Can you describe your use case more?

I don't think format matters - if you've got multiple processes writing simultaneously, you'll have a potential for corruption. What you want is a lock file. Basically, you just create a separate file called something like my process.lock. When a process wants to write to the other file, you check if the lock file exists - if yes, wait until it doesn't; if no, create it. In the lock file, store just the process id of the file that has the lock, so you can also add logic to check if the process exists (if it doesn't, it probably died - you may have to check the file you're writing to for corruption/recover). When done writing, delete the file to release the lock.

See https://en.wikipedia.org/wiki/File_locking, and specifically the section on lock files.

This is a common enough pattern there are probably libraries to handle a lot of the logic for you, though it's also simple enough to handle yourself.