note 29811 deleted from function.flock by danbrown

Note Submitter: derf (at) tamu (dott) edu ---- This addresses a few topics posted earlier: First (to sbsaylors), using a seperate lockfile has problems since you must seperately check the lockfile, then create the lockfile, then perform your action. In the event of two processes running at the same time, you may wind up in the situation where there is no file and execution proceeds as follows: A:checks-for-file B:checks-for-file A:creates-file B:creates-file A:open-file B:open-file A:writes B:writes ... now, both A and B have written different things to the file, which in most systems would be buffered in seperate memory for A and B. Thus, if A closes first, when B closes its buffer will overwrite A. If B closes first, A will overwrite B. Other situations may arise where everything looks ok, because B's entire execution happens before A opens the file. But then, B will delete the lock file and A will be unprotected. Second (to the nameless person): flock doesn't exist to guarantee that your program doesn't stop running. It only makes sure that other programs that bother to flock() won't mess with the file while you're messing with it. Proper method to do this would be to write into a temporary file, then copy that file over the original in a single operation. If the execution stops, only the temp file is corrupt.