL("https://github.com/extism/plugins/releases/"
                                "latest/download/count_vowels.wasm");
  manifest.setConfig("vowels", "aeiouyAEIOUY");
  extism::Plugin plugin(manifest, true);

  const std::string hello("Yellow, World!");
  auto out = plugin.call("count_vowels", hello);
  std::string response(out.string());
  std::cout << response << std::endl;
  // => {"count":4,"total":4,"vowels":"aeiouyAEIOUY"}
}

Host Functions

Let's extend our count-vowels example a little bit: Instead of storing the total in an ephemeral plug-in var, let's store it in a persistent key-value store!

Wasm can't use our KV store on it's own. This is where Host Functions come in.

Host functions allow us to grant new capabilities to our plug-ins from our application. In this case, they are native C++ functions that are passed to and can can be invoked by the plug-in.

Let's load the manifest like usual but load up the count_vowels_kvstore plug-in:

  const auto manifest =
      extism::Manifest::wasmURL("https://github.com/extism/plugins/releases/"
                                "latest/download/count_vowels_kvstore.wasm");

Note: The source code for this is here and is written in rust, but it could be written in any of our PDK languages.

Unlike our previous plug-in, this plug-in expects you to provide host functions that satisfy its import interface for a KV store.

We want to expose two functions to our plugin, kv_write which writes a bytes value to a key and kv_read which reads the bytes at the given key:

  // pretend this is Redis or something :)
  std::map<std::string, std::vector<uint8_t>> kvStore;

  auto t = std::vector<extism::ValType>{extism::ValType::I64};
  auto kvRead = extism::Function(
      "kv_read", t, t,
      [&kvStore](extism::CurrentPlugin plugin, void *user_data) {
        const auto it = kvStore.find(plugin.inputString());
        if (it == kvStore.end()) {
          const std::vector<uint8_t> zeros{0, 0, 0, 0};
          plugin.output(zeros.data(), zeros.size());
          return;
        }
        plugin.output(it->second.data(), it->second.size());
      });
  auto kvWrite = extism::Function(
      "kv_write", {extism::ValType::I64, extism::ValType::I64}, {},
      [&kvStore](extism::CurrentPlugin plugin, void *user_data) {
        const auto key = plugin.inputString(0);
        auto value = plugin.inputBuffer(1);
        kvStore[key] = value.vector();
      });

We need to pass these imports to the plug-in to create them. All imports of a plug-in must be satisfied for it to be initialized:

  extism::Plugin plugin(manifest, true, {kvRead, kvWrite});

Now, we can demo it:

  const std::string hello("Hello, World!");
  auto out = plugin.call("count_vowels", hello);
  std::string response(out.string());
  std::cout << response << std::endl;
  // => {"count":3,"total":3,"vowels":"aeiouAEIOU"}

  out = plugin.call("count_vowels", hello);
  response = out.string();
  std::cout << response << std::endl;
  // => {"count":3,"total":6,"vowels":"aeiouAEIOU"}

Linking

CMake

find_package(extism-cpp)
target_link_libraries(target_name extism-cpp)

or statically:

find_package(extism-cpp)
target_link_libraries(target_name extism-cpp-static)

`pkg-config`

pkg-config --libs extism-cpp

or statically:

pkg-config --static --libs extism-cpp-static

Alternative Dependency Strategies

link jsoncpp statically

For builds using extism-cpp-static to be static other than glibc, etc. jsoncpp must be linked statically. jsoncpp provided by package managers often isn't provided as a static library.

Building and installing jsoncpp from source (including jsoncpp_static):

git clone https://github.com/open-source-parsers/jsoncpp
cd jsoncpp
cmake -B build && cmake --build build -j
make -C build install

In-Tree builds

If you wish, instead of using installed deps, you can do an in-tree build!

For example:

mkdir extism-cpp-project
cd extism-cpp-project
git init
git submodule add https://github.com/extism/extism.git
git submodule add https://github.com/open-source-parsers/jsoncpp.git
git submodule add https://github.com/extism/cpp-sdk.git
cd cpp-sdk
cmake -DEXTISM_CPP_BUILD_IN_TREE=1 -B build && cmake --build build

Fallback Dependencies

If EXTISM_CPP_BUILD_IN_TREE is not set and find_package fails, the dependency is fetched from the internet using FetchContent and built from source.

The author believes this is the worst way to deal with the deps as it requires building them from source and doesn't use a centrally managed, flat tree of dependencies. It's provided just as a cross-platform convenience.

Testing

cmake --build build --target test