docs: add file format note to utmpdump manual page

Tell that the old textual format that does not use full utmp time precision,
and has issues with timezones.  Warn also that the textual format may become
incompatible, although there are no plans in foreseeable future to do so.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

diff --git a/login-utils/utmpdump.1 b/login-utils/utmpdump.1
index 68516d8bdb6266b4699b77e8cac8057afa2a7b02..c7a843d1a526b3422f7861b3dadc656f241f5a15 100644 (file)
--- a/login-utils/utmpdump.1
+++ b/login-utils/utmpdump.1
@@ -57,6 +57,18 @@ bogus entries, and reintegrated using:
 But be warned,
 .B utmpdump
 was written for debugging purposes only.
+.SS File formats
+.PP
+The only binary version of the
+.BR utmp (5)
+is standardised.  Textual dumps may become incompatible in future.
+.PP
+The version 2.28 was the last one that printed text output using
+.BR ctime (3)
+timestamp format.  Newer dumps use millisecond precision ISO-8601 timestamp
+format in UTC-0 timezone.  Conversion from former timestamp format can be
+made to binary, although attempt to do so can lead the timestamps to drift
+amount of timezone offset.
 .SH BUGS
 You may
 .B not