BSD 4_3_Tahoe development

Work on file usr/src/ucb/ex/FLOCK_README

Synthesized-from: CSRG/cd2/4.3tahoe

diff --git a/usr/src/ucb/ex/FLOCK_README b/usr/src/ucb/ex/FLOCK_README
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+Also added to vi is support for advisory file locking. Vi now assigns
+a sharable lock when reading files, and an exclusive lock when editting
+or writing on files.
+
+This means that if the file is open by another editor, vi will warn
+the user of the conflict. Multiple readers of the same file are
+of course never warned, but any attempt to modify the file from
+vi will produce an error message. The error message can be
+bypassed of course with "w!" et al.
+
+Note that seperate locks are kept on alternate and original files
+used within vi, so all files referenced by vi are safe.
+
+The main motivation of this is to allow "safe" use of possibly
+backgrounded vi's, but it is in general quite useful on a busy system.
+
+This also obviates things like vipw.
+
+The changes work on 4.2 as well, but watch out for flock system bugs.
+
+       Bill Jolitz
+       Symmetric
+       william@Ernie.Berkeley.EDU