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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