Packio - An asynchronous msgpack-RPC and JSON-RPC library built on top of Boost.Asio.


Header-only | JSON-RPC | msgpack-RPC | asio | coroutines

This library requires C++17 and is designed as an extension to boost.asio. It will let you build asynchronous servers or client for JSON-RPC or msgpack-RPC.

The project is hosted on GitHub and available on Conan Center. Documentation is available on GitHub Pages.


#include <iostream>

#include <packio/packio.h>

using packio::arg;
using packio::nl_json_rpc::completion_handler;
using packio::nl_json_rpc::make_client;
using packio::nl_json_rpc::make_server;
using packio::nl_json_rpc::rpc;

int main(int, char**)
    using namespace packio::arg_literals;

    // Declare a server and a client, sharing the same io_context
    packio::net::io_context io;
    packio::net::ip::tcp::endpoint bind_ep{
        packio::net::ip::make_address(""), 0};
    auto server = make_server(packio::net::ip::tcp::acceptor{io, bind_ep});
    auto client = make_client(packio::net::ip::tcp::socket{io});

    // Declare a synchronous callback with named arguments
        "add", {"a", "b"}, [](int a, int b) { return a + b; });
    // Declare an asynchronous callback with named arguments
        "multiply", {"a", "b"}, [&io](completion_handler complete, int a, int b) {
            // Call the completion handler later
                io, [a, b, complete = std::move(complete)]() mutable {
                    complete(a * b);
    // Declare a coroutine with unnamed arguments
        "pow", io, [](int a, int b) -> packio::net::awaitable<int> {
            co_return std::pow(a, b);

    // Connect the client
    // Accept connections
    // Run the io_context
    std::thread thread{[&] {; }};

    // Make an asynchronous call with named arguments
    std::promise<int> add1_result, multiply_result;
        std::tuple{arg("a") = 42, arg("b") = 24},
        [&](packio::error_code, const rpc::response_type& r) {
    std::cout << "42 + 24 = " << add1_result.get_future().get() << std::endl;

    // Use packio::net::use_future with named arguments and literals
    auto add_future = client->async_call(
        std::tuple{"a"_arg = 12, "b"_arg = 23},
    std::cout << "12 * 23 = " << add_future.get().result.get<int>() << std::endl;

    // Spawn the coroutine and wait for its completion
    std::promise<int> pow_result;
        [&]() -> packio::net::awaitable<void> {
            // Call using an awaitable and positional arguments
            auto res = co_await client->async_call(
                "pow", std::tuple{2, 8}, packio::net::use_awaitable);
    std::cout << "2 ** 8 = " << pow_result.get_future().get() << std::endl;


    return 0;


  • C++17 or C++20
  • msgpack >= 3.2.1
  • nlohmann_json >= 3.9.1
  • boost.asio >= 1.70.0 or asio >= 1.13.0

Older versions of msgpack and nlohmann_json are probably compatible but they are not tested on the CI.


Standalone or Boost.Asio

By default, packio uses boost::asio. It is also compatible with standalone asio. To use the standalone version, the preprocessor macro PACKIO_STANDALONE_ASIO=1 must be defined.

If you are using the conan package, you can use the option standalone_asio=True.

Depending on your choice, the namespace packio::net will be an alias for either boost::asio or asio.

RPC components

You can define the following preprocessor macros to either 0 or 1 to force-disable or force-enable components of packio:


If you're using the conan package, use the associated options instead, conan will define these macros accordingly.

If you're not using the conan package, packio will try to auto-detect whether these components are available on your system. Define the macros to the appropriate value if you encounter any issue.

Boost before 1.75

If you're using the conan package with a boost version older than 1.75, you need to manually disable Boost.Json with the options boost_json=False.

Tested compilers

  • gcc-7
  • gcc-8
  • gcc-9
  • gcc-10
  • clang-6
  • clang-7
  • clang-8
  • clang-9
  • clang-10
  • clang-11
  • Apple clang-12
  • Visual Studio 2019 Version 16.8

Older compilers may be compatible but are not tested.

Install with conan

conan install packio/x.x.x


packio is compatible with C++20 coroutines:

  • calls can use the packio::asio::use_awaitable completion token
  • coroutines can be registered in the server

Coroutines are tested for the following compilers:

  • gcc-10 (with -fcoroutines)
  • clang-11 (with libc++)
  • Apple clang-12


You will find some samples in test_package/samples/ to help you get a hand on packio.


Let's compute fibonacci's numbers recursively over websockets with coroutines on a single thread ... in 65 lines of code.

#include <iostream>

#include <packio/extra/websocket.h>
#include <packio/packio.h>

using packio::msgpack_rpc::make_client;
using packio::msgpack_rpc::make_server;
using packio::net::ip::make_address;

using awaitable_tcp_stream = decltype(packio::net::use_awaitable_t<>::as_default_on(
using websocket = packio::extra::
    websocket_adapter<boost::beast::websocket::stream<awaitable_tcp_stream>, true>;
using ws_acceptor =
    packio::extra::websocket_acceptor_adapter<packio::net::ip::tcp::acceptor, websocket>;

int main(int argc, char** argv)
    if (argc < 2) {
        std::cerr << "I require one argument" << std::endl;
        return 1;
    const int n = std::atoi(argv[1]);

    packio::net::io_context io;
    packio::net::ip::tcp::endpoint bind_ep{make_address(""), 0};

    auto server = make_server(ws_acceptor{io, bind_ep});
    auto client = make_client(websocket{io});

        "fibonacci", io, [&](int n) -> packio::net::awaitable<int> {
            if (n <= 1) {
                co_return n;

            auto r1 = co_await client->async_call("fibonacci", std::tuple{n - 1});
            auto r2 = co_await client->async_call("fibonacci", std::tuple{n - 2});

            co_return<int>() +<int>();

    int result = 0;
        [&]() -> packio::net::awaitable<void> {
            auto ep = server->acceptor().local_endpoint();
            co_await client->socket().next_layer().async_connect(ep);
            co_await client->socket().async_handshake(
                "" + std::to_string(ep.port()), "/");
            auto ret = co_await client->async_call("fibonacci", std::tuple{n});
            result = ret.result.template as<int>();


    std::cout << "F{" << n << "} = " << result << std::endl;

    return 0;
  • Separated client and server

    Separated client and server

    Based on my previous request Closed Issue #65 i have a further question. I'm using the nl_json_rpc client and I tried to separate the client and server in two different programs (executable). From the client i can connect to the server socket (in the code below i get "The connection has been established!")

    packio::net::io_context io_context;
    packio::net::ip::tcp::socket socket{io_context};
    packio::net::ip::tcp::resolver resolver{io_context};
    auto endpoints = resolver.resolve("localhost", std::to_string(6543));
    packio::error_code error;
    auto client = make_client(std::move(socket));
    client->socket().connect(endpoints->endpoint(), error);
    if(not error)
        std::cout << "The connection has been established!";
        std::cerr << "Something went wrong :(";

    But for some reason the following RPC async_call does not work anymore, e.g. it seems it waits forever for the add_result.get_future().get()

    std::promise<int> add_result, multiply_result;
        std::tuple{arg("a") = 42, arg("b") = 24},
        [&](packio::error_code, const rpc::response_type& r) {
    std::cout << "42 + 24 = " << add_result.get_future().get() << std::endl;

    May i ask you if you know what the problem is or you can add an example with separated client & server?

    opened by tinu73 7
  • RPC client as class member variable

    RPC client as class member variable

    How do i define the rpc client as a class member variable? I cannot use auto. I have to setup a rpc connection to serveral devices and i thought the best to do this is to instantiate a device object from a device class, each holding the rpc client as a member variable.

    opened by tinu73 4
  • Progress support with WebSocket adapter

    Progress support with WebSocket adapter

    I have WebSocket server over JSON-RPC (MR, that can do long (from 1 to 30 minutes) the API call and can generate progress (%) with extra data (remaining time, sec). Can I do progress somehow?

    opened by dmitryuk 2
  • Missing optional header

    Missing optional header

    The optional header isn't included in traits.h. It causes some errors with MSVC:

    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2065: 'optional': undeclared identifier
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(89): note: see reference to class template instantiation 'packio::traits::AsCallHandler<T,Result>' being compiled
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C3544: '_Args': parameter pack expects a type template argument
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2993: 'unknown-type': is not a valid type for non-type template parameter 'condition'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2955: 'packio::traits::Trait': use of class template requires template argument list
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(62): note: see declaration of 'packio::traits::Trait'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2143: syntax error: missing ',' before '>'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(166): error C2061: syntax error: identifier 'optional'
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(483): note: see reference to class template instantiation 'packio::client<Socket,Map>' being compiled
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(182): error C2061: syntax error: identifier 'optional'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(414): error C2061: syntax error: identifier 'optional'
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(472): note: see reference to class template instantiation 'packio::client<Socket,Map>::initiate_async_call<Buffer>' being compiled
    opened by intelligide 2
  • C++20 should use coroutines by default

    C++20 should use coroutines by default

    ATM, C++20 and coroutines usage is differenciated in tests because compilers do not properly support coroutines. In a (close?) future, C++20 and coroutines usage should go hand in hand.

    Compiler status:

    • gcc: need gcc10.3 or gcc11 because of a bug in coroutines which has already been fixed upstream
    • clang: need clang-11 for non experimental support
    • msvc: need version 16.8 (released but not yet available in Github Actions)
    • apple-clang: probably in the next version ?
    opened by qchateau 0
  • 2.2.0(Dec 29, 2021)

    What's Changed

    • Ws by @qchateau in
    • Ssl by @qchateau in
    • Ws by @qchateau in
    • Ci by @qchateau in

    Full Changelog:

    Source code(tar.gz)
    Source code(zip)
  • 2.1.0(Dec 29, 2021)

    What's Changed

    • ci: remove caching by @qchateau in
    • Auto detect asio by @qchateau in
    • packio: add boost json, add perf program by @qchateau in
    • Fix by @qchateau in
    • tests: cross-platform get_endpoint for local sockets by @qchateau in
    • ci: test asio 1.18.1 by @qchateau in
    • ci: use boost.json on macos by @qchateau in
    • ci: faster build on macos by @qchateau in

    Full Changelog:

    Source code(tar.gz)
    Source code(zip)
  • 2.0.1(Dec 29, 2021)

    What's Changed

    • Split tests by @qchateau in
    • Modernize by @qchateau in

    Full Changelog:

    Source code(tar.gz)
    Source code(zip)
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