如何从正在使用的文件中读取

Question

There's a file I wanted to get into, but whenever I try to open it I get the message "The process cannot access the file because it is being used by another process".

Well, I want in! So, how can i do it?

I've been brainstorming a few ways to try, I'm hoping to get some input on other ways, or if my ideas wouldn't work for some reason that is not apparent to me.

• Idea 1 The folder knows where the file is, it just won't open it. What if I create a program to read from the memory address of the file, copy it, then rebuild it somewhere else? I'm not sure if this has hope, because it relies on the file being the issue.
• Idea 2 How does my process know that another process is using the file? If it's checking against all the other processes, maybe I can also figure out which process is using that file and pause it or end it.

Either of these ideas will probably take me weeks. Is anyone more creative and can think of another way; or more knowledgeable and eliminate an impractical idea?

Answer 1

In Windows, applications are allowed to obtain exclusive locks on files . When the process opens the file , one thing you specify is who else can access it while your process does (those are the .NET methods, but equivalents exist in other languages). Excel, for example, is notorious for getting an exclusive lock when you open a file. The way around it is usually to find the offending process and kill it to break the lock. Unlocker is the app that I'm most familiar with to accomplish this. If the process is a System process, however, you may not be able to kill it. You'd have to reboot to reset the lock.

Reading directly from another process's memory is unlikely to be reliable. The application may not have an in-memory copy, may not have a complete in memory copy, may not have a consistent in memory copy, and may not have an in memory copy that matches what's on disk (If they're editing the document, for example).

Your process knows that the file is locked because when it tries to open the file, it does so by asking the operating system for access to the file. The operating system responds saying, "Request denied. Another process has this file open and locked." The OS doesn't tell your process what process has the file open because trying to open a file doesn't include asking for who already has it open. Your process must ask the right question to get the answer you're looking for.

Answer 2

Windows makes you specify a sharing modes when opening a file. The sharing mode may prevent the file from being read, written, or deleted while you have it open. If you want to allow simultaneous read access you should include FILE_SHARE_READ in the dwShareMode parameter when you call CreateFile ( http://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).aspx ).

In other words, if you want to enable concurrent access to an open file you must modify the way the file is opened in the first place.

The portable standard libraries in C and Java don't offer a way to set the sharing mode when opening a file, but their usual implementations on windows set the sharing mode to READ+WRITE.