PC-Online 1998 February/PCOnline_02_1998.iso/filesbbs/dos/uudecenc.arj/UUDECENC/UUDECODE.DOC

Text File | 1991-10-06 | 12.1 KB | 261 lines

UU-encoding is a way to code a file which may contain any characters into
a standard character set that can be reliably sent over diverse networks.


THE CHARACTER ENCODING:

The basic scheme is to break groups of 3 eight bit characters (24 bits) into
4 six bit characters and then add 32 (a space) to each six bit character
which maps it into the readily transmittable character.  Another way of
phrasing this is to say that the encoded 6 bit characters are mapped into
the set:
        `!"#$%&'()*+,-./012356789:;<=>?@ABC...XYZ[\]^_
for transmission over communications lines.

As some transmission mechanisms compress or remove spaces, spaces are changed
into back-quote characters (a 96).  (A better scheme might be to use a bias
of 33 so the space is not created, but this is not done.)

Another newer less popular encoding method, called XX-encoding uses the set:
        +-01..89ABC...XYZabc...xyz

In my opinion, XX-encoding is superior to UU-encoding because it uses more
"normal" characters that are less likely to get corrupted.  In fact several
of the special characters in the UU set do not get thru an EBCDIC to ASCII
translation correctly.  Conversely, an advantage of the UU set is that it does
not use lower case characters.  Now-a-days both upper and lower case are sent
with no problems; maybe in the communications dark ages, there was a problem
with lower case.

This "UU" encode/decode pair can handle either XX or UU encoding.  The encode
program defaults to creating a UU encoded file; but can be run with a "-x"
option to create an XX encoding.

The decode program defaults to autodetect.  However the program can get
confused by comment lines preceeding the actual encoded data.  The decode
mode can be forced to UU or XX with the "-u" or "-x" parameter.

Another option is for the actual map table to be inserted at the front of the
file.  The format for this is discussed later.  The table parameters are
detected and used by this decode program.  (A table will override the "-x" or
"-u" parameters.)  The encode program can be run with a "-t" option which
tells it to put the table into the encoded file.

A third encode mapping is the one used by Brad Templeton's ABE program.  This
is not handled by these programs as the check and control information
surrounding the actual encoded data is in a different form.

From a theoritical view, this encoding is breaking down 24 bits modulo 64.  Note
that 64**3 is = 2**24.  The result is 24 bits in for 32 bits out, a 33% size
increase.  Note that 85**5 > 2**32.  Also note that there are 94 transmittable
ASCII characters (from 0x21 thru 0x7e).  Thus modulo 85 encoding (the atob
encoder) transforms 32 bits to 5 ASCII chars or 40 bits for a 25% size increase.

The trade off in the module 85 encoding is that many communications systems do
not reliably transmit 85 ASCII characters.  The tidla, carrot, brackets, and
sometimes upper or lower case frequently get corrupted.

COMPOSING A LINE OF ENCODED CHARACTERS:

A small number of eight bit characters are encoded into a single line and a
count is put at the start of the line.  (Most lines in an encoded file have 45
encoded characters.  When you look at a UU-encoded file note that most lines
start with the letter "M".  "M" is decimal 77 which, minus the 32 bias, is 45.)

This encode program puts a check character at the end of each line.  The check
is the sum of all the encoded characters, before adding the mapping, modulo 64.

Note: Horton 9/1/87 UUENCODE has a bug in the line check algorithm; it uses the
sum of the original, not the encoded characters.  This decode program accepts
either form of line check character.

In previous versions (4.13 and lower) the line check characters was generated by
default by this encode program and was supressed with the "-L" option.  One
reason to supress them is if they will be decoded by one of the old Horton
decoders.  Most decoders either accept this form of check or simply stop looking
after the line length is exhausted.  My feelings are mixed about the line
checksums because errors of this type essentially never occur.

However with modern, error-free communications systems and with the CRC checks
on the entire file (see below) I have made the default for uuencoding to have NO
line level check characters effective version 4.21.  The "-L" option on uuencode
turns on generation of line checksums.  If you have a really bad communications
system and you want to isolate a problem, turn them on.

Uudecode automatically checks for the presense line checksums, so the default
for uudecode is to leave line level checks on; if there are some problems the
"-L" option for uudecode turns them off.  Sometimes there is junk at the end of
the line which causes spurious line checksum errors.

I have encountered various other ways that encoders end lines.  One encoder
put a "M" at both the start and end of the line.  Another used a line count
character.  This decode program checks all of these.  I would not be surprised
if some encoder out there ends lines with astrological symbols.  If you
encounter some other wierd form of encoded file, let me know.


PACKAGING THE LINES INTO FILES:

The lines of encoded data can be preceded by comments and by network addressing
information.  The encoded data is directly preceded by a line containing:

             begin <file-mode> <file-name>

This line is created by the encoding program.  The decode program scans the
file looking for "begin" in column 1.

The final end of encoded data is an encoded line with zero encoded characters
(a back-quote), followed by a line containing "end".

For integrity checking, various encode programs insert checksums for the
entire file.  This decode tries to check for all known types of file checksums.
This is discussed in more detail later.

This encode program puts a header line, containing the section number and
file name, in front of every section:

         "section <number> of uuencode of file <file name>"

At the end of a section the encode program inserts a line containing checksum
and file size information.  This can be suppressed with the "-c" option.

All the "integrity fields" (the checksum, the line check, and the section
header line) are inserted in a way that they will be ignored by other UUDECODE
programs that cannot handle them.  This decode program does not require any of
these fields; if not present, integrity checking is not done.  This program
pair is 100% downward compatible!


SPLITING UP LONG FILES:

Long files are broken into several sections before transmission.  This is
done because very large files are cumbersome to handle and because some
networks require files to be less than 64K bytes.

This encode program automatically breaks large encoded files into sections.
This split is controlled by several options.  First the "-s" option tells
encode not to split the file.  The "-s nnn" option tells encode to split the
file into hunks of "nnn" lines.  The default is 950 lines which is about 59k.
Sometimes extensive comments are put into the first file; thus it may be
necessary for the first file to contain fewer encoded lines.  The "-h nnn"
option tells encode to leave room for "nnn" additional lines in the first file.

If the data file being encoded is called FOO.ZIP, then the encode program
names the encoded files FOO1.UUE, FOO2.UUE, etc.

The decode program searches for the various sections, scans over preliminary
comments and decodes all as if they were one big file.  Decode is passed the
base file name "FOO".

Decode can (but rarely does) get confused and thinks header lines are encoded
data.  If so, edit the file and try again.  This has only happened once to me
and I have decoded a lot of files.

When decode encounters a premature end-of-file or some data which is not
decodable, it assumes the end of a file section.  decode is conservative when
it encounters data it cannot decode.

Usually this undecodable data is valid "trailer" data put at the end of file
for data transmission purposes.  However the file may be bad.  So decode
continues to scan the file, if decode then encounters a line which is decodable
it assumes the file is bad.  Or if there are more than 30 lines remaining in
the file, decode assumes the file is bad.

When decode encounters a valid end of file section it must get the next file
in sequence.  If the file name ends with a number, decode tries the next file
name in numeric sequence.  Otherwise decode asks for a file name.  If this
file does not contain decodable data, decode asks for another file to try.

The "SECTION" line inserted by the encode program is used for validity checking
only.  If not present, decode will accept any file containing encoded lines.


OTHER FILE FORMS:

Sometimes files come across in shell archives that automatically check
sequencing and call uudecode for you on the UNIX systems.  If you prefer to
download the raw files to MS-DOS, this decode program will filter thru simple
shell scripts and decode the data automatically.

There is one more rarely used feature of ENCODE: many input files can be
encoded into one large encode file.  (I have never seen this used.)  The end
of an input file is a zero length encoded line, followed by another "begin"
line instead of by an "end" line.  This decode program will decode this sort of
file; but the encode will only handle a single input file.


FILE LEVEL CHECKSUMS:

There are three types of file checksums found in encoded files:

       UUENCODE 2.14 and below inserted lines that gave the section
       size and the original input file size.  This is supplanted
       by a better technique in 3.07; but 3.07 UUDECODE still checks
       and validates the old form

       UUENCODE 3.07 and Rahul Dhesi''s encode scripts compute a Unix
       "sum -r" on the encoded sections and on the original input file.
       A difference is that UUENCODE 3.07 puts the expected "sum -r"
       and size at the end of a file while Rahul''s scripts put them at
       begining.  This UUDECODE analyzes either.

       The third form of checksum is a full 32 bit CRC that Rahul''s script
       inserts.  My code does not handle this.  Rahul has written the
       BRIK program to check them.  If there is a "sum -r" failure, BRIK
       analysis should be considered.

Also Unisys Unix platforms put a line containing just the original file size at
the end of the encoded file.  My code checks this.


TABLE LINES:

Some encoded files but the mapping used at the front of the encoded file,
just in front of the "begin" line.  The format for this is:

                   table
                   first 32 characters
                   second 32 characters

All this starts in column 1.

If decode encounters a table specification, it uses it and overrides any
command line parameters.  Encode will create the table lines if run with
the "-t" parameter.


COMPLETION CODES:

On successful completion, UUDECODE sets ERRORLEVEL to 0.  If there are any
problems, ERRORLEVEL is set to non-zero.

Most of the options to UUDECODE are obvious.  However, the "-e" option needs
more explanation.  The purpose of "-e" is to automatically run an un-archiver
(like ZOO or PKUNPAK) when UUDECODE successfully completes.  If the "-e"
option is given, UUDECODE calls BAT file UNARCUUE on successful completion
UNARCUUE is passed two parameters:

                the filename decoded (with no extension)
                the file extension.

Normally the file extension tells which un-archiver to call.  The UNARCUUE
BAT file, can test these parameters and call the necessary un-archiver.


This works well for me.  On UNIX I find a program I want in three sections:
              PRG1, PRG2, PRG3.
I copy the three files down to my PC as PRG1.UUE, PRG2.UUE, and PRG3.UUE.  I
then just enter UUDECODE PRG and the thing decodes.


Done privately and not for profit (freeware).  Suggestions appreciated.
The programs are written in Turbo Pascal 5.5 with about 5% TASM for speed.
The source is not public domain.  I would entertain consulting contracts
for porting to other hardware platforms.


Richard Marks
931 Sulgrave Lane
Bryn Mawr, PA 19010