github.com/mvdan/u-root-coreutils@v0.0.0-20230122170626-c2eef2898555/cmds/core/cpio/cpio.formats (about) 1 CPIO(5) FreeBSD File Formats Manual CPIO(5) 2 3 NAME 4 cpio -- format of cpio archive files 5 6 DESCRIPTION 7 The cpio archive format collects any number of files, directories, and 8 other file system objects (symbolic links, device nodes, etc.) into a 9 single stream of bytes. 10 11 General Format 12 Each file system object in a cpio archive comprises a header record with 13 basic numeric metadata followed by the full pathname of the entry and the 14 file data. The header record stores a series of integer values that gen- 15 erally follow the fields in struct stat. (See stat(2) for details.) The 16 variants differ primarily in how they store those integers (binary, 17 octal, or hexadecimal). The header is followed by the pathname of the 18 entry (the length of the pathname is stored in the header) and any file 19 data. The end of the archive is indicated by a special record with the 20 pathname ``TRAILER!!!''. 21 22 PWB format 23 XXX Any documentation of the original PWB/UNIX 1.0 format? XXX 24 25 Old Binary Format 26 The old binary cpio format stores numbers as 2-byte and 4-byte binary 27 values. Each entry begins with a header in the following format: 28 29 struct header_old_cpio { 30 unsigned short c_magic; 31 unsigned short c_dev; 32 unsigned short c_ino; 33 unsigned short c_mode; 34 unsigned short c_uid; 35 unsigned short c_gid; 36 unsigned short c_nlink; 37 unsigned short c_rdev; 38 unsigned short c_mtime[2]; 39 unsigned short c_namesize; 40 unsigned short c_filesize[2]; 41 }; 42 43 The unsigned short fields here are 16-bit integer values; the unsigned 44 int fields are 32-bit integer values. The fields are as follows 45 46 magic The integer value octal 070707. This value can be used to deter- 47 mine whether this archive is written with little-endian or big- 48 endian integers. 49 50 dev, ino 51 The device and inode numbers from the disk. These are used by 52 programs that read cpio archives to determine when two entries 53 refer to the same file. Programs that synthesize cpio archives 54 should be careful to set these to distinct values for each entry. 55 56 mode The mode specifies both the regular permissions and the file 57 type. It consists of several bit fields as follows: 58 0170000 This masks the file type bits. 59 0140000 File type value for sockets. 60 0120000 File type value for symbolic links. For symbolic links, 61 the link body is stored as file data. 62 0100000 File type value for regular files. 63 0060000 File type value for block special devices. 64 0040000 File type value for directories. 65 0020000 File type value for character special devices. 66 0010000 File type value for named pipes or FIFOs. 67 0004000 SUID bit. 68 0002000 SGID bit. 69 0001000 Sticky bit. On some systems, this modifies the behavior 70 of executables and/or directories. 71 0000777 The lower 9 bits specify read/write/execute permissions 72 for world, group, and user following standard POSIX con- 73 ventions. 74 75 uid, gid 76 The numeric user id and group id of the owner. 77 78 nlink The number of links to this file. Directories always have a 79 value of at least two here. Note that hardlinked files include 80 file data with every copy in the archive. 81 82 rdev For block special and character special entries, this field con- 83 tains the associated device number. For all other entry types, 84 it should be set to zero by writers and ignored by readers. 85 86 mtime Modification time of the file, indicated as the number of seconds 87 since the start of the epoch, 00:00:00 UTC January 1, 1970. The 88 four-byte integer is stored with the most-significant 16 bits 89 first followed by the least-significant 16 bits. Each of the two 90 16 bit values are stored in machine-native byte order. 91 92 namesize 93 The number of bytes in the pathname that follows the header. 94 This count includes the trailing NUL byte. 95 96 filesize 97 The size of the file. Note that this archive format is limited 98 to four gigabyte file sizes. See mtime above for a description 99 of the storage of four-byte integers. 100 101 The pathname immediately follows the fixed header. If the namesize is 102 odd, an additional NUL byte is added after the pathname. The file data 103 is then appended, padded with NUL bytes to an even length. 104 105 Hardlinked files are not given special treatment; the full file contents 106 are included with each copy of the file. 107 108 Portable ASCII Format 109 Version 2 of the Single UNIX Specification (``SUSv2'') standardized an 110 ASCII variant that is portable across all platforms. It is commonly 111 known as the ``old character'' format or as the ``odc'' format. It 112 stores the same numeric fields as the old binary format, but represents 113 them as 6-character or 11-character octal values. 114 115 struct cpio_odc_header { 116 char c_magic[6]; 117 char c_dev[6]; 118 char c_ino[6]; 119 char c_mode[6]; 120 char c_uid[6]; 121 char c_gid[6]; 122 char c_nlink[6]; 123 char c_rdev[6]; 124 char c_mtime[11]; 125 char c_namesize[6]; 126 char c_filesize[11]; 127 }; 128 129 The fields are identical to those in the old binary format. The name and 130 file body follow the fixed header. Unlike the old binary format, there 131 is no additional padding after the pathname or file contents. If the 132 files being archived are themselves entirely ASCII, then the resulting 133 archive will be entirely ASCII, except for the NUL byte that terminates 134 the name field. 135 136 New ASCII Format 137 The "new" ASCII format uses 8-byte hexadecimal fields for all numbers and 138 separates device numbers into separate fields for major and minor num- 139 bers. 140 141 struct cpio_newc_header { 142 char c_magic[6]; 143 char c_ino[8]; 144 char c_mode[8]; 145 char c_uid[8]; 146 char c_gid[8]; 147 char c_nlink[8]; 148 char c_mtime[8]; 149 char c_filesize[8]; 150 char c_devmajor[8]; 151 char c_devminor[8]; 152 char c_rdevmajor[8]; 153 char c_rdevminor[8]; 154 char c_namesize[8]; 155 char c_check[8]; 156 }; 157 158 Except as specified below, the fields here match those specified for the 159 old binary format above. 160 161 magic The string ``070701''. 162 163 check This field is always set to zero by writers and ignored by read- 164 ers. See the next section for more details. 165 166 The pathname is followed by NUL bytes so that the total size of the fixed 167 header plus pathname is a multiple of four. Likewise, the file data is 168 padded to a multiple of four bytes. Note that this format supports only 169 4 gigabyte files (unlike the older ASCII format, which supports 8 giga- 170 byte files). 171 172 In this format, hardlinked files are handled by setting the filesize to 173 zero for each entry except the last one that appears in the archive. 174 175 New CRC Format 176 The CRC format is identical to the new ASCII format described in the pre- 177 vious section except that the magic field is set to ``070702'' and the 178 check field is set to the sum of all bytes in the file data. This sum is 179 computed treating all bytes as unsigned values and using unsigned arith- 180 metic. Only the least-significant 32 bits of the sum are stored. 181 182 HP variants 183 The cpio implementation distributed with HPUX used XXXX but stored device 184 numbers differently XXX. 185 186 Other Extensions and Variants 187 Sun Solaris uses additional file types to store extended file data, 188 including ACLs and extended attributes, as special entries in cpio ar- 189 chives. 190 191 XXX Others? XXX 192 193 BUGS 194 The ``CRC'' format is mis-named, as it uses a simple checksum and not a 195 cyclic redundancy check. 196 197 The old binary format is limited to 16 bits for user id, group id, 198 device, and inode numbers. It is limited to 4 gigabyte file sizes. 199 200 The old ASCII format is limited to 18 bits for the user id, group id, 201 device, and inode numbers. It is limited to 8 gigabyte file sizes. 202 203 The new ASCII format is limited to 4 gigabyte file sizes. 204 205 None of the cpio formats store user or group names, which are essential 206 when moving files between systems with dissimilar user or group number- 207 ing. 208 209 Especially when writing older cpio variants, it may be necessary to map 210 actual device/inode values to synthesized values that fit the available 211 fields. With very large filesystems, this may be necessary even for the 212 newer formats. 213 214 SEE ALSO 215 cpio(1), tar(5) 216 217 STANDARDS 218 The cpio utility is no longer a part of POSIX or the Single Unix Stan- 219 dard. It last appeared in Version 2 of the Single UNIX Specification 220 (``SUSv2''). It has been supplanted in subsequent standards by pax(1). 221 The portable ASCII format is currently part of the specification for the 222 pax(1) utility. 223 224 HISTORY 225 The original cpio utility was written by Dick Haight while working in 226 AT&T's Unix Support Group. It appeared in 1977 as part of PWB/UNIX 1.0, 227 the ``Programmer's Work Bench'' derived from Version 6 AT&T UNIX that was 228 used internally at AT&T. Both the old binary and old character formats 229 were in use by 1980, according to the System III source released by SCO 230 under their ``Ancient Unix'' license. The character format was adopted 231 as part of IEEE Std 1003.1-1988 (``POSIX.1''). XXX when did "newc" 232 appear? Who invented it? When did HP come out with their variant? When 233 did Sun introduce ACLs and extended attributes? XXX 234 235 FreeBSD 6.0 October 5, 2007 FreeBSD 6.0 236 237 238