cpio

format of cpio archive files

TLDR

Take a list of file names from standard input and add them [o]nto an archive in cpio's binary format

>_ echo "[file1] [file2] [file3]" | cpio -o > [archive.cpio]
copy

Copy all files and directories in a directory and add them [o]nto an archive, in [v]erbose mode

>_ find [path/to/directory] | cpio -ov > [archive.cpio]
copy

P[i]ck all files from an archive, generating [d]irectories where needed, in [v]erbose mode

>_ cpio -idv < [archive.cpio]
copy

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: -literal -offset indent 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

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. -literal -offset indent 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. -literal -offset indent 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.

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

STANDARDS

The cpio utility is no longer a part of POSIX or the Single Unix Standard. It last appeared in 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 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.

SEE ALSO

cpio(1), tar(5)

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