基于 IOCP 的协程调度器——零基础深入浅出 C++20 协程

基于完成端口的 IO 多路复用

上文中提到了 Unix 系统中多路复用接口的发展历程：分别经历了 select -> poll -> epoll/kqueue，Windows 则通过完成端口一统江山，其实它俩调用方式差不太多：

而在可等待对象上，IOCP 则丰富的多：

* 文件 I/O 事件​​
* 文件系统变更
* 套接字（Socket）事件​​
* 命名管道（Named Pipe）事件​​
* 设备 I/O 事件​​
* 定时器事件（结合 Waitable Timer）​​

这方面能与它相提并论的恐怕只有 kqueue 了。有了上面的铺垫再参考之前 epoll 的实现，直接上 demo 源码：

#include <coroutine> #include <unordered_map> #include <windows.h> #include <vector> #include <stdexcept> #include <iostream> #include <sstream> #include <memory> struct Task { struct promise_type { Task get_return_object() { return {}; } std::suspend_never initial_suspend() { return {}; } std::suspend_never final_suspend() noexcept { return {}; } void return_void() {} void unhandled_exception() { std::terminate(); } }; }; class IocpScheduler { private: HANDLE iocp_handle; std::unordered_map<HANDLE, std::coroutine_handle<>> io_handles; public: IocpScheduler() { iocp_handle = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0); if (iocp_handle == NULL) { throw std::runtime_error("CreateIoCompletionPort failed"); } } ~IocpScheduler() { CloseHandle(iocp_handle); } void register_io(HANDLE file_handle, std::coroutine_handle<> handle) { if (io_handles.find(file_handle) == io_handles.end()) { io_handles[file_handle] = handle; if (CreateIoCompletionPort(file_handle, iocp_handle, (ULONG_PTR)file_handle, 0) == NULL) { throw std::runtime_error("CreateIoCompletionPort failed to associate file handle"); } } } void run() { while (true) { DWORD bytes_transferred = 0; ULONG_PTR completion_key = 0; LPOVERLAPPED overlapped = nullptr; BOOL success = GetQueuedCompletionStatus( iocp_handle, &bytes_transferred, &completion_key, &overlapped, INFINITE); if (completion_key != 0) { HANDLE ready_handle = (HANDLE)completion_key; if (auto it = io_handles.find(ready_handle); it != io_handles.end()) { it->second.resume(); } } } } }; struct AsyncReadAwaiter { IocpScheduler& sched; HANDLE file_handle; std::unique_ptr<char[]> buffer; DWORD buffer_size; OVERLAPPED overlapped; DWORD bytes_read; AsyncReadAwaiter(IocpScheduler& s, HANDLE file, DWORD size) : sched(s), file_handle(file), buffer_size(size), bytes_read(0) { buffer = std::make_unique<char[]>(size); ZeroMemory(&overlapped, sizeof(OVERLAPPED)); } bool await_ready() const { return false; } void await_suspend(std::coroutine_handle<> h) { sched.register_io(file_handle, h); if (!ReadFile(file_handle, buffer.get(), buffer_size, &bytes_read, &overlapped)) { DWORD error = GetLastError(); if (error != ERROR_IO_PENDING) { std::stringstream ss; ss << "ReadFile failed, error " << error; throw std::runtime_error(ss.str()); } } } std::string await_resume() { DWORD bytes_transferred = 0; if (!GetOverlappedResult(file_handle, &overlapped, &bytes_transferred, FALSE)) { DWORD error = GetLastError(); std::stringstream ss; ss << "GetOverlappedResult failed, error " << error; throw std::runtime_error(ss.str()); } return std::string(buffer.get(), bytes_transferred); } }; Task async_read_file(IocpScheduler& sched, const char* path) { HANDLE file_handle = CreateFileA( path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL); if (file_handle == INVALID_HANDLE_VALUE) { std::stringstream ss; ss << "CreateFile failed, error " << GetLastError(); throw std::runtime_error(ss.str()); } while (true) { auto data = co_await AsyncReadAwaiter(sched, file_handle, 4096); std::cout << "Read " << data.size() << " bytes\n"; if (data.size() == 0) { break; } } CloseHandle(file_handle); } int main(int argc, char* argv[]) { if (argc < 2) { std::cout << "Usage: sample file_path" << std::endl; return 1; } IocpScheduler scheduler; async_read_file(scheduler, argv[1]); scheduler.run(); return 0; }

先看编译：

Compile Explorer 中指定最新的 msvc 编译器和 C++20 选项可以编译通过，注意在 Windows 中选项指定的语法与 Unix 大相径庭，别弄错了。

一点一点降低版本尝试，发现能编译这段代码的最低版本是 msvc19.29，对应 vs16.11，如果你需要在本地安装测试环境的话，稳妥起见安装 msvc19.30、对应 vs17.0 也就是 VS2022 比较好，如果本地只有 VS2019，需要升级到第五个也就是最后一个发行版才可以。

接下来创建一个简单的控制台应用包含上面的源文件，需要配置一下 C++ 语言标准：