libcrosscoro/examples/coro_semaphore.cpp

#include <coro/coro.hpp>
#include <iostream>

int main()
{
    // Have more threads/tasks than the semaphore will allow for at any given point in time.
    coro::thread_pool tp{coro::thread_pool::options{.thread_count = 8}};
    coro::semaphore   semaphore{2};

    auto make_rate_limited_task = [&](uint64_t task_num) -> coro::task<void> {
        co_await tp.schedule();

        // This will only allow 2 tasks through at any given point in time, all other tasks will
        // await the resource to be available before proceeding.
        co_await semaphore.acquire();
        std::cout << task_num << ", ";
        semaphore.release();
        co_return;
    };

    const size_t                  num_tasks{100};
    std::vector<coro::task<void>> tasks{};
    for (size_t i = 1; i <= num_tasks; ++i)
    {
        tasks.emplace_back(make_rate_limited_task(i));
    }

    coro::sync_wait(coro::when_all(std::move(tasks)));
}