Cpio Formats

   A Cpio archive consists of one or more concatenated member files. Each member file contains a header optionally followed by file contents as indicated in the header. The end of the archive is indicated by another header describing an (empty) file named TRAILER!!.

A Cpio archive stores each entry as a fixed-size header followed by a variable-length filename and variable-length data. Unlike tar, cpio does only minimal padding of the header or file data. There are a variety of cpio formats, which differ primarily in how they store the initial header: some store the values as octal or hexadecimal numbers in ASCII, others as binary values of varying byte order and length.

   WinTar can read a number of common cpio variants and can write odc and newc format archives.

binary

   WinTar can read and extract both big-endian and little-endian variants of the original binary cpio format. This format used 32-bit binary values for file size and mtime, and 16-bit binary values for the other fields.

odc

   WinTar can both read and write this POSIX-standard format. This format stores the header contents as octal values in ASCII. It is standard, portable, and immune from byte- order confusion. File sizes and mtime are limited to 33 bits (8GB file size), other fields are limited to 18 bits.

SVR4

   WinTar can read both CRC and non-CRC variants of this format. The SVR4 format uses eight-digit hexadecimal values for all header fields. This limits file size to 4GB, and also limits the mtime and other fields to 32 bits. The SVR4 format can optionally include a CRC of the file contents.

   Cpio first appeared in PWB/UNIX 1.0, which was released within AT&T in 1977. PWB/UNIX 1.0 formed the basis of System III Unix, released outside of AT&T in 1981. This makes cpio older than tar, although cpio was not included in Version 7 AT&T Unix. As a result, the tar command became much better known in universities and research groups that used Version 7. The combination of the find and cpio utilities provided very precise control over file selection. Unfortunately, the format has many limitations that make it unsuitable for widespread use. Only the POSIX format permits files over 4GB, and its 18-bit limit for most other fields makes it unsuitable for modern systems. In addition, cpio formats only store numeric UID/GID values (not usernames and group names), which can make it very difficult to correctly transfer archives across systems with dissimilar user numbering.

Related file formats

---

•  archive file formats

•  CAB archive format

•  Zip archive format

•  Tar archive format

•  Pax archive format