Asynchronous reading and writing of files

Files are read and written asynchronously

1: Ordinary read and write files and open files are synchronous,比如C的fopen, fclose, fread等;
2: The access speed of disk is much lower than that of memory,所以OSTo wait for the disk device to read and write.
3: 如果采用同步,Then the task will hang,Wait for the disk to read the data,通知OS.
4: 高性能的服务器,提高并发,Reading and writing files will use asynchronous mode.
5: 异步的模式:

1>Issue a request to read the file;
2>Notify the app when you're done,并处理;

win同步读

1: Open a file synchronously:

HANDLE hFile = CreateFile(路径, GENERIC_READ, 0,
	NULL,OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL,NULL);
GENERIC_READ只读的方式;

2: Read a file synchronously:ReadFile(hFile, buf, max_len,&dwRead,&overlap);
3: 关闭一个文件: CloseHandle(hFile);

Win异步读

1: Open a file asynchronously:

HANDLE hFile = CreateFile(路径, GENERIC_READ, 0,
NULL,OPEN_EXISTING, 
FILE_FLAG_OVERLAPPED|FILE_ATTRIBUTE_NORMAL,NULL);

FILE_FLAG_OVERLAPPED: Flag to open the file asynchronously, GENERIC_READ只读的方式;

2: 创建一个OVERLAPPED对象,传递给OS,Carry an event,when reading is complete,触发事件;

OVERLAPPED overlap;
overlap.Offset = 0; //The offset at which to start reading and writing the file,The offset is calculated from the beginning of the file
overlap.OffsetHigh =0;  //64The higher of the file offset position of the bit32位
overlap.hEvent = hEvent;

3: 读文件: ReadFile(hFile, buf, max_len,&dwRead,&overlap);
4:Add the event to the wait collection to wait for completion.

WaitForSingleObject/WaitForMultipleObjects,GetLastError获取错误信息

5: 关闭一个文件: CloseHandle(hFile);

同步与异步的区别

1: all have to wait,One is to control in the user plane,One is in kernel control;
2: Synchronization in the kernel etc,灵活度不够, Really can only wait for one,但是简单;
3: The user controls the wait,Multiple processing can be waited at the same time;
4: 异步:可以同时处理多个请求,发出请求后,Wait for all these events,Deal with them as they end,继续等待;

Waiting for multiple handles at the same time

1:

DWORD WaitForMultipleObjects(  
  DWORD nCount,             // Specifies the number of handles in the list 最大值为MAXIMUM_WAIT_OBJECTS 
  CONST HANDLE *lpHandles,  // 句柄数组的指针 
  BOOL fWaitAll,            // 等待的类型,如果为TRUE,Indicates unless the object both emits a signal,否则就一直等待下去;如果FALSE,Indicates that any object emits a signal
  DWORD dwMilliseconds      // Specifies the number of milliseconds to wait.如设为零,表示立即返回.such as specifying constantsINFINITE,You can wait indefinitely according to the actual situation 
);  

2: 返回值:

WAIT_TIMEOUT: The object remains in an unsignaled state,But the specified wait timeout time has been exceeded
WAIT_OBJECT_0: All objects emit signals;
WAIT_FAILED: 执行失败,可以通过GetLastError获取错误信息;
nIndex : WAIT_OBJECT_0 + 5, 第5object emits an event;

demo

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include <Windows.h>

int main(int argc, char** argv)
{
    
#if 0
	//同步
	char buf[1024];
	DWORD readed = 1024;
	int ret;
	//HANDLE 句柄: The unique identifier of the current object
	HANDLE hfile = INVALID_HANDLE_VALUE; // Open failure or return failure will beINVALID_HANDLE_VALUE
	hfile = CreateFile(L"bin//in.txt", GENERIC_READ, 0, NULL,
		OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL); // OPEN_EXISTING Open an existing file
	if (hfile == INVALID_HANDLE_VALUE)
	{
    
		printf("open error\n");
		goto failed;
	}


	ret = ReadFile(hfile, buf, 1024, &readed, NULL);
	if (ret == 0)
	{
    
		goto failed;
	}
	buf[readed] = 0;
	printf("%s\n", buf);
#else
	//异步 FILE_FLAG_OVERLAPPED 模式
	char buf[1024];
	DWORD readed = 1024;
	OVERLAPPED overlap;
	memset(&overlap, 0, sizeof(overlap));
	HANDLE hevent = CreateEvent(NULL, false, false, NULL);

	HANDLE hfile = INVALID_HANDLE_VALUE; // Open failure or return failure will beINVALID_HANDLE_VALUE
	hfile = CreateFile(L"bin//in.txt", GENERIC_READ, 0, NULL,
		OPEN_EXISTING, FILE_FLAG_OVERLAPPED | FILE_ATTRIBUTE_NORMAL, NULL); // OPEN_EXISTING Open an existing file
	if (hfile == INVALID_HANDLE_VALUE)
	{
    
		printf("open error\n");
		goto failed;
	}

	//读的时候, 发送请求, 准备一个OVERLAPPED对象
	// 传给OS, 等OS读完以后, 会通过OVERLAPPEDSend us an event
	overlap.hEvent = hevent;
	overlap.Offset = 0;  // 从第0个字符开始读起

	// 马上返回, IO挂起, 没有读到数据,ERROR_IO_PENDING状态
	ReadFile(hfile, buf, 1024, &readed, &overlap);
	if (GetLastError() == ERROR_IO_PENDING)
	{
    
		WaitForSingleObject(hevent, INFINITE); //一直等待
		readed = overlap.InternalHigh;  //One byte of data was read;
		buf[readed] = 0;
		printf("async %s\n", buf);
	}


#endif

	CloseHandle(hfile);
failed:
	return 0;
}