c++ - 刷新不调用文件更改

Question

我正在编写一个程序，其中一个进程 A 读取另一个进程 B 附加到文件的数据。我正在使用 ReadDirectoryChangesW 进行通知。问题是在我关闭 B 中的句柄之前不会生成通知，尽管我正在刷新使用 fflush 将内容写入文件。代码如下所示

文件编写器:

int _tmain(int argc, _TCHAR* argv[])
{
    FILE *fp;

    fp=_fsopen("log.txt", "a", _SH_DENYNO);

    char str[4096];

    for(int i=1;i<4096;i++)
        str[i]=i;

    while(true){
        fwrite(str,1,4096,fp);
        fflush(fp);
        Sleep(2000);
    }

    return 0;
}

文件阅读器:

#include "stdafx.h"
#include <windows.h>
#include <stdlib.h>
#include <stdio.h>
#include <tchar.h>
#include <conio.h>
#include <assert.h> 
#include <share.h>

void _tmain(int argc, TCHAR *argv[])
{
    FILE *fp;
    fp=_fsopen("C:\\Users\\dell\\Documents\\Visual Studio 2012\\Projects\\FileWriter\\FileWriter\\log.txt", "r", _SH_DENYNO);
    int last_size=0,new_size=0;

    if(fp==NULL)
        return ;

    HANDLE m_hMonitoredDir = CreateFile(TEXT("C:\\Users\\dell\\Documents\\Visual Studio 2012\\Projects\\FileWriter\\FileWriter"), FILE_LIST_DIRECTORY, 
        FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE, 
        NULL, OPEN_EXISTING, 
        FILE_FLAG_BACKUP_SEMANTICS, NULL );

    if ( m_hMonitoredDir == INVALID_HANDLE_VALUE )
    {
        DWORD dwErr = GetLastError();
        printf("error");
        return;
    }

    char szBuf[ MAX_PATH ];
    DWORD dwBytesRead = 0;
    int flag=0;
    char *buffer;

    while ( ReadDirectoryChangesW( m_hMonitoredDir, szBuf, MAX_PATH, FALSE, FILE_NOTIFY_CHANGE_LAST_WRITE,&dwBytesRead, NULL, NULL ))
    {
        PFILE_NOTIFY_INFORMATION pstFileNotif = (PFILE_NOTIFY_INFORMATION)( szBuf );
        if ( pstFileNotif->Action == FILE_ACTION_MODIFIED ) 
        {
            char szNotifFilename[ MAX_PATH ] = { 0 };
            if ( int iNotifFilenameLen = WideCharToMultiByte( CP_OEMCP, NULL, 
                pstFileNotif->FileName, 
                pstFileNotif->FileNameLength / sizeof( WCHAR ), 
                szNotifFilename, sizeof( szNotifFilename ) / sizeof( char ), 
                NULL, NULL ) )
            {

                if ( strcmp("log.txt", szNotifFilename ) == 0 )
                {   
                    fseek(fp, 0, SEEK_END);
                    new_size = ftell(fp);   
                    fseek(fp,last_size,SEEK_SET);
                    int size=new_size-last_size;
                    buffer=new char[size+1];
                    fread(buffer,1,size,fp);
                    buffer[size]='\0';
                    printf("%s",buffer);
                    free(buffer);
                }
            }
        }
    }

}

谁能帮我在 B 中使用 fflush 后立即收到通知？

Answer 1

我认为这是不可能的。根据the documentation在 FILE_NOTIFY_CHANGE_LAST_WRITE 上(强调我的):

Any change to the last write-time of files in the watched directory or subtree causes a change notification wait operation to return. The operating system detects a change to the last write-time only when the file is written to the disk. For operating systems that use extensive caching, detection occurs only when the cache is sufficiently flushed.

fflush()确保文件数据传回操作系统，但不保证数据写入磁盘，因为通常涉及大量缓存:

Buffers are normally maintained by the operating system, which determines the optimal time to write the data automatically to disk: when a buffer is full, when a stream is closed, or when a program terminates normally without closing the stream. The commit-to-disk feature of the run-time library lets you ensure that critical data is written directly to disk rather than to the operating-system buffers.

正如其他人在评论中所说，您最好使用命名管道来实现您的目标，因为您只处理一个已知文件。