https://pastebin.com/B7GexCCy

 

Offers:

- Thread Safe (same object can be used by multiple threads)

- Throws lock exception if two or more distinct objects pointing to the same file try to acquire a lock (locks acquired by performing read/write)

 

(One minor thing: Assumes EOF contains new line - you can put some hack to fix it but I didn't bother)

 

Notes (from java lib):

This file-locking API is intended to map directly to the native locking facility of the underlying operating system. Thus the locks held on a file should be visible to all programs that have access to the file, regardless of the language in which those programs are written.

Whether or not a lock actually prevents another program from accessing the content of the locked region is system-dependent and therefore unspecified. The native file-locking facilities of some systems are merely advisory, meaning that programs must cooperatively observe a known locking protocol in order to guarantee data integrity. On other systems native file locks are mandatory, meaning that if one program locks a region of a file then other programs are actually prevented from accessing that region in a way that would violate the lock. On yet other systems, whether native file locks are advisory or mandatory is configurable on a per-file basis. To ensure consistent and correct behavior across platforms, it is strongly recommended that the locks provided by this API be used as if they were advisory locks.

looks good