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,

     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.

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

             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];


           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-

     XXX Others? XXX

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

     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.

     cpio(1), tar(5)

     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.

     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

BSD                             October 5, 2007                            BSD
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