Documentation/technical/index-format.txt

   1 Git index format
   2 ================
   3
   4 == The Git index file has the following format
   5
   6   All binary numbers are in network byte order. Version 2 is described
   7   here unless stated otherwise.
   8
   9    - A 12-byte header consisting of
  10
  11      4-byte signature:
  12        The signature is { 'D', 'I', 'R', 'C' } (stands for "dircache")
  13
  14      4-byte version number:
  15        The current supported versions are 2, 3 and 4.
  16
  17      32-bit number of index entries.
  18
  19    - A number of sorted index entries (see below).
  20
  21    - Extensions
  22
  23      Extensions are identified by signature. Optional extensions can
  24      be ignored if Git does not understand them.
  25
  26      Git currently supports cached tree and resolve undo extensions.
  27
  28      4-byte extension signature. If the first byte is 'A'..'Z' the
  29      extension is optional and can be ignored.
  30
  31      32-bit size of the extension
  32
  33      Extension data
  34
  35    - 160-bit SHA-1 over the content of the index file before this
  36      checksum.
  37
  38 == Index entry
  39
  40   Index entries are sorted in ascending order on the name field,
  41   interpreted as a string of unsigned bytes (i.e. memcmp() order, no
  42   localization, no special casing of directory separator '/'). Entries
  43   with the same name are sorted by their stage field.
  44
  45   32-bit ctime seconds, the last time a file's metadata changed
  46     this is stat(2) data
  47
  48   32-bit ctime nanosecond fractions
  49     this is stat(2) data
  50
  51   32-bit mtime seconds, the last time a file's data changed
  52     this is stat(2) data
  53
  54   32-bit mtime nanosecond fractions
  55     this is stat(2) data
  56
  57   32-bit dev
  58     this is stat(2) data
  59
  60   32-bit ino
  61     this is stat(2) data
  62
  63   32-bit mode, split into (high to low bits)
  64
  65     4-bit object type
  66       valid values in binary are 1000 (regular file), 1010 (symbolic link)
  67       and 1110 (gitlink)
  68
  69     3-bit unused
  70
  71     9-bit unix permission. Only 0755 and 0644 are valid for regular files.
  72     Symbolic links and gitlinks have value 0 in this field.
  73
  74   32-bit uid
  75     this is stat(2) data
  76
  77   32-bit gid
  78     this is stat(2) data
  79
  80   32-bit file size
  81     This is the on-disk size from stat(2), truncated to 32-bit.
  82
  83   160-bit SHA-1 for the represented object
  84
  85   A 16-bit 'flags' field split into (high to low bits)
  86
  87     1-bit assume-valid flag
  88
  89     1-bit extended flag (must be zero in version 2)
  90
  91     2-bit stage (during merge)
  92
  93     12-bit name length if the length is less than 0xFFF; otherwise 0xFFF
  94     is stored in this field.
  95
  96   (Version 3 or later) A 16-bit field, only applicable if the
  97   "extended flag" above is 1, split into (high to low bits).
  98
  99     1-bit reserved for future
 100
 101     1-bit skip-worktree flag (used by sparse checkout)
 102
 103     1-bit intent-to-add flag (used by "git add -N")
 104
 105     13-bit unused, must be zero
 106
 107   Entry path name (variable length) relative to top level directory
 108     (without leading slash). '/' is used as path separator. The special
 109     path components ".", ".." and ".git" (without quotes) are disallowed.
 110     Trailing slash is also disallowed.
 111
 112     The exact encoding is undefined, but the '.' and '/' characters
 113     are encoded in 7-bit ASCII and the encoding cannot contain a NUL
 114     byte (iow, this is a UNIX pathname).
 115
 116   (Version 4) In version 4, the entry path name is prefix-compressed
 117     relative to the path name for the previous entry (the very first
 118     entry is encoded as if the path name for the previous entry is an
 119     empty string).  At the beginning of an entry, an integer N in the
 120     variable width encoding (the same encoding as the offset is encoded
 121     for OFS_DELTA pack entries; see pack-format.txt) is stored, followed
 122     by a NUL-terminated string S.  Removing N bytes from the end of the
 123     path name for the previous entry, and replacing it with the string S
 124     yields the path name for this entry.
 125
 126   1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes
 127   while keeping the name NUL-terminated.
 128
 129   (Version 4) In version 4, the padding after the pathname does not
 130   exist.
 131
 132   Interpretation of index entries in split index mode is completely
 133   different. See below for details.
 134
 135 == Extensions
 136
 137 === Cached tree
 138
 139   Cached tree extension contains pre-computed hashes for trees that can
 140   be derived from the index. It helps speed up tree object generation
 141   from index for a new commit.
 142
 143   When a path is updated in index, the path must be invalidated and
 144   removed from tree cache.
 145
 146   The signature for this extension is { 'T', 'R', 'E', 'E' }.
 147
 148   A series of entries fill the entire extension; each of which
 149   consists of:
 150
 151   - NUL-terminated path component (relative to its parent directory);
 152
 153   - ASCII decimal number of entries in the index that is covered by the
 154     tree this entry represents (entry_count);
 155
 156   - A space (ASCII 32);
 157
 158   - ASCII decimal number that represents the number of subtrees this
 159     tree has;
 160
 161   - A newline (ASCII 10); and
 162
 163   - 160-bit object name for the object that would result from writing
 164     this span of index as a tree.
 165
 166   An entry can be in an invalidated state and is represented by having
 167   a negative number in the entry_count field. In this case, there is no
 168   object name and the next entry starts immediately after the newline.
 169   When writing an invalid entry, -1 should always be used as entry_count.
 170
 171   The entries are written out in the top-down, depth-first order.  The
 172   first entry represents the root level of the repository, followed by the
 173   first subtree---let's call this A---of the root level (with its name
 174   relative to the root level), followed by the first subtree of A (with
 175   its name relative to A), ...
 176
 177 === Resolve undo
 178
 179   A conflict is represented in the index as a set of higher stage entries.
 180   When a conflict is resolved (e.g. with "git add path"), these higher
 181   stage entries will be removed and a stage-0 entry with proper resolution
 182   is added.
 183
 184   When these higher stage entries are removed, they are saved in the
 185   resolve undo extension, so that conflicts can be recreated (e.g. with
 186   "git checkout -m"), in case users want to redo a conflict resolution
 187   from scratch.
 188
 189   The signature for this extension is { 'R', 'E', 'U', 'C' }.
 190
 191   A series of entries fill the entire extension; each of which
 192   consists of:
 193
 194   - NUL-terminated pathname the entry describes (relative to the root of
 195     the repository, i.e. full pathname);
 196
 197   - Three NUL-terminated ASCII octal numbers, entry mode of entries in
 198     stage 1 to 3 (a missing stage is represented by "0" in this field);
 199     and
 200
 201   - At most three 160-bit object names of the entry in stages from 1 to 3
 202     (nothing is written for a missing stage).
 203
 204 === Split index
 205
 206   In split index mode, the majority of index entries could be stored
 207   in a separate file. This extension records the changes to be made on
 208   top of that to produce the final index.
 209
 210   The signature for this extension is { 'l', 'i', 'n', 'k' }.
 211
 212   The extension consists of:
 213
 214   - 160-bit SHA-1 of the shared index file. The shared index file path
 215     is $GIT_DIR/sharedindex.<SHA-1>. If all 160 bits are zero, the
 216     index does not require a shared index file.
 217
 218   - An ewah-encoded delete bitmap, each bit represents an entry in the
 219     shared index. If a bit is set, its corresponding entry in the
 220     shared index will be removed from the final index.  Note, because
 221     a delete operation changes index entry positions, but we do need
 222     original positions in replace phase, it's best to just mark
 223     entries for removal, then do a mass deletion after replacement.
 224
 225   - An ewah-encoded replace bitmap, each bit represents an entry in
 226     the shared index. If a bit is set, its corresponding entry in the
 227     shared index will be replaced with an entry in this index
 228     file. All replaced entries are stored in sorted order in this
 229     index. The first "1" bit in the replace bitmap corresponds to the
 230     first index entry, the second "1" bit to the second entry and so
 231     on. Replaced entries may have empty path names to save space.
 232
 233   The remaining index entries after replaced ones will be added to the
 234   final index. These added entries are also sorted by entry name then
 235   stage.
 236
 237 == Untracked cache
 238
 239   Untracked cache saves the untracked file list and necessary data to
 240   verify the cache. The signature for this extension is { 'U', 'N',
 241   'T', 'R' }.
 242
 243   The extension starts with
 244
 245   - A sequence of NUL-terminated strings, preceded by the size of the
 246     sequence in variable width encoding. Each string describes the
 247     environment where the cache can be used.
 248
 249   - Stat data of $GIT_DIR/info/exclude. See "Index entry" section from
 250     ctime field until "file size".
 251
 252   - Stat data of core.excludesfile
 253
 254   - 32-bit dir_flags (see struct dir_struct)
 255
 256   - 160-bit SHA-1 of $GIT_DIR/info/exclude. Null SHA-1 means the file
 257     does not exist.
 258
 259   - 160-bit SHA-1 of core.excludesfile. Null SHA-1 means the file does
 260     not exist.
 261
 262   - NUL-terminated string of per-dir exclude file name. This usually
 263     is ".gitignore".
 264
 265   - The number of following directory blocks, variable width
 266     encoding. If this number is zero, the extension ends here with a
 267     following NUL.
 268
 269   - A number of directory blocks in depth-first-search order, each
 270     consists of
 271
 272     - The number of untracked entries, variable width encoding.
 273
 274     - The number of sub-directory blocks, variable width encoding.
 275
 276     - The directory name terminated by NUL.
 277
 278     - A number of untrached file/dir names terminated by NUL.
 279
 280 The remaining data of each directory block is grouped by type:
 281
 282   - An ewah bitmap, the n-th bit marks whether the n-th directory has
 283     valid untracked cache entries.
 284
 285   - An ewah bitmap, the n-th bit records "check-only" bit of
 286     read_directory_recursive() for the n-th directory.
 287
 288   - An ewah bitmap, the n-th bit indicates whether SHA-1 and stat data
 289     is valid for the n-th directory and exists in the next data.
 290
 291   - An array of stat data. The n-th data corresponds with the n-th
 292     "one" bit in the previous ewah bitmap.
 293
 294   - An array of SHA-1. The n-th SHA-1 corresponds with the n-th "one" bit
 295     in the previous ewah bitmap.
 296
 297   - One NUL.