Provided by: libarchive-dev_3.1.2-7ubuntu2.8_amd64 
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 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
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 determine 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 conventions.
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 contains 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 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 old binary format
above.
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 last 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
SEE ALSO
cpio(1), tar(5)
STANDARDS
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.
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 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.