General :: Detect If A Program Is Writing To A File?

I have custom software that writes to a sensitive large file when the user does something. I would like to make backup copies of The file that gets written to, but if I make a gzip of the file at the same time someone is changing something, it will corrupt the backup because some of the data will be missing, as its backed up during being written to.

a) Is there a way to detect if a file is currently being accessed/written to?
That way if its currently being accessed, I can just make the script wait until its done and then finally back it up.

b) Instead of backing up the large file while it has potential to get written to, would it be better to make a copy of the file first, then gzip the copy? This idea comes from the fact that gzipping the original takes 5-10 seconds, whereas making a copy only takes 1-2 seconds. The less time, the less chance of corruption.

c) Is there anyway to freeze a program or a file to stop it from being written to for an amount of time?

With a, b, and c together. The best solution I have to my problem would be a script that first detects rather the file is being accessed. If not, it would then freeze the file/program and then make a quick copy of it. Once the copy is created, it will unfreeze the original file/program and then go about gzipping the copy.