如何判断文件是否在Linux上的其他位置打开？

Question

How can I tell if a file is open in C? I think the more technical question would be how can I retrieve the number of references to a existing file and determine with that info if it is safe to open.

The idea I am implementing is a file queue. You dump some files, my code processes the files. I don't want to start processing until the producer closes the file descriptor.

Everything is being done in linux.

Thanks, Chenz

Answer 1

Digging out that info is a lot of work(you'd have to search thorugh /proc/*/fd You'd be better off with any of:

• Save to temp then rename. Either write your files to a temporary filename or directory, when you're done writinh, rename it into the directory where your app reads them. Renaming is atomic, so when the file is present you know it's safe to read.
• Maybe a variant of the above , when you're done writing the file foo you create an empty file named foo.finished . You look for the presence of *.finished when processing files.
• Lock the files while writing, that way reading the file will just block until the writer unlocks it. See the flock/lockf functions, they're advisory locks though so both the reader and writer have to lock , and honor the locks.

Answer 2

I don't think there is any way to do this in pure C (it wouldn't be cross platform).

If you know what files you are using ahead of time, you can use inotify to be notified when they open.

Answer 3

C has facilities for handling files, but not much for getting information on them. In portable C, about the only thing you can do is try to open the file in the desired way and see if it works.

Answer 4

generally you can't do that for variuos reasons (eg you cannot say if the file is opened with another user).

If you can control the processes that open the file and you are try to avoid collisions by locking the file (there are many libraries on linux in order do that)

Answer 5

Use the lsof command. (List Open Files).

Answer 6

如果你同时控制生产者和消费者，你可以使用flock（）的lockf（）来锁定文件。

Answer 7

在大多数发行版上都有lsof命令，它显示了所有当前打开的文件，如果你的文件在同一个目录中或者有一些可识别的名称模式，你可以使用grep它的输出。