Felspar IO

C++ coroutine based asynchronous IO

felspar-io is a C++20 library to help you use coroutines to perform input and output operations (IOPs). The library should not yet be considered stable. It allows for the use of either io_uring or poll to be used as the event loop.

Using the library

The library is designed to be easy to use from CMake projects. To use it add it in your source tree somewhere and then use add_subdirectory to include it in your project. The library also requires the use of some other libraries which can get by using an include. So if you have the library in a folder called felspar-io, you can add the following to your CMakeLists.txt:


To make use of the library add felspar-io in your target_link_libraries directive.

target_link_libraries(your-app otherlib1 otherlib2 felspar-io)

I/O functionality

To echo data received on a socket the following code works:

#include <felspar/io.hpp>

felspar::io::warden::task<void> echo_connection(
        felspar::io::warden &ward, felspar::posix::fd sock) {
    std::array<std::byte, 256> buffer;
    while (auto bytes = co_await ward.read_some(sock, buffer, 20s)) {
        std::span writing{buffer};
        auto written = co_await felspar::io::write_all(
                ward, sock, writing.first(bytes), 20s);

A coroutine for running an accept loop for an echo server would look like this:

        echo_server(felspar::io::warden &ward, std::uint16_t port) {
    auto fd = ward.create_socket(AF_INET, SOCK_STREAM, 0);
    felspar::posix::bind_to_any_address(fd, port);
    int constexpr backlog = 64;

    felspar::io::warden::starter<void> co;
    for (auto acceptor = felspar::io::accept(ward, fd);
            auto cnx = co_await acceptor.next();) {
        co.post(echo_connection, ward, felspar::posix::fd{*cnx});
        co.garbage_collect_completed(); // ignores errors

The default behaviour for all errors is to throw an exception, but this can be altered by wrapping the IOP in an felspar::io::ec call:

if (auto result = co_await ward.read_some(fd, buffer, 200ms); result) {
} else {
    log_error("read error", result.error);

This only works for IOPs (direct APIs on the warden). Compound convenience APIs will always throw exceptions. felspar-io uses felspar-exception in order to track source code locations for errors thrown -- this means the call site of the IO API will be in the exception what() string.


The library is built around the notion of "wardens". There is an abstract felspar::io::warden type that provides an API for various IOPs (and in the future) polymorphic allocation for memory required to execute the IOPs and coroutines that make use of them.

Concrete warden implementation make use of a particular API family to implement the required asynchronous IO and timing APIs. At the moment there are the felspar::io::poll_warden and the felspar::io::uring_warden. The first makes use of the poll() system call and the latter the io_uring facilities. The library design is centred around the capabilities of io_uring rather than poll, with poll being treated as a compatibility fallback for use on platforms where io_uring is not available. The intention is not to expose the entirety of the io_uring API space, just those IOPs that are most useful to a wide range of applications (e.g. a focus on network and file IO).

Different wardens will have slight differences in observable behaviour as a consequence of their differing APIs. For example, sleeps using poll will have best case jitter of just over 1 millisecond, wheres on io_uring this figure should be at least one order of magnitude lower.

Time outs

All of the operations support time outs which can be passed as an extra final parameter:

co_await felspar::io::read_exactly(ward, fd, buffer, 100ms);

The timeouts operate on a per-IOP basis on the warden in use, so compound APIs like read_exactly (that may issue several IOPs) can take longer to time out. When a time out expires an exception of type felspar::io::timeout (a sub-class of std::system_error) is thrown. The error code in the exception will be equal to felspar::io::timeout::error.