Linux：如何确定文件是否已被其他进程更新？

Question

I am currently watching an XML file from log4j output. I have a custom viewer that displays the log-output in GUI. I need to watch this file as to when it gets updated so that the GUI can re-parse and update itself. In C# there is a FileWatcher concept so on Windows no-probs, but what options do i have using C on Linux.

Is there a standard way of doing this on both unix and linux flavours (POSIX maybe)?

Thanks

Answer 1

Are you looking for something like inotify ?

Alternatively you could poll the file using stat .

Answer 2

John mentioned the two main routes you can take for watching files under Unix/Linux systems: notification and polling.

Notification is when the system itself (usually the kernel) triggers a message to registered applications whenever the file is accessed or written to. This approach requires a compliant system and might not be available on older machines.

The primary implementation of notification under Linux is inotify , which is built into kernels 2.6.13 and later and can be patched into 2.6.12 and earlier. There's a nice IBM guide on using inotify as well as a basic set of tools and C library for accessing inotify.

Polling is when your program repeatedly checks a file at intervals to see if anything about that file - its size, last modification time, owner, etc. - has been changed.

There's no standard implementation of this, as it's written on a program-level rather than system-level basis, although the system call poll exists and seems to do something useful for this purpose (the man page says it's similar to select , and it waits for an event to become available on a file descriptor). You could also take the more basic approach with stat and do your checking on your own.

Answer 3

If you want something more portable than inotify, which is a Linux-ism, look into the File Alteration Monitor , which is older and more portable technology, but can be less performant. On the other hand, it does something useful with NFS-mounted filesystems, which depending on your application could be important.