[need help] boost asio example code echo server failed to work if we enable io_uring

[gaowayne]

hello expert

I run boost asio echo example with io_uring. but I saw below error code, same code works fine with epoll mode.

Exception: io_uring_get_sqe: No buffer space available [system:105]

[gaowayne]

boost asio expert,
here is my example echo server from asio example

//
// echo_server.cpp - Based on Boost 1.90 example
// echo_server_with_as_single_default.cpp
//

#include <boost/asio/experimental/as_single.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/signal_set.hpp>
#include <boost/asio/write.hpp>
#include <boost/asio/executor_work_guard.hpp>
#include <cstdio>
#include <iostream>
#include <unistd.h>  // for getpid()

using boost::asio::experimental::as_single_t;
using boost::asio::ip::tcp;
using boost::asio::awaitable;
using boost::asio::co_spawn;
using boost::asio::detached;
using boost::asio::use_awaitable_t;
using default_token = as_single_t<use_awaitable_t<>>;
using tcp_acceptor = default_token::as_default_on_t<tcp::acceptor>;
using tcp_socket = default_token::as_default_on_t<tcp::socket>;
namespace this_coro = boost::asio::this_coro;

awaitable<void> echo(tcp_socket socket)
{
  char data[1024];
  for (;;)
  {
    auto [e1, nread] = co_await socket.async_read_some(boost::asio::buffer(data));
    if (nread == 0) break;
    auto [e2, nwritten] = co_await async_write(socket, boost::asio::buffer(data, nread));
    if (nwritten != nread) break;
  }
}

awaitable<void> listener()
{
  auto executor = co_await this_coro::executor;
  tcp_acceptor acceptor(executor, {tcp::v4(), 55555});
  std::cout << "Echo server listening on port 55555\n";
  for (;;)
  {
    if (auto [e, socket] = co_await acceptor.async_accept(); socket.is_open())
    {
      std::cout << "New connection accepted\n";
      co_spawn(executor, echo(std::move(socket)), detached);
    }
  }
}

int main()
{
  try
  {
    std::cout << "Starting echo server...\n";
    std::cout << "Press Ctrl+C to stop\n";

    boost::asio::io_context io_context(1);

    // TEMPORARY WORKAROUND: Disable signal_set to test if io_uring works without it
    // The issue is that signal_set creation triggers register_internal_io_object
    // which calls get_sqe() and fails with ENOBUFS
    // Use 'kill' command to stop the server: kill <pid>
    std::cout << "WARNING: Signal handling disabled for io_uring testing\n";
    std::cout << "Use 'kill <pid>' to stop the server\n";
    std::cout << "Server PID: " << getpid() << "\n";

    // Create a work guard to keep io_context alive
    auto work_guard = boost::asio::make_work_guard(io_context);

    // Start listener
    co_spawn(io_context, listener(), detached);

    // NOTE: signal_set creation is disabled - this is a known issue with io_uring
    // boost::asio::signal_set signals(io_context, SIGINT, SIGTERM);
    // signals.async_wait([&](auto, auto){ io_context.stop(); });

    std::cout << "Starting event loop...\n";
    io_context.run();

    std::cout << "Server stopped\n";
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
    return 1;
  }

  return 0;
}

here is my test client code

#!/bin/bash
# Simple test client for echo server

HOST=${1:-localhost}
PORT=${2:-55555}

echo "Connecting to $HOST:$PORT..."
echo "Type messages and press Enter. Type 'quit' to exit."
echo ""

while true; do
    read -p "> " message
    if [ "$message" = "quit" ]; then
        break
    fi
    echo "$message" | nc $HOST $PORT
done

echo "Disconnected"

after I run test client, it is just hanging never hello never get response back.

==client side==
root@salab-smcamd02:/mnt/nvme4n1/wayne/s3proxy/S3LightBoost/rpcs3_io_uring/test_boost_example# sh test_client.sh
Connecting to localhost:55555...
Type messages and press Enter. Type 'quit' to exit.

hello

===server side===
(s3proxy) root@salab-smcamd02:/mnt/nvme4n1/wayne/s3proxy/S3LightBoost/rpcs3_io_uring/test_boost_example/build# ./echo_server_io_uring
Starting echo server...
Press Ctrl+C to stop
WARNING: Signal handling disabled for io_uring testing
Use 'kill ' to stop the server
Server PID: 1091794
Starting event loop...
Echo server listening on port 55555

===my cmakelist.txt===

cmake_minimum_required(VERSION 3.16)

project(BoostEchoServerTest
  VERSION 1.0.0
  LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

# Find Boost
if(DEFINED ENV{Boost_ROOT})
    set(Boost_ROOT $ENV{Boost_ROOT})
    message(STATUS "Using Boost_ROOT from environment: ${Boost_ROOT}")
elseif(DEFINED ENV{BOOST_ROOT})
    set(Boost_ROOT $ENV{BOOST_ROOT})
    message(STATUS "Using BOOST_ROOT from environment: ${Boost_ROOT}")
endif()

set(Boost_USE_STATIC_LIBS OFF)
set(Boost_USE_DEBUG_RUNTIME OFF)
set(Boost_USE_MULTITHREADED ON)
set(Boost_USE_STATIC_RUNTIME OFF)

find_package(Boost 1.90 QUIET)
if(NOT Boost_FOUND)
    find_package(Boost 1.82 QUIET)
endif()

if(NOT Boost_FOUND)
    message(FATAL_ERROR "Boost 1.82+ not found. Please install Boost or set Boost_ROOT")
endif()

message(STATUS "Boost version: ${Boost_VERSION}")
message(STATUS "Boost include dirs: ${Boost_INCLUDE_DIRS}")

# Version 1: Default (epoll) - no special flags
add_executable(echo_server_epoll echo_server.cpp)
target_include_directories(echo_server_epoll PRIVATE ${Boost_INCLUDE_DIRS})
target_compile_definitions(echo_server_epoll PRIVATE BOOST_ASIO_NO_DEPRECATED)
if(TARGET Boost::system)
    target_link_libraries(echo_server_epoll PRIVATE Boost::system)
endif()

# Version 2: io_uring version
add_executable(echo_server_io_uring echo_server.cpp)
target_include_directories(echo_server_io_uring PRIVATE ${Boost_INCLUDE_DIRS})
target_compile_definitions(echo_server_io_uring PRIVATE 
    BOOST_ASIO_NO_DEPRECATED
    BOOST_ASIO_HAS_IO_URING=1
    BOOST_ASIO_DISABLE_EPOLL=1
    BOOST_ASIO_HAS_IO_URING_AS_DEFAULT=1
)
if(TARGET Boost::system)
    target_link_libraries(echo_server_io_uring PRIVATE Boost::system)
endif()

# Find liburing for io_uring version
find_library(URING_LIBRARY NAMES uring)
if(URING_LIBRARY)
    message(STATUS "Found liburing: ${URING_LIBRARY}")
    target_link_libraries(echo_server_io_uring PRIVATE ${URING_LIBRARY})
else()
    message(WARNING "liburing not found - io_uring version may not work")
endif()