🚀 go-pugleaf

Yesterday was 16 June, which was the day I said I would collect
tar and cpio comments until.  Included below is the revised note
for P1003.1, incorporating those comments.  I will deliver it
to P1003.1 in Seattle Monday.



                                  tar vs. cpio      IEEE P1003.1 N.___
                                                          17 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 the current 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 (always 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.

          This note is addressed to P1003.1, and is concerned with
          data interchange formats.  Although user interface issues
          may be of interest to P1003.2, they are not addressed here.

          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>







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



            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
                System III, data blocks and file names are byte
                swapped; in System V, only data blocks are byte
                swapped.  <V11N09 Guy Harris> <V11N47 Andrew
                Tannenbaum>

            3.  The two-byte integer format limits the range of inode
                numbers to 0..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 only constrains this
                number to the range 0..262143, the format 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 is 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 directly record that two files are linked.
                If two files that are linked are written in cpio
                format, two copies will be written.  The cpio program
                detects duplicate files by matching pairs of (h_dev,
                h_ino) and producing links, but that is done after the
                fact.  <V11N09 Guy Harris> <V11N45 Guy Harris> <V11N54
                Ian Donaldson> (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>







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



            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.  Although
                it is possible to extend cpio to handle symbolic
                links, and at least one vendor has done this, <V11N45
                Guy Harris> the format proposed to P1003.1 is the
                format in the SVID, and does not handle symbolic
                links.

            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> The cpio program was first available outside
                AT&T with PWB/UNIX 1.0, <V11N45 Guy Harris> <V11N63
                Joseph S. D. Yao> and later with System III.  However,
                in the interim, Version 7, which did not include cpio
                <V11N53 Bill Jones> <V11N62 Guy Harris> but did
                include tar, became the most influential system.
                There was a V7 addendum tape, but it also did not
                include cpio (according to its README file); <V11N65
                Rick Adams> the addendum tape was in tar format.
                Also, it appears that the cpio format of PWB was not
                the same as that of System III.  <V11N39 Henry
                Spencer> And System III and all releases of System V
                include tar.  <V11N26 Joseph S. D. Yao> <V11N63 Joseph
                S. D. Yao> <V11N45 Guy Harris> <V11N47 Andrew
                Tannenbaum>

            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>

          Advantages of cpio format include:

            1.  Both X/OPEN <N.043 X/OPEN> <V11N13> and the SVID
                <N.048 Lorraine C. Kevra> <V11N14> use it, although
                evidently defined somewhat differently.  <N.064
                Dominic Dunlop> <V11N15>

            2.  Archives made in cpio format are often smaller than
                ones in tar format.  <V11N44 Mark Horton> But this is
                only because of the headers, and thus the effect






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



                diminishes with larger files.

            3.  On a local (non-networked) system, cpio is more
                efficient at copying directory trees than tar.
                <V11N46 Steve Blasingame> However, this is really an
                implementation issue.

          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>

            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, long file names,
                <V11N49 Jerry Schwarz> <V11N50 Michael MacDonald> and
                contiguous files) 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>






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



            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> and
                at least one vendor sells a version of tar that can do
                this.  <V11N48 Michael Gersten> 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>

          Disadvantages of tar format:

            1.  If an attempt is made to extract only the second of a
                pair of hard linked files the tar program will attempt
                to link the second file to the nonexistent first file,
                and nothing will be extracted.  Although a
                sufficiently clever implementation could avoid this,
                the problem can be considered to be in the archive
                format.  <V11N66 Kenneth Almquist>

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

            1.  File names still longer than the length of PATH_MAX
                (at least 255) <V11N50 Michael MacDonald> that the
                POSIX format allows (and than the 128 that cpio
                permits or than the 100 that historical tar allows)
                would be preferable, although the POSIX limit is
                useful for most cases.  <V11N54 Ian Donaldson>

            2.  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.

            3.  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.








          Page 6                  tar vs. cpio      IEEE P1003.1 N.___



          There is nothing that the proposed cpio can handle that the
          tar-based format already in POSIX sS10.1 cannot handle; in
          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 67