RxRsync Manual

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By Daniel Hellerstein (15 May 2000)

RxRsync ver 1.03: Rexx procedures for the rsync differencing protocol


RxRsync contains several OS/2 classic REXX procedures that implement the rsync "differencing" protocol. This document describes their use.

I) Introduction

The Rsync protocol is a "client/server" differencing protocol that does not require that both parties have the same copy of a prior version. Instead, the client sends a synopsis of a prior version, information which the server can use to create a difference file (it uses this synopsis in lieu of the actual contents of the prior version).

Thus, rsync trades off some extra exchange of information (the synopsis the client sends to the server), in return for removing the need for both sides of the transaction having identical copies of the prior version. Of course, the efficiency of rsync is a function of how close the prior version (on the client side) is to the current version (on the server side). Nevertheless, in even the worst case (comparison of two different random files), the penalty is small; wheras in the best case (slight changes), 10 to 1 reductions in total (both ways) message size are not uncommon.

The rsync protocol has several steps. Assume a client wants a current version of a file; and that the client has a prior version of the file available. Then:

1) A "client" creates a "synopsis" of it's prior version.

2) The client requests the "server" for the current version, and sends a copy of this synopsis along with this request

3) The server creates a "difference" file by comparing the synopsis to it's copy of the current version.

4) The server send this difference file back to the client

5) The client combines the difference file with it's copy of the prior version to create an exact duplicate of the new version.

Note that the server is NOT expected to have a copy of the prior version!

The procedures in RxRsync can be used to implement this chain of events. About all you need to do is worry about the communication steps (steps 2 and 4).

II) Installing RxRsync

First, unzip RXRSYNC.ZIP to an empty temporary directory.

1) Copy RXRsync.DLL to your LIBPATH (say, copy it to x:\OS2\DLL). You will also need a copy of REXXUTIL, but that's part of most OS/2 installations.

2) Write a REXX program, and either:

a)include a copy of RxRSYNC.REX (say, put it at the end of your rexx program file)

b) Load RxRSYNC.RXL into "macrospace", and load a few dlls. See the notes in section IV below for the details.

3) Within your REXX programs, call the procedures.

Alternatively, you can use the RXSYNC.CMD program - it's an "all rexx" implementation of rsync. It's very slow, but it should work on any REXX system.

III) Description of procedures

There are three REXX procedures:

creates a synopsis of an old version.
uses this synopsis, and the new version, to create a gdiff-formatted difference file
uses a gdiff-formatted difference file, and the old version, to build a copy of the new version

Rsync_Synopsis: create a synopsis of an old version




oldver_file: a fully qualified file name (the old version). 
      synopsis_file: a fully qualified file name (the synopsis file)
comment: an optional comment 
quiet: set to 1 to suppress runtime status messages
       This is an optional parameter (the default is 0).
blocksize: blocksize to be used when creating synopsis file.
           Sizes between 500 and 1000 seem to work best.
           This is an optional parameter (the default is 500)


status: A status message of the form:
          stat multi-word message
          stat= OK for success
                ERROR for failure


  • Examples of status returned values:
    ERROR no such old version
    OK 5151 bytes written to C1FILE.RSY
  • The synopsis_file will be created in "overwrite mode". If a file with this name exists, it will be deleted (that is, synopsis_file is written in "overwrite" mode).
  • The comment can be up to 80 characters long. If not specified, a timestamp is used

Rsync_Gdiff: use a synopsis to create a GDIFF-formatted difference file




synopsis_file: a synopsis file 
newver_file: a fully qualified file name (the new version)
diff_file: a fully qualified file name  (the difference file)
quiet: set to 1 to suppress runtime status messages. 
       This is an optional parameter (the default is 0).


status: A status message
The status message is either:
 OK  md4_value
 ERROR Error message

The md4_value is a 32 hex character MD4 hash of the newver_file. See Rsync_unGdiff for an example of how it can be used.


  • The synopsis_file should exist - say, due to a prior call to Rsync_Synopsis; or because a client "sent it to you").
  • The newver_file should exist - it's the "new version" of the oldver_file
  • The diff_file will be created in "overwrite" mode.

rsync_unGdiff: create a duplicate of a new file from a difference file




oldver_file: a fully qualified file name (the old version)
diff_file: a fully qualified file name  (the difference file)
newver_file: a fully qualified file name (the "duplicate" new ver)
amd4: (optional) md4 of the "server's new" version of the file.
quiet:(optional) set to 1 to suppress runtime status messages (The default is 0).


status: A status message


  • the oldver_file must exist and be available
  • the diff_file must exist - say, it's a file that a "server" created using rsync_ungdiff and sent to you.
  • the newver_file will be created in "overwrite" mode.
  • the status has the same structure as status in Rsync_Synopsis
  • if you specify amd4, and the md4 hash of the newver_file (that is created) does NOT match amd4, then an error message is generated (and newver_file is NOT created).
  • rsync_unGdiff can be used for ANY "gdiff-formatted" difference file - not just "gdiff-formatted" difference files produced by rsync_Gdiff. That is, if a program produced a GDiff formatted difference file by comparing oldver (which you have available) and newver (which you don't have), then you can recreate this newever by rsync_ungdiff (with oldver_file and this "difference file" as arguments)

IV) The rxRsync.Dll dynamic link library.

Since REXX is very slow at repetitive math, the above rexx procedures use several procedures in rxRsync.dll.

RxRsyncLoad: Loads the rxRsync procedures.

For example:

if rxfuncquery('rx_md4')=1  then do
     call RXFuncAdd 'RXRsyncLoad', 'RXRSYNC', 'RxRsyncLoad'
     call RxRsyncLoad
 if rxfuncquery('rx_md4')=1  then do
    return "ERROR could not load  RxRsync.DLL"

RxRsyncDrop: unload the rxRsync procedures

For example:

call RxRsyncDrop

RX_RSYNC32: Compute a 32 bit rolling checksum of a string

For example:

csum32=rx_rsync32('some kind of string of any length')
csum32 will be an 8 character hex number

RX_ADLER32: Compute an adler-32 checksum

  This is the standard adler-32 checksum (which differs from the
  "modified adler 32 checksum" used by rsync).




a32 is an 8 hex character string (possibly padded with 0s on the left)


a32x=rx_adler32('This is a very long string. Really!')

Alternatively, you can build the checksum in pieces; by sequentially calling rx_adler32 with substrings. This is useful if you want the checksum of a large file.




olda32 is an 8 hex character string returned by an earlier call to 


a32a=rx_adler32('This is a very long string.')
a32b=rx_adler32(' Really!',a32a)

Note: a32b will equal a32x

RX_MD4: Compute an MD4 hash of a string or a file

   If fileflag not specified, or not equal to 1, then stuff is a string.
   Otherwise, stuff should be a fully qualified filename.
      amd4=rx_md4('some kind of string of any length')
   amd4 (or fmd4) will be a 32 hex character md4.   

RX_RSYNC32_MD4: Compute a 32 bit rolling checksum, and an md4 hash

  For example: 
      csum32_md4=rx_rsync32_md4('some kind of string of any length')
      csum32 will be an 20 characters. The first 4 are the rolling
      checksum, characters 5 to 20 are the MD4. 
      Thus:  csum32=c2x(substr(csum_32_md4),1,4)

RX_Rsync_Gdiff: Compute a gdiff-formatted "difference", given a

               "current instance and a "synopsis".
       newverfile: a filename, pointing to the "current instance"
       synopsis: the string containing the  "synopsis" of the "old 
                 instance" (as may be produced by Rsync_Synopsis)
                 Note: this is a character string, it's NOT a filename!
       outfile: a file name, the "difference" file will be written
                to this file name (in overwrite mode)
       use4: Optional. If set to 1, then only the first 4 characters of
             the md4 checksum are used to verify. This is useful
             when using the http version of rsync (smaller request
             headers, with some small risk of an incorrect "undifferencing",
             which may necessitate a re-request).
    status is a status message. It can be:
        OK md4_value
       ERROR error message
       The md4_value is a 32 hex character md4 hash of newverfile

V) Notes and disclaimer

  • Ver 1.03 fixed a "sharing violation" (RX_RSYNC_UNGDIFF was neglecting to close an input file). Thanks to Heiko Scheidt for finding the problem and the solution!
  • RSYNCtst.CMD demonstrates the use of these procedures as text inclusions.
  • RSyncts2.CMD demonstrates their use as a macrospace library.
  •  !!! If you use the "macrospace" version, you MUST be sure to load several dlls, and to load the macrospace library.
    RsyncTs2 contains a simple procedure (LOAD_LIBS) that will do this.
  • The Rsync protocol was invented by Andrew Tridgell. For more information, see http://samba.org.au/rsync/
  • A description of the GDIFF format can be found at: http://www.w3.org/TR/NOTE-gdiff-19970901.html
  • Users of the SRE-http web server (http://www.srehttp.org) can use the sreRsync "pre-reply procedure", and the DoGET.CMD http requester, as an http implementation of rsync.
  • Structure of a synopsis file
 Comment  - 80 characters (i.e.; a requested file name)
 1 space
 Blocksize  - 6 digit integer (i.e; 500)
 1 space
 #Blocks    - 8 digit character (N)
 1 space
 md4        - 32 digit md4
 3 spaces
 chksum1||md41||..||chksumN||md4N -- chksum and md4 values 
                                     (machine integer format, 
                                      high order bytes first)
this is subject to change (it may be standardized to be compatible with unix implementations of rsync)
  • Contents of rxRsync.zip
read.me       - a small read.me file
RxRSYNC.RXL   - REXX "macrospace" version of the three REXX procedures
rxrsync.rex   - REXX code version of the three REXX procedures
rxsync.cmd    - An all REXX implementation of Rsync (for demo purposes)
rsyncts2.cmd  - Demo of rsync, using RxRSYNC.DLL
rsynctst.cmd  - Demo of rsync, using (local copy of) rxrsync.rex
rxrsync.doc   - this documentation file
RxRsync.dll   - a Rexx callable dll containing several procedures
dllsrc.zip    - Source code (watcom fortran, and rexx) used to create
                RxRsync.dll and RxRsync.rxl


This is freeware that is to be used at your own risk - the author and any potentially affiliated institutions disclaim all responsibilties for any consequence arising from the use, misuse, or abuse of this software (or pieces of this software).

You may use this (or subsets of this) program as you see fit, including for commercial purposes; so long as proper attribution is made, and so long as such use does not in any way preclude others from making use of this code.


Daniel Hellerstein (danielh@crosslink.net or danielh@econ.ag.gov)