zic now supports links to links

* NEWS, backward, zic.8: Mention this.
* zic.c (make_links):
New function, which supports links to later links.
In this function, improve quality of warnings about
links to links.
(main): Use it.

1 files changed, 25 insertions, 4 deletions

diff --git a/zic.8 b/zic.8
index e46d0ab..f79148f 100644
--- a/zic.8
+++ b/zic.8
@@ -190,7 +190,10 @@ the named file rather than in the standard location.
 Be more verbose, and complain about the following situations:
 .RS
 .PP
-The input specifies a link to a link.
+The input specifies a link to a link,
+something not supported by some older parsers, including
+.B zic
+itself through release 2022e.
 .PP
 A year that appears in a data file is outside the range
 of representable years.
@@ -691,19 +694,37 @@ The
 .B TARGET
 field should appear as the
 .B NAME
-field in some zone line.
+field in some zone line or as the
+.B LINK-NAME
+field in some link line.
 The
 .B LINK-NAME
 field is used as an alternative name for that zone;
 it has the same syntax as a zone line's
 .B NAME
 field.
+Links can chain together, although the behavior is unspecified if a
+chain of one or more links does not terminate in a Zone name.
+A link line can appear before the line that defines the link target.
+For example:
+.sp
+.ne 3
+.nf
+.in +2m
+.ta \w'Zone\0\0'u +\w'Greenwich\0\0'u
+Link	Greenwich	G_M_T
+Link	Etc/GMT	Greenwich
+Zone	Etc/GMT\0\00\0\0\*-\0\0GMT
+.sp
+.in
+.fi
+The two links are chained together, and G_M_T, Greenwich, and Etc/GMT
+all name the same zone.
 .PP
 Except for continuation lines,
 lines may appear in any order in the input.
 However, the behavior is unspecified if multiple zone or link lines
-define the same name, or if the source of one link line is the target
-of another.
+define the same name.
 .PP
 The file that describes leap seconds can have leap lines and an
 expiration line.