| CPIO(5) | File Formats Manual | CPIO(5) |
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:
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.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:
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.
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
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 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.
| December 23, 2011 | macOS 15.0 |