Question

I want known if a determinate file is in use by process, i.e. if file is open in read-only mode by that process.

I thought about searching through /proc/[pid]/[fd] directory, but this way I waste a lot of time, and I think that doing this is not beautiful.

Is there any way using some Linux API to determinate if X file is open by any process? Or maybe some structures data like /proc but for files?

Answer 1

Determining if a process is using a file is easy. The inverse less so. The reason is that the kernel does not keep track of the inverse directly. The information that IS kept is:

1. A file knows how many links refer to itself (inode table)
2. A processes knows what files it has open (file descriptor table)

This is why lsof‘s /proc walking is necessary. The file descriptors in use by a particular process are kept in /proj/$PID (among other things), and so lsof can use this (and other things) to spit out all of the pid <-> fd <-> inode relationships.

This is a nice article on lsof. As with any Linux util, you can always check out its source code for all of the details 🙂