It would be nice for the RCS file format (which is implemented by a
great many tools, both free and non-free, both by calling GNU RCS and
by reimplementing access to RCS files) were documented in some
standard separate from any one tool.  But as far as I know no such
standard exists.  Hence this file.

The place to start is the rcsfile.5 manpage in the GNU RCS 5.7
distribution.  Then look at the diff at the end of this file (which
contains a few fixes and clarifications to that manpage).

If you are interested in MKS RCS, src/ci.c in GNU RCS 5.7 has a
comment about their date format.  However, as far as we know there
isn't really any document describing MKS's changes to the RCS file
format.

The rcsfile.5 manpage does not document what goes in the "text" field
for each revision.  The answer is that the head revision contains the
contents of that revision and every other revision contain a bunch of
edits to produce that revision ("a" and "d" lines).  The GNU diff
manual (the version I looked at was for GNU diff 2.4) documents this
format somewhat (as the "RCS output format"), but the presentation is
a bit confusing as it is all tangled up with the documentation of
several other output formats.  If you just want some source code to
look at, the part of CVS which applies these is RCS_deltas in
src/rcs.c.

The first time I read rcsfile.5 I didn't really notice the part about
the order of the revisions.  This order _is_ important and CVS relies
on it.  It is documented but it would be clearer if the example in
rcsfile.5 also showed the order of the revisions (and the "next" and
"branch" fields and anything else where it would be useful to have an
example of how a revision tree is represented in an RCS file).

There is one case where CVS uses CVS-specific, non-compatible changes
to the RCS file format, and this is magic branches.  See cvs.texinfo
for more information on them.  CVS also sets the RCS state to "dead"
to indicate that a file does not exist in a given revision (this is
stored just as any other RCS state is).

The rules regarding keyword expansion are not documented along with
the rest of the RCS file format; they are documented in the co(1)
manpage in the RCS 5.7 distribution.  See also the "Keyword
substitution" chapter of cvs.texinfo.  The co(1) manpage refers to
special behavior if the log prefix for the $Log keyword is /* or (*.
RCS 5.7 produces a warning whenever it behaves that way, and current
versions of CVS do not handle this case in a special way (CVS 1.9 and
earlier invoke RCS to perform keyword expansion).

Note that the "comment {string};" syntax from rcsfile.5 specifies a
comment leader, which affects expansion of the $Log keyword for old
versions of RCS.  The comment leader is not used by RCS 5.7 or current
versions of CVS.

Both RCS 5.7 and current versions of CVS handle the $Log keyword in a
different way if the log message starts with "checked in with -k by ".
I don't think this behavior is documented anywhere.

Diff follows:

(Note that in the following diff the old value for the Id keyword was:
    Id: rcsfile.5in,v 5.6 1995/06/05 08:28:35 eggert Exp 
and the new one was:
    Id: rcsfile.5in,v 5.7 1996/12/09 17:31:44 eggert Exp 
but since this file itself might be subject to keyword expansion I
haven't included a diff for that fact).

===================================================================
RCS file: RCS/rcsfile.5in,v
retrieving revision 5.6
retrieving revision 5.7
diff -u -r5.6 -r5.7
--- rcsfile.5in	1995/06/05 08:28:35	5.6
+++ rcsfile.5in	1996/12/09 17:31:44	5.7
@@ -85,7 +85,8 @@
 .LP
 \f2sym\fP	::=	{\f2digit\fP}* \f2idchar\fP {\f2idchar\fP | \f2digit\fP}*
 .LP
-\f2idchar\fP	::=	any visible graphic character except \f2special\fP
+\f2idchar\fP	::=	any visible graphic character,
+		except \f2digit\fP or \f2special\fP
 .LP
 \f2special\fP	::=	\f3$\fP | \f3,\fP | \f3.\fP | \f3:\fP | \f3;\fP | \f3@\fP
 .LP
@@ -119,12 +120,23 @@
 the minute (00\-59),
 and
 .I ss
-the second (00\-60).
+the second (00\-59).
+If
 .I Y
-contains just the last two digits of the year
-for years from 1900 through 1999,
-and all the digits of years thereafter.
-Dates use the Gregorian calendar; times use UTC.
+contains exactly two digits,
+they are the last two digits of a year from 1900 through 1999;
+otherwise,
+.I Y
+contains all the digits of the year.
+Dates use the Gregorian calendar.
+Times use UTC, except that for portability's sake leap seconds are not allowed;
+implementations that support leap seconds should output
+.B 59
+for
+.I ss
+during an inserted leap second, and should accept
+.B 59
+for a deleted leap second.
 .PP
 The
 .I newphrase
@@ -144,16 +156,23 @@
 field in order of decreasing numbers.
 The
 .B head
-field in the
-.I admin
-node points to the head of that sequence (i.e., contains
+field points to the head of that sequence (i.e., contains
 the highest pair).
 The
 .B branch
-node in the admin node indicates the default
+field indicates the default
 branch (or revision) for most \*r operations.
 If empty, the default
 branch is the highest branch on the trunk.
+The
+.B symbols
+field associates symbolic names with revisions.
+For example, if the file contains
+.B "symbols rr:1.1;"
+then
+.B rr
+is a name for revision
+.BR 1.1 .
 .PP
 All
 .I delta