How do filesystems handle concurrent read/write?

Question

User A asks the system to read file foo and at the same time user B wants to save his or her data onto the same file. How is this situation handled on the files

Answer 1

For Linux, the short answer is you could get some strange information back from a file if there is a concurrent writer. The kernel does use locking internally to run each read() and write() operation serially. (Although, I forget whether the whole file is locked or if it's on a per-page granularity.) But if the application uses multiple write() calls to write information to the file, a read() could happen between any of those calls, so it could see inconsistent data. This is an atomicity violation in the operating system.

As mdma has mentioned, you could use file locking to make sure there is only one reader and one writer at a time. It sounds like NTFS uses mandatory locking, where if one program locks the file, all other programs get error messages when they try to access it.

Unix programs generally don't use locking at all, and when they do, the lock is usually advisory. An advisory lock only prevents other processes from getting an advisory lock on the same file; it doesn't actually prevent the read or write. (That is, it only locks the file for those who check the lock.)