I have compared several tar implementations with the standard.

	(IEEE/Posix1003/IEC-9945-1 Standard Data Interchange format)

Although the POSIX.1-1988 standard now also defines cpio as an exchange format,
I cannot recommend the cpio archive format for data exchange. There are at
least 6 totally incompatible archive formats - all covered by the name "cpio".
It is most likely, that you are using an archive format that other cpio
implementations will not understand at all.

Note that POSIX.1-2001 will drop the cpio format from the standard as it
is not extendible (e.g. for large files > 8 GB and UID's > 2097151).

Tar in general will at least extract most of the files if you are using a
different implementation to extract the archive.

I've had a look at the following implementations:

	Index:	Program description:		Source of program:
	=====	====================		==================
	1)	bsd 4.3 tar			(Regents of UCB)
	2)	pax / ustar on SunOS 4.1	(USENIX)
	3)	tar on  Solaris 2.3/2.4/2.5	(Sun/AT&T ??)
	4)	gnutar	1.11.8			(gnu)
	5)	gnucpio 2.3			(gnu)


Summary:
	1)	bsd 4.3 tar
		Pre Posix 1003.1
	
		- Miscomputes the checksum. Therefore it is not able to extract
		  standard conforming tar archives if they contain 8bit chars
		  in the filename. This is a common bug found in many other
		  implementations too.

		No additional problems on portability except with gnutar
		archives. But this is not a problem of BSD tar.


	2)	pax / ustar (found on SunOS-4.x)

		- Dumps core on every even/odd use.
		- Computes checksums only on the first 500 bytes of the 
		  tar header: not conforming to Posix 1003.1 standard.

		Note:	This claims to be a reference implementation for
			the Posix 1003.1 standard!


	3)	tar distributed with Solaris 2.3/2.4/2.5

		- Transfers more than 12 Bit from stat.st_mode (violating Posix)
		- Complains about "impossible file type" when reading 
		  tar archives which do not contain these illegal upper bits.
		  This problem is still present in Solaris 7 & Solaris 8

		- Does not handle non null terminated filenames correctly.
		  The standard allows filenames that are exactly 100 chars
		  and therefore are not null terminated. (Fixed in Solaris 2.5)

		For the above reasons, Sun's tar is not conforming to
		Posix 1003.1.

		- Loops infinitely when trying to dump /dev/fd.
		  Caused by incorrect handling of nested directories (assumes 
		  all directories seekable).
		  This makes it impossible to use Solaris tar on the root file
		  system.


	4)	gnutar
		Claims not to be conforming to Posix 1003.1. (gnu is not tar)

		- Many bugs in implementation and design.
		  (e.g. when handling/creating multi volume archives)

		- The second logical EOF block in GNU-tar archives is missing
		  with a 50% chance.
		  This will cause correctly working tar implementations to
		  complain about an illegal/missing EOF in the tar archive.
		  This bug seems to be fixed with newer 1.13 releases

		- Deeply nested directory trees will not be dumped:
		  Error message is: Too many open files
		  (This is a similar implementation bug as found in Solaris tar
		  with the /dev/fd loop) caused by the fact that GNU-tar
		  assumes infinite resources in file descriptors.

		- Hard links with long names to files with long names do not
		  work. This bug seems to be fixed with newer 1.13 releases.

		- GNU-tar cannot read Posix compliant tar archives with 
		  long file names if the filename prefix it at least
		  138 characters. GNU-tar will think that it found an extended
		  sparse GNU tar archive and gets out of sync for the rest of
		  the archive.
		  See --sparse design bug description below.
		  This bug seems to be partially fixed with newer 1.13 releases
		  Even GNU-tar-1.13.19 does not seem to evaluate USTAR magic
		  and version to distinguish between a POSIX tar archive and a
		  non-standard GNU-tar archive.

		- GNU-tar even has a not yet identified bug which causes GNUtar
		  not to be able to partially read star archives if these
		  archives are not created with star -Hustar
		  May be this is caused by aspects of the topic above.

		- Option --sparse produces archives which cannot be read by any
		  other tar implementation known to me (except star), because
		  they will get "out of sync".
		  Posix 1003.1 conforming tar archives let gnutar get
		  "out of sync" even if the --portability option is used (see
		  above). This is a severe design bug in GNU-tar.

			Description:
			The size field in a tar archive cannot reflect the
			real size of a sparse file to have compatibility to 
			other implementations (this is also true for "star" 
			archives but star archives use a value in the size 
			field that is understood by other tar implementations).

			If the "sparse" file contains more than 4 holes,
			the "size" field in the GNU-tar control block does not
			reflect the total size of the (shrunk) sparse file in
			the archive because it does not count the 'sparse'
			extension headers. Posix compliant archives that use
			the name prefix field with more than 137 characters
			will have a value != 0 on a field that that makes
			gnutar believe that such an extension header is
			present - GNU-tar will get out of sync.

			Note: The general rule for any tar is that it should
			be able to read any "tar" compliant data stream with
			the exception that enhancements to the standard
			only will fail on the files that contain the extension.
			Those files should be extracted as if they were 
			regular files.

		- When GNU-tar writes archives it is not able to write long
		  filenames correctly according to POSIX.1-1988 or to 
		  POSIX.1-2001. As GNU-tar uses a non-standard extension to 
		  handle filenames > 100 chars, GNU-tar is a frequent problem 
		  of the portability of archives. Is is not uncommon that the
		  length of filenames exceeds 100 chars, while > 99% of the 
		  long filenames do not exceed ~ 230 chars. So most of the 
		  long filenames may be handled by the POSIX.1-1988 method 
		  which has been first documented in the 1987 draft of the 
		  POSIX.1 standard. I strongly recommend not to use GNU-tar 
		  to create archives for source exchange for this reason.

		It is bad to see that now (in 2001), 11 years after the 
		POSIX.1-1988 standard has become accepted, GNU-tar still does
		not conform to this POSIX standard. Even worse: the first 
		draft of the POSIX.1-1988 standard that did not deviate from 
		the final in important things, appeared in autumn 1987. This 
		is about the first time when PD-tar which was the base for
		GNU-tar appeared. PD-tar (in 1987) _did_ follow the POSIX.1 
		standard with one single exception: it did not implement long 
		filenames (filenames > 100 chars) at all. The non-standard GNU 
		method of handling long filenames has been introduced in 1989 
		by people from FSF. At this time, GNU-tar did not yet use the 
		POSIX.1 filename prefix for other non-POSIX purposes, so there
		is no excuse for the non-standard way that FSF went. Don't 
		believe the false GNU-tar history from FSF. I send a correct 
		GNU-tar history to FSF in 1994, FSF still has to correct their 
		false claims about GNU-tar history.

		See also http://www.geocrawler.com/archives/3/92/1997/2/0/2217471/
		as a proof that a previous GNU tar maintainer did admit the
		wrong design done by FSF members in the past.

		Summary: The main problem with GNU-tar, when it is reading TAR
		archives, is that assumes all tar archives to be non-standard
		GNU-tar archives. It does not implement a TAR format detection
		based on the actual header format (as found in star) in total. 
		Instead, it seems to have peep-hole based decisions on how to
		interpret parts of the TAR haeder. This can never work 
		correctly.

		Note: I do not recommend GNU tar as an exchange format.
		      Use star -Hustar for maximum portability instead.
		      If you like to write archives compliant to POSIX-1.2001
		      use star -Hexustar to create archives with extended POSIX
		      headers.

	5)	gnucpio

		- Splits long filenames at the leftmost '/' instead of the 
		  rightmost position of '/' required by my copy of the 
		  Posix standard.

		- The docs claim compatibility with gnutar.
		  But extraction of gnutar archives containing 'atime' gives 
		  funny filenames! (try this ...)

		- Octal numbers are left padded with ' ' instead of '0'.
		  The mode field contains more than the lower 12 bits from 
		  stat.st_mode.
		
