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
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.
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 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.
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
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 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
BUGS
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.
BSD December 23, 2011 BSD