ftp: how to prevent access to file currently being accessed?

[Oliver Hardt]

hello:  i really have not much knowledge about how ftp works, so 
forgive the question in case it was just too obvious.
	problem:  a file is kept on a ftp-server.  this file is being 
accessed by n clients, and all clients will modify the file and 
upload it to the server.
	question: how can i monitor from the client side whether the 
file is currently being used by another client, so that the it is not 
accessed until the  currently accessing client finished uploading?

	thank you!  best, oliver hardt.