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.
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!!!”.
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:
- h_magic
- 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 entry.
- 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.
- h_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.
- h_majmin
- 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.
- h_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.
- h_namesize
- The number of bytes in the pathname that follows the
header. This count includes the trailing NUL byte.
- h_filesize
- 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.
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.
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.
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 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 first one that appears in the archive.
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.
The
cpio implementation distributed with HPUX used
XXXX but stored device numbers differently XXX.
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
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
The “CRC” format is mis-named, as it uses a simple checksum and
not a cyclic redundancy check.
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.
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.