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* New: unison2.48+4.04.2, unison2.48+4.08.1 [test]
@ 2020-09-08 20:57 Andrew Schulman
  0 siblings, 0 replies; only message in thread
From: Andrew Schulman @ 2020-09-08 20:57 UTC (permalink / raw)
  To: cygwin-announce

Two new Unison packages are now available in test:


Both of these are Unison 2.48.4, but compiled with OCaml 4.04.2 and 4.08.1,
respectively. For the reasons explained below, we now need separate Unison
packages for compatible versions of both Unison and OCaml.

If you use Unison 2.48, please test these packages to see if one of them
works to synchronize with your server, and report the results here. If they
don't work for you, we can add other combinations. Once these packages move
to the current release, they'll obsolete the current unison2.48 package, so
please test them now.

Unison is a file synchronizer for Unix and Windows. It allows two replicas
of a collection of files and directories to be stored on different hosts
(or different disks on the same host), modified separately, and then
brought up to date by propagating the changes in each replica to the other.

== Unison versions and packages

Unison comes in several complementary packages for Cygwin:

                   Unison   OCaml
Package name       version  version  Unison executable
------------       -------  -------  -----------------
unison2.27         2.27.*   4.01.0   /usr/bin/unison-2.27
unison2.32         2.32.*   4.01.0   /usr/bin/unison-2.32
unison2.40         2.40.*   4.02.3   /usr/bin/unison-2.40
unison2.45         2.45.*   4.01.1   /usr/bin/unison-2.45
unison2.48+4.04.2  2.48.*   4.04.2   /usr/bin/unison-2.48+4.04.2
unison2.48+4.08.1  2.48.*   4.08.1   /usr/bin/unison-2.48+4.08.1
unison2.49         2.49.*   4.02.3   /usr/bin/unison-2.49
unison2.51         2.51.*   4.04.2   /usr/bin/unison-2.51

You can install any number of these packages side-by-side. Separate
packages are needed because in order to synchronize your files, you have to
run compatible versions of Unison on the client and server. Two Unison
executables are compatible if and only if:

(1) They have the same first two numbers of the Unison version. For
example, all Unison versions 2.48.* are compatible with each other. But if
you try to use version 2.51.x to sync with a server running version 2.48.y,
Unison will issue an error message about incompatible versions and quit.


(2) They were built with compatible versions of the OCaml compiler. OCaml
has changed its format over time for "marshaling" or serializing data. If
you run Unison executables that were built with OCaml versions that use
different marshaling formats, even if the Unison versions are the same,
you'll get the dreaded error message

  Fatal error: Fatal error during unmarshaling (input_value: ill-formed
  message), possibly because client and server have been compiled with 
  different versions of the OCaml compiler.

Apparently OCaml introduced breaking changes to its marshaling format in
versions 4.08 and 4.11. So versions pre-4.08, between 4.08 and 4.11, or
post-4.11 should be mutually compatible. But this hasn't been tested much.
For discussion of OCaml version incompatibilities, see

By installing one or more of the packages listed above, you can run
whichever version you need in order to synchronize with your server. If you
need a different combination of Unison and OCaml versions than is available
in the current packages, please send a report to It may
be possible to create a new package for it.

== Setting a default version

The package postinstallation scripts use alternatives(8) to install a
symlink /usr/bin/unison that points to one of the above-named executables.
By default this symlink will track the highest-numbered version of Unison
that you install on your system. You can change that using alternatives:

  alternatives --config unison

(recommended) or manually. See "man alternatives" for details.

If the server provides multiple versions of Unison, then you can invoke
Unison on the client with e.g. '-servercmd /usr/bin/unison-2.48' to run the
version you want on the server, or put 'servercmd /usr/bin/unison-2.48'
into the client's preference file.

== User interface

All of the Unison packages for Cygwin use the text UI. There is also a GTK2
UI for Unison, but I haven't been able to get it working yet under Cygwin.
At some time in the future I may make unison*+gtk2 packages available for

Andrew E. Schulman

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