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     cpio — format of cpio archive files


     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 generally 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
     The PWB binary cpio format is the original format, when cpio was introduced as part of the
     Programmer's Work Bench system, a variant of 6th Edition UNIX.  It stores numbers as 2-byte
     and 4-byte binary values.  Each entry begins with a header in the following format:

           struct header_pwb_cpio {
                   short   h_magic;
                   short   h_dev;
                   short   h_ino;
                   short   h_mode;
                   short   h_uid;
                   short   h_gid;
                   short   h_nlink;
                   short   h_majmin;
                   long    h_mtime;
                   short   h_namesize;
                   long    h_filesize;

     The short fields here are 16-bit integer values, while the long fields are 32 bit integers.
     Since PWB UNIX, like the 6th Edition UNIX it was based on, only ran on PDP-11 computers,
     they are in PDP-endian format, which has little-endian shorts, and big-endian longs.  That
     is, the long integer whose hexadecimal representation is 0x12345678 would be stored in four
     successive bytes as 0x34, 0x12, 0x78, 0x56.  The fields are as follows:

             The integer value octal 070707.

     h_dev, h_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

     h_mode  The mode specifies both the regular permissions and the file type, and it also holds
             a couple of bits that are irrelevant to the cpio format, because the field is
             actually a raw copy of the mode field in the inode representing the file.  These are
             the IALLOC flag, which shows that the inode entry is in use, and the ILARG flag,
             which shows that the file it represents is large enough to have indirect blocks
             pointers in the inode.  The mode is decoded as follows:

             0100000  IALLOC flag - irrelevant to cpio.
             0060000  This masks the file type bits.
             0040000  File type value for directories.
             0020000  File type value for character special devices.
             0060000  File type value for block special devices.
             0010000  ILARG flag - irrelevant to cpio.
             0004000  SUID bit.
             0002000  SGID bit.
             0001000  Sticky bit.
             0000777  The lower 9 bits specify read/write/execute permissions for world, group,
                      and user following standard POSIX conventions.

     h_uid, h_gid
             The numeric user id and group id of the owner.

             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.

             For block special and character special entries, this field contains the associated
             device number, with the major number in the high byte, and the minor number in the
             low byte.  For all other entry types, it should be set to zero by writers and
             ignored by readers.

             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 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 16 megabyte file
             sizes, because PWB UNIX, like 6th Edition, only used an unsigned 24 bit integer for
             the file size internally.

     The pathname immediately follows the fixed header.  If h_namesize is odd, an additional NUL
     byte is added after the pathname.  The file data is then appended, again with an additional
     NUL appended if needed to get the next header at an even offset.

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

   New Binary Format
     The new binary cpio format showed up when cpio was adopted into late 7th Edition UNIX.  It
     is exactly like the PWB binary format, described above, except for three changes:

     First, UNIX now ran on more than one hardware type, so the endianness of 16 bit integers
     must be determined by observing the magic number at the start of the header.  The 32 bit
     integers are still always stored with the most significant word first, though, so each of
     those two, in the struct shown above, was stored as an array of two 16 bit integers, in the
     traditional order.  Those 16 bit integers, like all the others in the struct, were accessed
     using a macro that byte swapped them if necessary.

     Next, 7th Edition had more file types to store, and the IALLOC and ILARG flag bits were re-
     purposed to accommodate these.  The revised use of the various bits is 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.
     0000777  The lower 9 bits specify read/write/execute permissions for world, group, and user
              following standard POSIX conventions.

     Finally, the file size field now represents a signed 32 bit integer in the underlying file
     system, so the maximum file size has increased to 2 gigabytes.

     Note that there is no obvious way to tell which of the two binary formats an archive uses,
     other than to see which one makes more sense.  The typical error scenario is that a PWB
     format archive unpacked as if it were in the new format will create named sockets instead of
     directories, and then fail to unpack files that should go in those directories.  Running
     bsdcpio -itv on an unknown archive will make it obvious which it is: if it's PWB format,
     directories will be listed with an 's' instead of a 'd' as the first character of the mode
     string, and the larger files will have a '?' in that position.

   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 new binary format.  The name and file body follow
     the fixed header.  Unlike the binary formats, 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 numbers.

           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 new binary format

     magic   The string “070701”.

     check   This field is always set to zero by writers and ignored by readers.  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 gigabyte files).

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

   New CRC Format
     The CRC format is identical to the new ASCII format described in the previous 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 arithmetic.  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 archives.

     XXX Others? XXX


     cpio(1), tar(5)


     The cpio utility is no longer a part of POSIX or the Single Unix Standard.  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 AT&T UNIX 6th Edition UNIX that was used internally at AT&T.  Both the new 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


     The “CRC” format is mis-named, as it uses a simple checksum and not a cyclic redundancy

     The binary formats are limited to 16 bits for user id, group id, device, and inode numbers.
     They are limited to 16 megabyte and 2 gigabyte file sizes for the older and newer variants,

     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 numbering.

     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.