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(emacs)RCS and SCCS

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Options for RCS and SCCS
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   By default, RCS uses locking to coordinate the activities of several
users, but there is a mode called "non-strict locking" in which you can
check-in changes without locking the file first.  Use `rcs -U' to
switch to non-strict locking for a particular file, see the `rcs'
manual page for details.

   When deducing the version control state of an RCS file, VC first
looks for an RCS version header string in the file (Note: Version
Headers).  If there is no header string, VC normally looks at the
file permissions of the work file; this is fast.  But there might be
situations when the file permissions cannot be trusted.  In this case
the master file has to be consulted, which is rather expensive.  Also
the master file can only tell you _if_ there's any lock on the file,
but not whether your work file really contains that locked version.

   You can tell VC not to use version headers to determine the file
status by setting `vc-consult-headers' to `nil'.  VC then always uses
the file permissions (if it is supposed to trust them), or else checks
the master file.

   You can specify the criterion for whether to trust the file
permissions by setting the variable `vc-mistrust-permissions'.  Its
value can be `t' (always mistrust the file permissions and check the
master file), `nil' (always trust the file permissions), or a function
of one argument which makes the decision.  The argument is the
directory name of the `RCS' subdirectory.  A non-`nil' value from the
function says to mistrust the file permissions.  If you find that the
file permissions of work files are changed erroneously, set
`vc-mistrust-permissions' to `t'.  Then VC always checks the master
file to determine the file's status.

   VC determines the version control state of files under SCCS much as
with RCS.  It does not consider SCCS version headers, though.  Thus,
the variable `vc-mistrust-permissions' affects SCCS use, but
`vc-consult-headers' does not.