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CPIO(5)			  FreeBSD File Formats Manual		       CPIO(5)

NAME
     cpio -- format of cpio archive files

DESCRIPTION
     The cpio archive format collects any number of files, directories,	and
     other file	system objects (symbolic links,	device nodes, etc.) into a
     single stream of bytes.

   General Format
     Each file system object in	a cpio archive comprises a header record with
     basic numeric metadata followed by	the full pathname of the entry and the
     file data.	 The header record stores a series of integer values that gen-
     erally follow the fields in struct	stat.  (See stat(2) for	details.)  The
     variants differ primarily in how they store those integers	(binary,
     octal, or hexadecimal).  The header is followed by	the pathname of	the
     entry (the	length of the pathname is stored in the	header)	and any	file
     data.  The	end of the archive is indicated	by a special record with the
     pathname ``TRAILER!!!''.

   PWB format
     XXX Any documentation of the original PWB/UNIX 1.0	format?	XXX

   Old Binary Format
     The old binary cpio format	stores numbers as 2-byte and 4-byte binary
     values.  Each entry begins	with a header in the following format:

	   struct header_old_cpio {
		   unsigned short   c_magic;
		   unsigned short   c_dev;
		   unsigned short   c_ino;
		   unsigned short   c_mode;
		   unsigned short   c_uid;
		   unsigned short   c_gid;
		   unsigned short   c_nlink;
		   unsigned short   c_rdev;
		   unsigned short   c_mtime[2];
		   unsigned short   c_namesize;
		   unsigned short   c_filesize[2];
	   };

     The unsigned short	fields here are	16-bit integer values; the unsigned
     int fields	are 32-bit integer values.  The	fields are as follows

     magic   The integer value octal 070707.  This value can be	used to	deter-
	     mine whether this archive is written with little-endian or	big-
	     endian integers.

     dev, ino
	     The device	and inode numbers from the disk.  These	are used by
	     programs that read	cpio archives to determine when	two entries
	     refer to the same file.  Programs that synthesize cpio archives
	     should be careful to set these to distinct	values for each	entry.

     mode    The mode specifies	both the regular permissions and the file
	     type.  It consists	of several bit fields as follows:
	     0170000  This masks the file type bits.
	     0140000  File type	value for sockets.
	     0120000  File type	value for symbolic links.  For symbolic	links,
		      the link body is stored as file data.
	     0100000  File type	value for regular files.
	     0060000  File type	value for block	special	devices.
	     0040000  File type	value for directories.
	     0020000  File type	value for character special devices.
	     0010000  File type	value for named	pipes or FIFOs.
	     0004000  SUID bit.
	     0002000  SGID bit.
	     0001000  Sticky bit.  On some systems, this modifies the behavior
		      of executables and/or directories.
	     0000777  The lower	9 bits specify read/write/execute permissions
		      for world, group,	and user following standard POSIX con-
		      ventions.

     uid, gid
	     The numeric user id and group id of the owner.

     nlink   The number	of links to this file.	Directories always have	a
	     value of at least two here.  Note that hardlinked files include
	     file data with every copy in the archive.

     rdev    For block special and character special entries, this field con-
	     tains the associated device number.  For all other	entry types,
	     it	should be set to zero by writers and ignored by	readers.

     mtime   Modification time of the file, indicated as the number of seconds
	     since the start of	the epoch, 00:00:00 UTC	January	1, 1970.  The
	     four-byte integer is stored with the most-significant 16 bits
	     first followed by the least-significant 16	bits.  Each of the two
	     16	bit values are stored in machine-native	byte order.

     namesize
	     The number	of bytes in the	pathname that follows the header.
	     This count	includes the trailing NUL byte.

     filesize
	     The size of the file.  Note that this archive format is limited
	     to	four gigabyte file sizes.  See mtime above for a description
	     of	the storage of four-byte integers.

     The pathname immediately follows the fixed	header.	 If the	namesize is
     odd, an additional	NUL byte is added after	the pathname.  The file	data
     is	then appended, padded with NUL bytes to	an even	length.

     Hardlinked	files are not given special treatment; the full	file contents
     are included with each copy of the	file.

   Portable ASCII Format
     Version 2 of the Single UNIX Specification	(``SUSv2'') standardized an
     ASCII variant that	is portable across all platforms.  It is commonly
     known as the ``old	character'' format or as the ``odc'' format.  It
     stores the	same numeric fields as the old binary format, but represents
     them as 6-character or 11-character octal values.

	   struct cpio_odc_header {
		   char	   c_magic[6];
		   char	   c_dev[6];
		   char	   c_ino[6];
		   char	   c_mode[6];
		   char	   c_uid[6];
		   char	   c_gid[6];
		   char	   c_nlink[6];
		   char	   c_rdev[6];
		   char	   c_mtime[11];
		   char	   c_namesize[6];
		   char	   c_filesize[11];
	   };

     The fields	are identical to those in the old binary format.  The name and
     file body follow the fixed	header.	 Unlike	the old	binary format, there
     is	no additional padding after the	pathname or file contents.  If the
     files being archived are themselves entirely ASCII, then the resulting
     archive will be entirely ASCII, except for	the NUL	byte that terminates
     the name field.

   New ASCII Format
     The "new" ASCII format uses 8-byte	hexadecimal fields for all numbers and
     separates device numbers into separate fields for major and minor num-
     bers.

	   struct cpio_newc_header {
		   char	   c_magic[6];
		   char	   c_ino[8];
		   char	   c_mode[8];
		   char	   c_uid[8];
		   char	   c_gid[8];
		   char	   c_nlink[8];
		   char	   c_mtime[8];
		   char	   c_filesize[8];
		   char	   c_devmajor[8];
		   char	   c_devminor[8];
		   char	   c_rdevmajor[8];
		   char	   c_rdevminor[8];
		   char	   c_namesize[8];
		   char	   c_check[8];
	   };

     Except as specified below,	the fields here	match those specified for the
     old binary	format above.

     magic   The string	``070701''.

     check   This field	is always set to zero by writers and ignored by	read-
	     ers.  See the next	section	for more details.

     The pathname is followed by NUL bytes so that the total size of the fixed
     header plus pathname is a multiple	of four.  Likewise, the	file data is
     padded to a multiple of four bytes.  Note that this format	supports only
     4 gigabyte	files (unlike the older	ASCII format, which supports 8 giga-
     byte files).

     In	this format, hardlinked	files are handled by setting the filesize to
     zero for each entry except	the last one that appears in the archive.

   New CRC Format
     The CRC format is identical to the	new ASCII format described in the pre-
     vious section except that the magic field is set to ``070702'' and	the
     check field is set	to the sum of all bytes	in the file data.  This	sum is
     computed treating all bytes as unsigned values and	using unsigned arith-
     metic.  Only the least-significant	32 bits	of the sum are stored.

   HP variants
     The cpio implementation distributed with HPUX used	XXXX but stored	device
     numbers differently XXX.

   Other Extensions and	Variants
     Sun Solaris uses additional file types to store extended file data,
     including ACLs and	extended attributes, as	special	entries	in cpio	ar-
     chives.

     XXX Others? XXX

SEE ALSO
     cpio(1), tar(5)

STANDARDS
     The cpio utility is no longer a part of POSIX or the Single Unix Stan-
     dard.  It last appeared in	Version	2 of the Single	UNIX Specification
     (``SUSv2'').  It has been supplanted in subsequent	standards by pax(1).
     The portable ASCII	format is currently part of the	specification for the
     pax(1) utility.

HISTORY
     The original cpio utility was written by Dick Haight while	working	in
     AT&T's Unix Support Group.	 It appeared in	1977 as	part of	PWB/UNIX 1.0,
     the ``Programmer's	Work Bench'' derived from Version 6 AT&T UNIX that was
     used internally at	AT&T.  Both the	old binary and old character formats
     were in use by 1980, according to the System III source released by SCO
     under their ``Ancient Unix'' license.  The	character format was adopted
     as	part of	IEEE Std 1003.1-1988 (``POSIX.1'').  XXX when did "newc"
     appear?  Who invented it?	When did HP come out with their	variant?  When
     did Sun introduce ACLs and	extended attributes? XXX

BUGS
     The ``CRC'' format	is mis-named, as it uses a simple checksum and not a
     cyclic redundancy check.

     The old binary format is limited to 16 bits for user id, group id,
     device, and inode numbers.	 It is limited to 4 gigabyte file sizes.

     The old ASCII format is limited to	18 bits	for the	user id, group id,
     device, and inode numbers.	 It is limited to 8 gigabyte file sizes.

     The new ASCII format is limited to	4 gigabyte file	sizes.

     None of the cpio formats store user or group names, which are essential
     when moving files between systems with dissimilar user or group number-
     ing.

     Especially	when writing older cpio	variants, it may be necessary to map
     actual device/inode values	to synthesized values that fit the available
     fields.  With very	large filesystems, this	may be necessary even for the
     newer formats.

FreeBSD	9.3			October	5, 2007			   FreeBSD 9.3

NAME | DESCRIPTION | SEE ALSO | STANDARDS | HISTORY | BUGS

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