If you can’t/don’t want to keep the file contents in memory but want an atomic rewrite as noted in the other suggestions, you can first copy the original file to a temp directory to a unique path (Apple’s documentation suggests using a temporary directory), then use NSFileManager’s
-replaceItemAtURL:withItemAtURL:backupItemName:options:resultingItemURL:error:
According to the reference documentation, this method ‘replaces the contents of the item at the specified URL in a manner that insures no data loss occurs.’ (from reference documentation). The copying of the original to the temporary directory is needed because this method moves the original file.
Here’s the NSFileManager reference documentation about -replaceItemAtURL:withItemAtURL:backupItemName:options:resultingItemURL:error: