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<p><i>CPIO</i>(5) File Formats Manual <i>CPIO</i>(5)</p>
<p style="margin-top: 1em"><b>NAME</b></p>
<p style="margin-left:9%;">cpio — format of cpio
archive files</p>
<p style="margin-top: 1em"><b>DESCRIPTION</b></p>
<p style="margin-left:9%;">The <b>cpio</b> archive format
collects any number of files, directories, and other file
system objects (symbolic links, device nodes, etc.) into a
single stream of bytes.</p>
<p style="margin-left:4%; margin-top: 1em"><b>General
Format</b></p>
<p style="margin-left:9%;">Each file system object in a
<b>cpio</b> 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 <i>struct
stat</i>. (See <i>stat</i>(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!!!”.</p>
<p style="margin-left:4%; margin-top: 1em"><b>PWB
format</b></p>
<p style="margin-left:9%;">The PWB binary <b>cpio</b>
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:</p>
<p style="margin-left:17%; margin-top: 1em">struct
header_pwb_cpio { <br>
short h_magic; <br>
short h_dev; <br>
short h_ino; <br>
short h_mode; <br>
short h_uid; <br>
short h_gid; <br>
short h_nlink; <br>
short h_majmin; <br>
long h_mtime; <br>
short h_namesize; <br>
long h_filesize; <br>
};</p>
<p style="margin-left:9%; margin-top: 1em">The <i>short</i>
fields here are 16-bit integer values, while the <i>long</i>
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:</p>
<p style="margin-top: 1em"><i>h_magic</i></p>
<p style="margin-left:19%;">The integer value octal
070707.</p>
<p style="margin-top: 1em"><i>h_dev</i>, <i>h_ino</i></p>
<p style="margin-left:19%;">The device and inode numbers
from the disk. These are used by programs that read
<b>cpio</b> archives to determine when two entries refer to
the same file. Programs that synthesize <b>cpio</b> archives
should be careful to set these to distinct values for each
entry.</p>
<p style="margin-top: 1em"><i>h_mode</i></p>
<p style="margin-left:19%; margin-top: 1em">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:</p>
<p style="margin-top: 1em">0100000</p>
<p style="margin-left:31%; margin-top: 1em">IALLOC flag -
irrelevant to cpio.</p>
<p>0060000</p>
<p style="margin-left:31%; margin-top: 1em">This masks the
file type bits.</p>
<p>0040000</p>
<p style="margin-left:31%; margin-top: 1em">File type value
for directories.</p>
<p>0020000</p>
<p style="margin-left:31%; margin-top: 1em">File type value
for character special devices.</p>
<p>0060000</p>
<p style="margin-left:31%; margin-top: 1em">File type value
for block special devices.</p>
<p>0010000</p>
<p style="margin-left:31%; margin-top: 1em">ILARG flag -
irrelevant to cpio.</p>
<p>0004000</p>
<p style="margin-left:31%; margin-top: 1em">SUID bit.</p>
<p>0002000</p>
<p style="margin-left:31%; margin-top: 1em">SGID bit.</p>
<p>0001000</p>
<p style="margin-left:31%; margin-top: 1em">Sticky bit.</p>
<p>0000777</p>
<p style="margin-left:31%; margin-top: 1em">The lower 9
bits specify read/write/execute permissions for world,
group, and user following standard POSIX conventions.</p>
<p style="margin-top: 1em"><i>h_uid</i>, <i>h_gid</i></p>
<p style="margin-left:19%;">The numeric user id and group
id of the owner.</p>
<p style="margin-top: 1em"><i>h_nlink</i></p>
<p style="margin-left:19%;">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.</p>
<p style="margin-top: 1em"><i>h_majmin</i></p>
<p style="margin-left:19%;">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.</p>
<p style="margin-top: 1em"><i>h_mtime</i></p>
<p style="margin-left:19%;">Modification time of the file,
indicated as the number of seconds since the start of the
epoch, 00:00:00 UTC January 1, 1970.</p>
<p style="margin-top: 1em"><i>h_namesize</i></p>
<p style="margin-left:19%;">The number of bytes in the
pathname that follows the header. This count includes the
trailing NUL byte.</p>
<p style="margin-top: 1em"><i>h_filesize</i></p>
<p style="margin-left:19%;">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.</p>
<p style="margin-left:9%; margin-top: 1em">The pathname
immediately follows the fixed header. If <b>h_namesize</b>
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.</p>
<p style="margin-left:9%; margin-top: 1em">Hardlinked files
are not given special treatment; the full file contents are
included with each copy of the file.</p>
<p style="margin-left:4%; margin-top: 1em"><b>New Binary
Format</b></p>
<p style="margin-left:9%;">The new binary <b>cpio</b>
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:</p>
<p style="margin-left:9%; margin-top: 1em">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.</p>
<p style="margin-left:9%; margin-top: 1em">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:</p>
<p style="margin-top: 1em">0170000</p>
<p style="margin-left:21%; margin-top: 1em">This masks the
file type bits.</p>
<p>0140000</p>
<p style="margin-left:21%; margin-top: 1em">File type value
for sockets.</p>
<p>0120000</p>
<p style="margin-left:21%; margin-top: 1em">File type value
for symbolic links. For symbolic links, the link body is
stored as file data.</p>
<p>0100000</p>
<p style="margin-left:21%; margin-top: 1em">File type value
for regular files.</p>
<p>0060000</p>
<p style="margin-left:21%; margin-top: 1em">File type value
for block special devices.</p>
<p>0040000</p>
<p style="margin-left:21%; margin-top: 1em">File type value
for directories.</p>
<p>0020000</p>
<p style="margin-left:21%; margin-top: 1em">File type value
for character special devices.</p>
<p>0010000</p>
<p style="margin-left:21%; margin-top: 1em">File type value
for named pipes or FIFOs.</p>
<p>0004000</p>
<p style="margin-left:21%; margin-top: 1em">SUID bit.</p>
<p>0002000</p>
<p style="margin-left:21%; margin-top: 1em">SGID bit.</p>
<p>0001000</p>
<p style="margin-left:21%; margin-top: 1em">Sticky bit.</p>
<p>0000777</p>
<p style="margin-left:21%; margin-top: 1em">The lower 9
bits specify read/write/execute permissions for world,
group, and user following standard POSIX conventions.</p>
<p style="margin-left:9%; margin-top: 1em">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.</p>
<p style="margin-left:9%; margin-top: 1em">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
<i>bsdcpio -itv</i> 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.</p>
<p style="margin-left:4%; margin-top: 1em"><b>Portable
ASCII Format</b></p>
<p style="margin-left:9%;">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.</p>
<p style="margin-left:17%; margin-top: 1em">struct
cpio_odc_header { <br>
char c_magic[6]; <br>
char c_dev[6]; <br>
char c_ino[6]; <br>
char c_mode[6]; <br>
char c_uid[6]; <br>
char c_gid[6]; <br>
char c_nlink[6]; <br>
char c_rdev[6]; <br>
char c_mtime[11]; <br>
char c_namesize[6]; <br>
char c_filesize[11]; <br>
};</p>
<p style="margin-left:9%; margin-top: 1em">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.</p>
<p style="margin-left:4%; margin-top: 1em"><b>New ASCII
Format</b></p>
<p style="margin-left:9%;">The "new" ASCII format
uses 8-byte hexadecimal fields for all numbers and separates
device numbers into separate fields for major and minor
numbers.</p>
<p style="margin-left:17%; margin-top: 1em">struct
cpio_newc_header { <br>
char c_magic[6]; <br>
char c_ino[8]; <br>
char c_mode[8]; <br>
char c_uid[8]; <br>
char c_gid[8]; <br>
char c_nlink[8]; <br>
char c_mtime[8]; <br>
char c_filesize[8]; <br>
char c_devmajor[8]; <br>
char c_devminor[8]; <br>
char c_rdevmajor[8]; <br>
char c_rdevminor[8]; <br>
char c_namesize[8]; <br>
char c_check[8]; <br>
};</p>
<p style="margin-left:9%; margin-top: 1em">Except as
specified below, the fields here match those specified for
the new binary format above.</p>
<p style="margin-top: 1em"><i>magic</i></p>
<p style="margin-left:19%; margin-top: 1em">The string
“070701”.</p>
<p style="margin-top: 1em"><i>check</i></p>
<p style="margin-left:19%; margin-top: 1em">This field is
always set to zero by writers and ignored by readers. See
the next section for more details.</p>
<p style="margin-left:9%; margin-top: 1em">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).</p>
<p style="margin-left:9%; margin-top: 1em">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.</p>
<p style="margin-left:4%; margin-top: 1em"><b>New CRC
Format</b></p>
<p style="margin-left:9%;">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 <i>check</i> 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.</p>
<p style="margin-left:4%; margin-top: 1em"><b>HP
variants</b></p>
<p style="margin-left:9%;">The <b>cpio</b> implementation
distributed with HPUX used XXXX but stored device numbers
differently XXX.</p>
<p style="margin-left:4%; margin-top: 1em"><b>Other
Extensions and Variants</b></p>
<p style="margin-left:9%;">Sun Solaris uses additional file
types to store extended file data, including ACLs and
extended attributes, as special entries in cpio
archives.</p>
<p style="margin-left:9%; margin-top: 1em">XXX Others?
XXX</p>
<p style="margin-top: 1em"><b>SEE ALSO</b></p>
<p style="margin-left:9%;"><i>cpio</i>(1),
<i>tar</i>(5)</p>
<p style="margin-top: 1em"><b>STANDARDS</b></p>
<p style="margin-left:9%;">The <b>cpio</b> 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 <i>pax</i>(1). The portable ASCII format is
currently part of the specification for the <i>pax</i>(1)
utility.</p>
<p style="margin-top: 1em"><b>HISTORY</b></p>
<p style="margin-left:9%;">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 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</p>
<p style="margin-top: 1em"><b>BUGS</b></p>
<p style="margin-left:9%;">The “CRC” format is
mis-named, as it uses a simple checksum and not a cyclic
redundancy check.</p>
<p style="margin-left:9%; margin-top: 1em">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,
respectively.</p>
<p style="margin-left:9%; margin-top: 1em">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.</p>
<p style="margin-left:9%; margin-top: 1em">The new ASCII
format is limited to 4 gigabyte file sizes.</p>
<p style="margin-left:9%; margin-top: 1em">None of the cpio
formats store user or group names, which are essential when
moving files between systems with dissimilar user or group
numbering.</p>
<p style="margin-left:9%; margin-top: 1em">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. Debian December 23,
2011 <i>CPIO</i>(5)</p>
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