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From: std-unix@ut-sally.UUCP (Moderator, John Quarterman)
Newsgroups: comp.std.unix
Subject: tar vs. cpio
Message-ID: <8188@ut-sally.UUCP>
Date: Mon, 1-Jun-87 22:25:08 EDT
Article-I.D.: ut-sally.8188
Posted: Mon Jun  1 22:25:08 1987
Date-Received: Wed, 3-Jun-87 04:11:49 EDT
Reply-To: std-unix@sally.utexas.edu
Lines: 309
Approved: jsq@sally.utexas.edu (Moderator, John Quarterman)

Included below is a draft proposal for IEEE P1003.1 regarding
the recently raised issue of Archive/Data Interchange Format.
I will deliver a proposal resembling it to P1003.1 at their
next meeting, which is three weeks from today, in Seattle.

Note two things:  this is a proposal for P1003.1, not P1003.2,
or any other group; if you disagree with my conclusions, you
can submit your own proposal-- the address is below.

If you agree with my approach but think it needs adjusting,
you can send me mail or submit articles.  If you disagree, you
can also do those things.



                                  tar vs. cpio      IEEE P1003.1 N.___
                                                           1 June 1987

                               John S. Quarterman

                    Institutional Representative from USENIX
                                   usenix!jsq



          Secretary, IEEE Standards Board
          Attention: P1003 Working Group
          345 East 47th St.
          New York, NY 10017

          In both the Trial Use Standard and Draft 10, POSIX sS10.1
          describes a data interchange format based on the tar
          program.  That section has appeared in every draft of IEEE
          1003.1 in some form and has always been based on tar format.
          The P1003.1 Working Group has recently received two related
          proposals regarding that section: one to add cpio format
          (including old-style, non-ASCII (non c option) format);
          <N.048 Lorraine C. Kevra> <V11N14> <V11N25 Eric S. Raymond>
          the other to replace the existing tar-based format with cpio
          format.  <N.043 X/OPEN> <V11N13> Some clarifications were
          received to the former.  <N.064 Dominic Dunlop> <V11N15> It
          was also proposed verbally in the latest Working Group
          meeting to drop sS10.1 altogether and let P1003.2 handle the
          issue.  <V11N08> <V11N11> <V11N09 Guy Harris> <V11N12 Doug
          Gwyn>

          The present note is a response to those proposals.  Much of
          the detail in it is derived from articles posted in the
          USENET newsgroup comp.std.unix.  Those articles are
          referenced with this format: <V11N09 Guy Harris> which gives
          the volume (11) and number of the article, and the name of
          the submittor.  If no submittor name is given, the posting
          was by the moderator, John S. Quarterman.  Thanks to those
          who submitted articles.  However, the content of this note
          is solely the responsibility of the author.

          There are a number of problems with both cpio formats.
          First, those related to the non-ASCII format:

            1.  Numerous parameters, including inode numbers, mode
                bits, and user and group IDs, are kept in two-byte
                binary integers.  This has historically produced
                serious byte-order problems when data is moved among
                systems with different byte orders.  <V11N09 Guy
                Harris>

            2.  The byte-swapping and word-swapping options to the
                cpio program are inadequate patches; with an ASCII
                format the problem would not be present.  The options
                are not consistent across versions of the program: in







          Page 2                  tar vs. cpio      IEEE P1003.1 N.___



                System III, data blocks and file names are byte
                swapped; in System V, only data blocks are byte
                swapped.  <V11N09 Guy Harris>

            3.  The two-byte integer format limits the range of inode
                numbers to 1..65535.  Many current file systems are
                bigger than that.  <V11N37 Paul Eggert> <V11N39 Henry
                Spencer>

          Non-ASCII cpio format is clearly not portable and should not
          even be considered for standardization.  <V11N12 Doug Gwyn>

          There are several problems that occur even with the ASCII
          cpio format:

            1.  Many implementations of cpio only look at the lower 16
                (or even 15) bits of the inode number, even in ASCII
                format.  <V11N39 Henry Spencer> This is because the
                variable that is used to contain the value is declared
                to be unsigned short, just as in binary format.  Thus,
                even though ASCII cpio format does not constrain this
                number, it is still less than portable.  <V11N37 Paul
                Eggert>

            2.  The proposed cpio ASCII format as specified, <N.048
                Lorraine C. Kevra> <V11N14> is not portable because
                the proposal assumes that sizeof(int) == sizeof(long).
                <N.064 Dominic Dunlop> <V11N15>

            3.  The file type written in a numerical format, making it
                UNIX specific rather than POSIX specific, since POSIX
                (and tar) specifies symbolic, rather than numerical,
                values for file types.  <V11N09 Guy Harris>

            4.  Hard links are not handled well, since cpio format
                does not record that two files are linked.  If two
                files that are linked are written in cpio format, two
                copies will be written.  There is an option to the
                cpio program to detect duplicate files by matching
                pairs of (h_dev, h_ino) and producing links, but that
                is done after the fact.  <V11N09 Guy Harris> (There is
                a program, afio, that handles cpio format more
                efficiently in this and other cases than the licensed
                versions of the program.) <V11N21 Chuck Forsberg>

            5.  Symbolic links are not handled at all, and no type
                value is reserved for them.  This makes cpio useless
                on a large class of historical implementations (those
                based on 4.2BSD or its file system) for one of the
                main purposes of POSIX sS10.1: archiving files for
                later retrieval and use on the same system.







          Page 3                  tar vs. cpio      IEEE P1003.1 N.___



            6.  The cpio format is less common than tar format: there
                are few historical implementations from Version 7 on
                that do not have tar; there are many that do not have
                cpio.  <V11N09 Guy Harris> <V11N10 Charles Hedrick>
                <V11N24 Jim Cottrell> It is true that cpio (non-ASCII
                format) was invented before tar, <V11N22 Joseph S. D.
                Yao> apparently in PWB System 1.0.  <V11N26 Joseph S.
                D. Yao> However, cpio was not available outside AT&T
                before the release of System III, while tar was in
                wide use with Version 7 and is still much more common.
                Also, it appears that the cpio format of PWB was not
                the same as that of System III.  <V11N39 Henry
                Spencer> Although System III and perhaps early
                releases of System V did not include tar, <V11N26
                Joseph S. D. Yao> current releases of System V do.

            7.  It is very late in the process to propose that P1003.1
                adopt cpio format now, especially considering that it
                was originally proposed to and rejected by the
                /usr/group committee before P1003.1 was even formed.
                <V11N39 Henry Spencer>

          There are several advantages to the current tar-based format
          as specified in sS10.1:

            1.  There are no byte- or word-swapping issues caused by
                the format, since all the header values are ASCII byte
                streams.  <V11N17 John Gilmore>

            2.  There are no inode numbers recorded, and file types
                are kept in symbolic form, so the format is less
                implementation-specific than cpio format.  <V11N17
                John Gilmore>

            3.  Historical tar format is the most widely used, as
                discussed in 6. above, despite apparent assertions to
                the contrary.  <N.043 X/OPEN> <V11N13>

            4.  The format specified in sS10.1 is upward-compatible
                with tar format.  Old tar archives can be extracted by
                a program that implements sS10.1.  Archives using some
                of the extensions of sS10.1 can be extracted with old
                (Version 7) tar programs, although symbolic links will
                not be extracted and contiguous files will not be
                handled properly (cpio does not handle these
                capabilities at all).  Files with very long names will
                not be handled properly (cpio does no better at this).
                All tar implementations are compatible to this extent.
                <V11N17 John Gilmore>









          Page 4                  tar vs. cpio      IEEE P1003.1 N.___



            5.  The /usr/group working group and P1003.1 have already
                done the work <P.061> <M.019 5.1.121 Pg.13> <RFC.003
                #121> <P.038> <P.006> required to add optional
                extensions (such as symbolic links, contiguous files,
                and long file names) that are needed on many
                historical implementations and that cpio format lacks.

            6.  The format is extensible for future facilities.
                <V11N39 Henry Spencer>

            7.  There is a public domain implementation of the format
                of sS10.1.  That implementation provided feedback which
                led to improvements in the current specification, and
                has been in use for years in transferring data with
                licensed tar implementations.  <V11N17 John Gilmore>

            8.  Many people prefer the user interface of the cpio
                program to that of the tar program, because the former
                can accept a list of pathnames to archive on standard
                input while the latter takes them as arguments,
                limiting the length of the list.  <V11N34 Andrew
                Tannenbaum> However, the above-mentioned public domain
                implementation of tar accepts pathnames on standard
                input.  <V11N17 John Gilmore> <V11N19 Jim Cottrell>
                Diffs to standard tar to add an option to accept
                pathnames on standard input when creating an archive
                have also been posted to USENET.  <V11N36 John
                Gilmore> The user interface is, in any case,
                irrelevant to P1003.1.  <V11N39 Henry Spencer> <V11N40
                Rahul Dhesi>

          There are some problems that neither tar nor cpio handles
          well.

            1.  An option to prevent crossing mount points would be
                useful for backups.  <V11N19 Jim Cottrell> <V11N22
                Joseph S. D. Yao> However, this appears to be more of
                an implementation issue than a format issue, <V11N28
                Dave Brower> <V11N32 Joseph S. D. Yao> especially
                considering that there are options to find in 4.2BSD,
                <V11N24 Jim Cottrell> SunOS 3.2, <V11N36 John Gilmore>
                and System V Release 3.0 <V11N35 Mike Akre> that take
                care of this.

            2.  The default block size in many tar implementations is
                too large for some tape controllers to read <V11N27
                Rob Lake> (the 3B20 has this problem).  This is not a
                problem with the interchange format, however.

          There is nothing that the proposed cpio can handle that the
          tar-based format already in POSIX sS10.1 cannot handle; in







          Page 5                  tar vs. cpio      IEEE P1003.1 N.___



          fact, the former is less capable.  If cpio format were
          augmented to handle missing capabilities, it would be
          subject to the same objections now aimed at the format given
          in sS10.1: that it was not identical with an existing format.

          There is no advantage in replacing the current tar-based
          format of sS10.1 with cpio format.  There is also no
          advantage in adding cpio format, because two standards are
          not as good as a single standard.

          Some have recommended removing sS10.1 from POSIX altogether,
          <V11N12 Doug Gwyn> perhaps with a recommendation for P1003.2
          to pick up the idea.  <V11N09 Guy Harris> While I believe
          that that would be preferable to adding cpio format, whether
          or not tar format remains, I recommend leaving sS10.1 as it
          is, because

             o+ The inclusion of an archive/interchange file format is
               in agreement with the purpose of POSIX to promote
               portability of application programs across interface
               implementations.  Some format will be used.  It is to
               the advantage of the users of the standard for there to
               be a standard format.

             o+ The de facto standard is tar format.  The current sS10.1
               standardizes that, and provides upward-compatible
               extensions in areas that were previously lacking.

          The Archive/Interchange File Format should be left as it is.

                                                  Thank you,



                                                  John S. Quarterman






Volume-Number: Volume 11, Number 41