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32 .Nd format of cpio archive files
36 archive format collects any number of files, directories, and other
37 file system objects (symbolic links, device nodes, etc.) into a single
40 Each file system object in a
42 archive comprises a header record with basic numeric metadata
43 followed by the full pathname of the entry and the file data.
44 The header record stores a series of integer values that generally
50 The variants differ primarily in how they store those integers
51 (binary, octal, or hexadecimal).
52 The header is followed by the pathname of the
53 entry (the length of the pathname is stored in the header)
55 The end of the archive is indicated by a special record with
59 XXX Any documentation of the original PWB/UNIX 1.0 format? XXX
63 format stores numbers as 2-byte and 4-byte binary values.
64 Each entry begins with a header in the following format:
65 .Bd -literal -offset indent
66 struct header_old_cpio {
67 unsigned short c_magic;
70 unsigned short c_mode;
73 unsigned short c_nlink;
74 unsigned short c_rdev;
76 unsigned short c_namesize;
77 unsigned int c_filesize;
83 fields here are 16-bit integer values; the
85 fields are 32-bit integer values.
86 The fields are as follows
87 .Bl -tag -width indent
89 The integer value octal 070707.
90 This value can be used to determine whether this archive is
91 written with little-endian or big-endian integers.
93 The device and inode numbers from the disk.
94 These are used by programs that read
96 archives to determine when two entries refer to the same file.
97 Programs that synthesize
99 archives should be careful to set these to distinct values for each entry.
101 The mode specifies both the regular permissions and the file type.
102 It consists of several bit fields as follows:
103 .Bl -tag -width "MMMMMMM" -compact
105 This masks the file type bits.
107 File type value for sockets.
109 File type value for symbolic links.
110 For symbolic links, the link body is stored as file data.
112 File type value for regular files.
114 File type value for block special devices.
116 File type value for directories.
118 File type value for character special devices.
120 File type value for named pipes or FIFOs.
127 On some systems, this modifies the behavior of executables and/or directories.
129 The lower 9 bits specify read/write/execute permissions
130 for world, group, and user following standard POSIX conventions.
133 The numeric user id and group id of the owner.
135 The number of links to this file.
136 Directories always have a value of at least two here.
137 Note that hardlinked files include file data with every copy in the archive.
139 For block special and character special entries,
140 this field contains the associated device number.
141 For all other entry types, it should be set to zero by writers
142 and ignored by readers.
144 Modification time of the file, indicated as the number
145 of seconds since the start of the epoch,
146 00:00:00 UTC January 1, 1970.
147 The four-byte integer is stored with the most-significant 16 bits first
148 followed by the least-significant 16 bits.
149 Each of the two 16 bit values are stored in machine-native byte order.
151 The number of bytes in the pathname that follows the header.
153 The size of the file.
154 Note that this archive format is limited to
155 four gigabyte file sizes.
158 above for notes on the storage of four-byte integers.
161 The pathname immediately follows the fixed header.
162 If the pathname has an odd length, an additional NULL
163 byte is added after the pathname.
164 The file data is then appended, padded with NULL
165 bytes to an even length.
166 .Ss Portable ASCII Format
168 standardized an ASCII variant that is portable across all
170 It is commonly known as the
173 It stores the numeric fields as 6-character or 11-character
175 .Bd -literal -offset indent
176 struct cpio_odc_header {
191 Except as specified below, the fields here match those specified
192 for the old binary format above.
193 .Bl -tag -width indent
198 The count here includes a terminating NULL byte.
201 The name and file body follow the fixed header.
202 If the files being archived are themselves entirely ASCII, then
203 the resulting archive will be entirely ASCII, except for the
204 NULL byte that terminates the name field.
206 The "new" ASCII format uses 8-byte hexadecimal fields for
207 all numbers and separates device numbers into separate fields
208 for major and minor numbers.
209 .Bd -literal -offset indent
210 struct cpio_newc_header {
228 Except as specified below, the fields here match those specified
229 for the old binary format above.
230 .Bl -tag -width indent
235 This field is always set to zero by writers and ignored by readers.
236 See the next section for more details.
239 The pathname is followed by NULL bytes so that the total size
240 of the fixed header plus pathname is a multiple of four.
241 Likewise, the file data is padded to a multiple of four bytes.
242 Note that this format supports only 4 gigabyte files (unlike the
243 older ASCII format, which supports 8 gigabyte files).
245 The CRC format is identical to the new ASCII format described
246 in the previous section except that the magic field is set
251 field is set to the sum of all bytes in the file data.
252 This sum is computed treating all bytes as unsigned values
253 and using unsigned arithmetic.
254 Only the least-significant 32 bits of the sum are stored.
258 implementation distributed with HPUX used XXXX but stored
259 device numbers differently XXX.
260 .Ss Other Extensions and Variants
261 Sun Solaris uses additional file types to store extended file
262 data, including ACLs and extended attributes, as special
263 entries in cpio archives.
269 format is mis-named, as it uses a simple checksum and
270 not a cyclic redundancy check.
272 The old binary format is limited to 16 bits for user id,
273 group id, device, and inode numbers.
274 It is limited to 4 gigabyte file sizes.
276 The old ASCII format is limited to 18 bits for
277 the user id, group id, device, and inode numbers.
278 It is limited to 8 gigabyte file sizes.
280 The new ASCII format is limited to 4 gigabyte file sizes.
282 None of the cpio formats store user or group names,
283 which are essential when moving files between systems with
284 dissimilar user or group numbering.
286 Especially when writing older cpio variants, it may be necessary
287 to map actual device/inode values to synthesized values that
288 fit the available fields.
289 With very large filesystems, this may be necessary even for
297 utility is no longer a part of POSIX or the Single Unix Standard.
300 It has been supplanted in subsequent standards by
302 The portable ASCII format is currently part of the specification for the
306 The original cpio utility was written by Dick Haight
307 while working in AT&T's Unix Support Group.
308 It was first available in PWB/UNIX 1.0, the
309 .Dq Programmer's Work Bench
312 that was used internally at AT&T.
313 It became more generally available as part of