CreateFile

功能

Creates or opens a file or I/O device.

函数原型

HANDLE CreateFile
(
    LPCSTR                lpFileName,
    DWORD                 dwDesiredAccess,
    DWORD                 dwShareMode,
    LPSECURITY_ATTRIBUTES lpSecurityAttributes,
    DWORD                 dwCreationDisposition,
    DWORD                 dwFlagsAndAttributes,
    HANDLE                hTemplateFile
);

• 第一个参数:lpFileName
  The name of the file or device to be created or opened

• 第二个参数:dwDesiredAccess
  The requested access to the file or device, which can be summarized as read, write, both or neither zero).

generic:通用

• 第三个参数:dwShareMode
  The requested sharing mode of the file or device, which can be read, write, both, delete, all of these, or none.

• 第四个参数:lpSecurityAttributes
  用于指定安全信息以及句柄的继承性，一般为NULL即可

• 第五个参数:dwCreationDisposition
  An action to take on a file or device that exists or does not exist.

• 第六个参数:dwFlagsAndAttributes

• 第七个参数:hTemplateFile
  一般为NULL

返回值

If the function succeeds, the return value is an open handle to the specified file, device, named pipe, or mail slot.
If the function fails, the return value is INVALID_HANDLE_VALUE

参考资料

• https://docs.microsoft.com/en-us/windows/desktop/api/FileAPI/nf-fileapi-createfilea
• 与设备相关的细节，务必参考上述网址
  
  

使用文件设备

异步I/O基础

• 一个线程向设备发出一个异步I/O请求，这个I/O 请求被传给设备驱动程序，后者负责完成实际的I/O操作。当驱动程序在等待设备响应时，应用程序的线程并不会因为要等到I/O请求完成而被挂起
• 在执行异步设备I/O的时候，必须传入一个已经初始化的OVERLAPPED结构

typedef struct _OVERLAPPED {
    ULONG_PTR Internal;
    ULONG_PTR InternalHigh;
    union {
        struct {
            DWORD Offset;
            DWORD OffsetHigh;
        } DUMMYSTRUCTNAME;
        PVOID Pointer;
    } DUMMYUNIONNAME;

    HANDLE  hEvent;
} OVERLAPPED, *LPOVERLAPPED;

注意点

• 设备驱动程序不会以先入先出的方式来处理队列中的I/O请求
• 如果请求的I/O操作是以同步方式进行的，那么ReadFile WriteFile返回非零值
• 如果请求的I/O操作是以异步方式进行的，那么调用上述两个函数返回FALSE，此时必须调用GetLastError来检查，若返回ERROR_IO_PENDING，那么I/O请求已经被成功添加到队列中，否则表示I/O请求无法被添加到设备驱动程序的队列中
• 在异步I/O请求完成前，一定不能移动或者销毁发出I/O请求时所使用的OVERLAPPED结构

取消队列中的异步I/O请求

• CancelIo Cancels all pending input and output (I/O) operations that are issued by the calling thread for the specified file. The function does not cancel I/O operations that other threads issue for a file handle.
• 关闭设备句柄，来取消已经添加到队列中的所有I/O请求，而不管他们是哪个线程添加的
• 当线程终止时，系统会自动取消该线程发出的所有I/O请求，除非发出I/O请求的设备具有与之关联的I/O完成端口
• CancelIoEx Marks any outstanding I/O operations for the specified file handle. The function only cancels I/O operations in the current process, regardless(不顾后果) of which thread created the I/O operation. 备注:此函数可以针对设备指定的某次I/O请求也可以针对设备的全部I/O请求

接收异步I/O请求完成通知

触发设备内核对象

• ReadFile WriteFile函数在将I/O请求添加到队列前，会先将设备内核对象设为非触发状态，当设备驱动完成请求后，驱动程序会将设备内核对象置为触发状态

触发事件内核对象

• 利用OVERLAPPED的hEvent来等待特定I/O请求的完成

可提醒I/O

• 当系统创建一个线程的时候，会同时创建一个与线程相关联的队列，称为异步过程调用队列(APC)。
• 当发出一个I/O请求的时候，我们可以告诉设备驱动程序在调用线程的APC队列中添加一项，需使用ReadFileEx WriteFileEx
• 当线程处于可提醒状态时，系统会检查他的APC队列，对队列中的每一项，系统会调用其回调函数(此调用过程是同一个线程进行的，不存在线程同步的问题)，并传入I/O错误码、已传输字节数、OVERLAPPED结构的地址
• 将线程置于可提醒状态的常用函数:SleepEx、WaitForSingleObjectEx、WaitForMultipleObjectsEx、GetQueuedCompletionStatusEx。在调用上述函数时，如果线程的APC队列不为空，则系统不会让线程进入睡眠状态。若线程至少处理了APC队列中的一项，上述函数会返回WAIT_IO_COMPLETION

可提醒I/O的劣势

• 必须使用回调函数，导致代码变的复杂
• 发出I/O请求的线程必须同时对完成通知进行处理，不存在负载均衡机制

• QueueUserApc:Adds a user-mode asynchronous procedure call (APC) object to the APC queue of the specified thread.(是一种高效的线程通信手段，甚至可以跨越进程界限)

I/O完成端口

传统的并发模型缺点

• 需要创建大量的线程，造成额外的开销
• 大量的线程并发执行，会造成严重的上下文切换问题，导致降低了性能

CreateIoCompletionPort

HANDLE WINAPI CreateIoCompletionPort
(
    HANDLE FileHandle,
    HANDLE ExistingCompletionPort,
    ULONG_PTR CompletionKey,
    DWORD NumberOfConcurrentThreads
);

• 功能
  Creates an input/output (I/O) completion port and associates it with a specified file handle, or creates an I/O completion port that is not yet associated with a file handle, allowing association at a later time.
  Associating an instance of an opened file handle with an I/O completion port allows a process to receive notification of the completion of asynchronous I/O operations involving that file handle.

• FileHandle
  An open file handle or INVALID_HANDLE_VALUE.
  The handle must be to an object that supports overlapped I/O.
  If a handle is provided, it has to have been opened for overlapped I/O completion(实现). For example, you must specify the FILE_FLAG_OVERLAPPED flag when using the CreateFile function to obtain the handle.
  If INVALID_HANDLE_VALUE is specified, the function creates an I/O completion port without associating it with a file handle. In this case, the ExistingCompletionPort parameter must be NULL and the CompletionKey parameter is ignored.

• ExistingCompletionPort
  A handle to an existing I/O completion port or NULL.
  If this parameter specifies an existing I/O completion port, the function associates it with the handle specified by the FileHandle parameter. The function returns the handle of the existing I/O completion port if successful; it does not create a new I/O completion port.
  If this parameter is NULL, the function creates a new I/O completion port and, if the FileHandle parameter is valid, associates it with the new I/O completion port. Otherwise no file handle association occurs. The function returns the handle to the new I/O completion port if successful.

• CompletionKey
  The per-handle user-defined completion key that is included in every I/O completion packet(信息包) for the specified file handle.

• NumberOfConcurrentThreads
  The maximum number of threads that the operating(操作) system can allow to concurrently(同时) process I/O completion packets for the I/O completion port. This parameter is ignored if the ExistingCompletionPort parameter is not NULL.
  If this parameter is zero, the system allows as many concurrently running threads as there are processors(中央处理器) in the system.

• 返回值:失败返回NULL。成功分为2种情况，在上面已经阐述

GetQueuedCompletionStatus

BOOL WINAPI GetQueuedCompletionStatus
(
    HANDLE CompletionPort,
    LPDWORD lpNumberOfBytesTransferred,
    PULONG_PTR lpCompletionKey,
    LPOVERLAPPED * lpOverlapped,
    DWORD dwMilliseconds
);

• 功能
  Attempts to dequeue(出列) an I/O completion packet from the specified I/O completion port. If there is no completion packet queued, the function waits for a pending I/O operation associated with the completion port to complete.

• CompletionPort
  A handle to the completion port.

• lpNumberOfBytesTransferred
  A pointer to a variable that receives the number of bytes transferred during an I/O operation that has completed.

• lpCompletionKey
  A pointer to a variable that receives the completion key value associated with the file handle whose I/O operation has completed. A completion key is a per-file key that is specified in a call to CreateIoCompletionPort.

• lpOverlapped
  A pointer to a variable that receives the address of the OVERLAPPED structure that was specified(指定) when the completed I/O operation was started.
  Even if you have passed the function a file handle associated with a completion port and a valid OVERLAPPED structure, an application can prevent completion port notification. This is done by specifying a valid event handle for the hEvent member of the OVERLAPPED structure, and setting its low-order bit. A valid event handle whose low-order bit is set keeps I/O completion from being queued to the completion port.(这段话的意思大致是:即使设备已经关联了一个I/O完成端口，也可以不将该设备的I/O请求添加到I/O完成端口的队列中，方式是为OVERLAPPED结构分配一个有效的事件内核对象并与1进行或操作)

• dwMilliseconds
  The number of milliseconds that the caller is willing to wait for a completion packet to appear at the completion port. If a completion packet does not appear within the specified time, the function times out, returns FALSE, and sets *lpOverlapped to NULL.
  If dwMilliseconds is INFINITE, the function will never time out. If dwMilliseconds is zero and there is no I/O operation to dequeue, the function will time out immediately.

• 返回值
  Returns nonzero (TRUE) if successful or zero (FALSE) otherwise.

• 备注
  This function associates a thread with the specified completion port. A thread can be associated with at most one completion port.

PostQueuedCompletionStatus

BOOL WINAPI PostQueuedCompletionStatus
(
    HANDLE CompletionPort,              //被投向模拟请求的I/O完成端口
    DWORD dwNumberOfBytesTransferred,   //以传递字节数
    ULONG_PTR dwCompletionKey,          //完成建值
    LPOVERLAPPED lpOverlapped           //被投递的OVERLAPPED结构地址
);

• 功能
  Posts an I/O completion packet to an I/O completion port.

I/O完成端口备注

• I/O完成端口对应线程池的线程个数一般是取主机的CPU数量乘以2
• 利用I/O完成端口写的一个TCP通信的例子:https://www.jianshu.com/p/09388ecaf47b