(emacs.info) Snapshot Caveats

 
 Snapshot Caveats
 ................
 
    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 the 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.
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(emacs.info) Making Snapshots
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