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(emacs)Snapshot Caveats

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Snapshot Caveats
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   VC's snapshot facilities are modeled on RCS's named-configuration
support.  They use RCS's native facilities for this, so under VC
snapshots made using RCS are visible even when you bypass VC.

   For SCCS, VC implements snapshots itself.  The files it uses contain
name/file/version-number triples.  These snapshots are visible only
through VC.

   A snapshot is a set of checked-in versions.  So make sure that all
the files are checked in and not locked when you make a snapshot.

   File renaming and deletion can create some difficulties with
snapshots.  This is not a VC-specific problem, but a general design
issue in version control systems that no one has solved very well yet.

   If you rename a registered file, you need to rename its master along
with it (the command `vc-rename-file' does this automatically).  If you
are using SCCS, you must also update the records of the snapshot, to
mention the file by its new name (`vc-rename-file' does this, too).  An
old snapshot that refers to a master file that no longer exists under
the recorded name is invalid; VC can no longer retrieve it.  It would
be beyond the scope of this manual to explain enough about RCS and SCCS
to explain how to update the snapshots by hand.

   Using `vc-rename-file' makes the snapshot remain valid for
retrieval, but it does not solve all problems.  For example, some of the
files in your program probably refer to others by name.  At the very
least, the makefile probably mentions the file that you renamed.  If you
retrieve an old snapshot, the renamed file is retrieved under its new
name, which is not the name that the makefile expects.  So the program
won't really work as retrieved.