iocp

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内核对象的概念，是什么？

设备内核对象？

事件内核对象？

IOCP 概念

An I/O completion port is associated with the process that created it and is not sharable between processes. However, a single handle is sharable between threads in the same process.

IOCP 相关的基础 API 只有这三个，相关类型和函数的描述：

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HANDLE WINAPI CreateIoCompletionPort(
  // An open file handle or INVALID_HANDLE_VALUE.
  _In_     HANDLE    FileHandle,
  // A handle to an existing I/O completion port or NULL.
  _In_opt_ HANDLE    ExistingCompletionPort,
  // Use the CompletionKey parameter to help your application track which I/O operations have completed.
  _In_     ULONG_PTR CompletionKey,
  _In_     DWORD     NumberOfConcurrentThreads
);
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BOOL GetQueuedCompletionStatus(
  HANDLE       CompletionPort,
  LPDWORD      lpNumberOfBytesTransferred,
  PULONG_PTR   lpCompletionKey,
  // 指针的指针
  LPOVERLAPPED *lpOverlapped,
  DWORD        dwMilliseconds
);
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BOOL WINAPI PostQueuedCompletionStatus(
  _In_     HANDLE       CompletionPort,
  // 后面三个参数直接转给 GetQueuedCompletionStatus，内部不使用也不做合法校验，可以不做有效赋值
  _In_     DWORD        dwNumberOfBytesTransferred,
  _In_     ULONG_PTR    dwCompletionKey,
  // In particular, the lpOverlapped parameter need not point to an OVERLAPPED structure.
  _In_opt_ LPOVERLAPPED lpOverlapped
);
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typedef struct _OVERLAPPED {
  // The status code for the I/O request.
  ULONG_PTR Internal;   //系统内部使用
  // The number of bytes transferred for the I/O request.
  ULONG_PTR InternalHigh;   //系统内部使用
  union {
    struct {
      DWORD Offset;
      DWORD OffsetHigh;
    } DUMMYSTRUCTNAME;
    PVOID Pointer;  // 系统内部使用
  } DUMMYUNIONNAME;
  HANDLE    hEvent;
} OVERLAPPED, *LPOVERLAPPED;

typedef struct _WSAOVERLAPPED {
  DWORD    Internal;
  DWORD    InternalHigh;
  DWORD    Offset;
  DWORD    OffsetHigh;
  WSAEVENT hEvent;
} WSAOVERLAPPED, *LPWSAOVERLAPPED;

Any unused members of this structure should always be initialized to zero before the structure is used in a function call.

其余三个属性一般置零 l.Offset = l.OffsetHigh = 0; l.hEvent = NULL;

The WSAOVERLAPPED structure is compatible with the Windows OVERLAPPED structure.

More

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BOOL ReadFile(
  HANDLE       hFile,
  LPVOID       lpBuffer,
  DWORD        nNumberOfBytesToRead,
  // 以下两个指针分属同步、异步场景中使用，否则置零
  LPDWORD      lpNumberOfBytesRead,
  LPOVERLAPPED lpOverlapped
);

The WSAStartup function initiates use of the Winsock DLL by a process.

The WSAStartup function must be the first Windows Sockets function called by an application or DLL.

By default, a socket created with the WSASocket function will not have this overlapped attribute set.
In contrast, the socket function creates a socket that supports overlapped I/O operations as the default behavior.

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int WSAAPI WSARecv(
  SOCKET                             s,
  // A pointer to an array of WSABUF structures.
  LPWSABUF                           lpBuffers,
  // The number of WSABUF structures in the lpBuffers array.
  DWORD                              dwBufferCount,
  // Use NULL for this parameter if the lpOverlapped parameter is not NULL to avoid potentially erroneous results.
  LPDWORD                            lpNumberOfBytesRecvd,
  LPDWORD                            lpFlags,
  // A pointer to a WSAOVERLAPPED structure
  LPWSAOVERLAPPED                    lpOverlapped,
  LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

int WSAAPI WSASend(
  SOCKET                             s,
  LPWSABUF                           lpBuffers,
  DWORD                              dwBufferCount,
  // Use NULL for this parameter if the lpOverlapped parameter is not NULL to avoid potentially erroneous results. 
  LPDWORD                            lpNumberOfBytesSent,
  DWORD                              dwFlags,
  LPWSAOVERLAPPED                    lpOverlapped,
  LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

Overlapped IO 重叠输入输出

异步 I/O 技术(Overlapped I/O)，避免使用多线程

  1. 激发的文件 handles / 设备内核对象

  2. 激发的 event 对象 / 事件内核对象

    IOEvent 用的是 Overlapped IO 模型。使用起来也比较简单，仅需要将 fd buf overlapped(event) 这4者绑在一起，由内核完成数据的收发，并通过 event 通知应用层，应用层再进行数据操作已经完成后的操作，不需要多余的内存拷贝，数据已经在指定的 buf 上了。C++ SOCKET通信模型（二）IOEvent

  3. 异步过程调用 APCs（Asynchronous Procedure Calls）

  4. I/O completion ports（重要）适用于高负载服务器

IOCP 输入输出完成端口

完成端口(Completion Port)详解

最后，我的建议是，想要使用 基于事件通知的 重叠 I/O 和基于完成例程的 重叠 I/O 的朋友，如果不是特别必要，就不要去使用了，因为这两种方式不仅使用和理解起来也不算简单，而且还有性能上的明显瓶颈