This document describes the format of archives read and written by the cpio utility. A cpio archive consists of the concatenation of one or more member files. Each member file contains a header (as described later in this reference page) 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!!.
There are two types of cpio archives, differing only in the style of the header. ASCII archives have totally printable header information; thus, if the files being archived are also ASCII files, the whole archive is ASCII. By default, cpio writes archives with binary headers.
The information in ASCII archive headers is stored in fixed-width, octal (base 8) numbers zero-padded on the left. Table 1 gives the order and field width for the information in the ASCII header.
|Field Width||Field Name||Meaning|
|6||magic||magic number "070707"|
|6||dev||device where file resides|
|6||ino||I-number of file|
|6||uid||owner user ID|
|6||gid||owner group ID|
|6||nlink||number of links to file|
|6||rdev||device major/minor for special file|
|11||mtime||modify time of file|
|6||namesize||length of file name|
|11||filesize||length of file to follow|
Table 1: ASCII Header Format for cpio File
Most of this information is compatible with that returned by the UNIX
Binary headers contain the same information in 2 byte (short) and 4 byte (long) integers as shown in Table 2.
Table 2: Binary Header Format for cpio File
After this information comes the file name (with namesize rounded up to the nearest 2 byte boundary). Then the file contents appear as in the ASCII archive. The byte ordering of the 2 and 4 byte integers in the binary format is machine dependent and thus portability of this format is not easily guaranteed.
cpio archives are fully compatible between UNIX and Windows systems. For maximum portability among different system architectures, only the ASCII archive format should be used.
Because the Windows file systems lack many features of the UNIX file system, much of the header information is meaningless.
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