I mean how to safely copy xlog files when postgresql is running? Any reply will be appreciated.
If you are hell bent on copying them manually then another way might be to look at the contents of the archive_status subdirectory. When you see a file in this directory with the suffix .ready it means that the corresponding WAL file is completed and should be safe to copy. However I don't think this would happen unless WAL archiving was turned on in the first place - if it is not turned on then postgres will just delete the WAL files soon after they are written (which will not give you much chance to copy them!)