Example where fiber yield and fiber sleep_for makes a difference.

fiber_asio_starvation.cc

#include <boost/asio.hpp>
#include <boost/fiber/all.hpp>
#include <chrono>
#include <memory>

#include "testing/experimental/round_robin/round_robin.hpp" // For asio round robin scheduler.

// Context:
// https://stackoverflow.com/questions/67276482/difference-between-boostfiberyield-and-boostfibersleep-for-when-using-fib
int main(int argc, char **argv) {
  auto io_context = std::make_shared<boost::asio::io_context>();
  boost::fibers::use_scheduling_algorithm<boost::fibers::asio::round_robin>(
      io_context);

  boost::fibers::fiber fiber_1([&io_context] {
    while (true) {
      boost::asio::high_resolution_timer timer(*io_context,
                                               std::chrono::seconds(1));

      boost::fibers::promise<bool> pr;
      boost::fibers::future<bool> fu = pr.get_future();
      timer.async_wait([&pr](const auto &timer) { pr.set_value(true); });
      fu.get(); // Suspends this fiber.

      std::cout << "We made it!\n";
      boost::this_fiber::yield();
    }
  });
  fiber_1.detach();

  boost::fibers::fiber fiber_2([] {
    while (true) {
      boost::this_fiber::yield();
      // If we comment out yield() and replace with sleep, we get printouts.
      // boost::this_fiber::sleep_for(std::chrono::milliseconds(10));
    }
  });
  fiber_2.detach();

  // Prevent io context from returning.
  boost::asio::executor_work_guard<decltype(io_context->get_executor())> work{
      io_context->get_executor()};
  io_context->run();
}