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CPIO(5) BSD File Formats Manual CPIO(5)
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NAME
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cpio — format of
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cpio archive files
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DESCRIPTION
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The cpio archive format
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collects any number of files, directories, and other file
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system objects (symbolic links, device nodes, etc.) into a
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single stream of bytes.
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General
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Format
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Each file system object in a cpio archive comprises a
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header record with basic numeric metadata followed by the
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full pathname of the entry and the file data. The header
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record stores a series of integer values that generally
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follow the fields in struct stat. (See stat(2) for
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details.) The variants differ primarily in how they store
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those integers (binary, octal, or hexadecimal). The header
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is followed by the pathname of the entry (the length of the
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pathname is stored in the header) and any file data. The end
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of the archive is indicated by a special record with the
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pathname ’’TRAILER!!!’’.
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PWB
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format
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XXX Any documentation of the original PWB/UNIX 1.0 format?
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XXX
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Old Binary
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Format
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The old binary cpio format stores numbers as 2-byte
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and 4-byte binary values. Each entry begins with a header in
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the following format:
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struct
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header_old_cpio {
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unsigned short c_magic;
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unsigned short c_dev;
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unsigned short c_ino;
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unsigned short c_mode;
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unsigned short c_uid;
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unsigned short c_gid;
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unsigned short c_nlink;
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unsigned short c_rdev;
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       cellspacing="0" cellpadding="0">
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unsigned short c_mtime[2];
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unsigned short c_namesize;
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       cellspacing="0" cellpadding="0">
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unsigned short c_filesize[2];
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};
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The unsigned
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short fields here are 16-bit integer values; the
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unsigned int fields are 32-bit integer values. The
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fields are as follows
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magic
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The integer
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value octal 070707. This value can be used to determine
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whether this archive is written with little-endian or
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big-endian integers.
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dev, ino
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The device and inode numbers
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from the disk. These are used by programs that read
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cpio archives to determine when two entries refer to
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the same file. Programs that synthesize cpio archives
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should be careful to set these to distinct values for each
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entry.
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mode
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The mode
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specifies both the regular permissions and the file type. It
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consists of several bit fields as follows:
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0170000
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This masks the
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file type bits.
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0140000
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File type value
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for sockets.
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0120000
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File type value
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for symbolic links. For symbolic links, the link body is
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stored as file data.
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0100000
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File type value
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for regular files.
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0060000
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File type value
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for block special devices.
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0040000
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File type value
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for directories.
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0020000
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File type value
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for character special devices.
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0010000
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File type value
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for named pipes or FIFOs.
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0004000
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SUID bit.
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0002000
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SGID bit.
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0001000
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Sticky bit. On
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some systems, this modifies the behavior of executables
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and/or directories.
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0000777
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The lower 9
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bits specify read/write/execute permissions for world,
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group, and user following standard POSIX conventions.
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uid, gid
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The numeric user id and group
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id of the owner.
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nlink
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The number of
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links to this file. Directories always have a value of at
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least two here. Note that hardlinked files include file data
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with every copy in the archive.
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rdev
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For block
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special and character special entries, this field contains
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the associated device number. For all other entry types, it
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should be set to zero by writers and ignored by readers.
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mtime
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Modification
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time of the file, indicated as the number of seconds since
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the start of the epoch, 00:00:00 UTC January 1, 1970. The
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four-byte integer is stored with the most-significant 16
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bits first followed by the least-significant 16 bits. Each
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of the two 16 bit values are stored in machine-native byte
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order.
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namesize
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The number of bytes in the
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pathname that follows the header. This count includes the
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trailing NUL byte.
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filesize
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The size of the file. Note that
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this archive format is limited to four gigabyte file sizes.
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See mtime above for a description of the storage of
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four-byte integers.
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The pathname
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immediately follows the fixed header. If the namesize
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is odd, an additional NUL byte is added after the pathname.
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The file data is then appended, padded with NUL bytes to an
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even length.
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Hardlinked files
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are not given special treatment; the full file contents are
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included with each copy of the file.
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Portable
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ASCII Format
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Version 2 of the Single UNIX Specification
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(’’SUSv2’’) standardized an ASCII
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variant that is portable across all platforms. It is
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commonly known as the ’’old
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character’’ format or as the
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’’odc’’ format. It stores the same
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numeric fields as the old binary format, but represents them
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as 6-character or 11-character octal values.
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struct
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cpio_odc_header {
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char c_magic[6];
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char c_dev[6];
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char c_ino[6];
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char c_mode[6];
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char c_uid[6];
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char c_gid[6];
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char c_nlink[6];
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char c_rdev[6];
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char c_mtime[11];
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char c_namesize[6];
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char c_filesize[11];
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};
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The fields are
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identical to those in the old binary format. The name and
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file body follow the fixed header. Unlike the old binary
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format, there is no additional padding after the pathname or
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file contents. If the files being archived are themselves
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entirely ASCII, then the resulting archive will be entirely
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ASCII, except for the NUL byte that terminates the name
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field.
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New ASCII
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Format
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The "new" ASCII format uses 8-byte hexadecimal
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fields for all numbers and separates device numbers into
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separate fields for major and minor numbers.
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struct
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cpio_newc_header {
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char c_magic[6];
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char c_ino[8];
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char c_mode[8];
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char c_uid[8];
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char c_gid[8];
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char c_nlink[8];
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char c_mtime[8];
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char c_filesize[8];
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char c_devmajor[8];
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char c_devminor[8];
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char c_rdevmajor[8];
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char c_rdevminor[8];
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char c_namesize[8];
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char c_check[8];
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};
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Except as
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specified below, the fields here match those specified for
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the old binary format above.
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magic
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The string
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’’070701’’.
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check
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This field is
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always set to zero by writers and ignored by readers. See
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the next section for more details.
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The pathname is
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followed by NUL bytes so that the total size of the fixed
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header plus pathname is a multiple of four. Likewise, the
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file data is padded to a multiple of four bytes. Note that
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this format supports only 4 gigabyte files (unlike the older
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ASCII format, which supports 8 gigabyte files).
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In this format,
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hardlinked files are handled by setting the filesize to zero
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for each entry except the last one that appears in the
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archive.
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New CRC
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Format
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The CRC format is identical to the new ASCII format
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described in the previous section except that the magic
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field is set to ’’070702’’ and the
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check field is set to the sum of all bytes in the
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file data. This sum is computed treating all bytes as
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unsigned values and using unsigned arithmetic. Only the
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least-significant 32 bits of the sum are stored.
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HP
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variants
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The cpio implementation distributed with HPUX used
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XXXX but stored device numbers differently XXX.
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Other
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Extensions and Variants
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Sun Solaris uses additional file types to store extended
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file data, including ACLs and extended attributes, as
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special entries in cpio archives.
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XXX Others?
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XXX
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SEE ALSO
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cpio(1), tar(5)
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STANDARDS
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The cpio utility is no
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longer a part of POSIX or the Single Unix Standard. It last
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appeared in Version 2 of the Single UNIX Specification
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(’’SUSv2’’). It has been supplanted
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in subsequent standards by pax(1). The portable ASCII format
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is currently part of the specification for the pax(1)
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utility.
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HISTORY
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The original cpio utility was
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written by Dick Haight while working in AT&T’s
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Unix Support Group. It appeared in 1977 as part of PWB/UNIX
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1.0, the ’’Programmer’s Work
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Bench’’ derived from Version 6 AT&T
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UNIX that was used internally at AT&T. Both the old
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binary and old character formats were in use by 1980,
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according to the System III source released by SCO under
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their ’’Ancient Unix’’ license. The
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character format was adopted as part of IEEE Std 1003.1-1988
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(’’POSIX.1’’). XXX when did
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"newc" appear? Who invented it? When did HP come
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out with their variant? When did Sun introduce ACLs and
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extended attributes? XXX
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BUGS
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The
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’’CRC’’ format is mis-named, as it
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uses a simple checksum and not a cyclic redundancy
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check.
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The old binary
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format is limited to 16 bits for user id, group id, device,
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and inode numbers. It is limited to 4 gigabyte file
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sizes.
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The old ASCII
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format is limited to 18 bits for the user id, group id,
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device, and inode numbers. It is limited to 8 gigabyte file
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sizes.
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The new ASCII
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format is limited to 4 gigabyte file sizes.
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None of the cpio
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formats store user or group names, which are essential when
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moving files between systems with dissimilar user or group
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numbering.
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Especially when
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writing older cpio variants, it may be necessary to map
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actual device/inode values to synthesized values that fit
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the available fields. With very large filesystems, this may
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be necessary even for the newer formats.
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BSD
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December 23, 2011 BSD
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