# Git index format

## The Git index file has the following format

    All binary numbers are in network byte order. Version 2 is described
    here unless stated otherwise.

- A 12-byte header consisting of

      4-byte signature:
        The signature is { 'D', 'I', 'R', 'C' } (stands for "dircache")

      4-byte version number:
        The current supported versions are 2, 3 and 4.

      32-bit number of index entries.

- A number of sorted index entries (see below).

- Extensions

      Extensions are identified by signature. Optional extensions can
      be ignored if Git does not understand them.

      Git currently supports cached tree and resolve undo extensions.

      4-byte extension signature. If the first byte is 'A'..'Z' the
      extension is optional and can be ignored.

      32-bit size of the extension

      Extension data

- 160-bit SHA-1 over the content of the index file before this checksum.

## Index entry

    Index entries are sorted in ascending order on the name field,
    interpreted as a string of unsigned bytes (i.e. memcmp() order, no
    localization, no special casing of directory separator '/'). Entries
    with the same name are sorted by their stage field.

    32-bit ctime seconds, the last time a file's metadata changed
      this is stat(2) data

    32-bit ctime nanosecond fractions
      this is stat(2) data

    32-bit mtime seconds, the last time a file's data changed
      this is stat(2) data

    32-bit mtime nanosecond fractions
      this is stat(2) data

    32-bit dev
      this is stat(2) data

    32-bit ino
      this is stat(2) data

    32-bit mode, split into (high to low bits)

    4-bit object type
      valid values in binary are 1000 (regular file), 1010 (symbolic link)
      and 1110 (gitlink)

    3-bit unused

    9-bit unix permission. Only 0755 and 0644 are valid for regular files.
    Symbolic links and gitlinks have value 0 in this field.

    32-bit uid
      this is stat(2) data

    32-bit gid
      this is stat(2) data

    32-bit file size
      This is the on-disk size from stat(2), truncated to 32-bit.

    160-bit SHA-1 for the represented object

    A 16-bit 'flags' field split into (high to low bits)

    1-bit assume-valid flag

    1-bit extended flag (must be zero in version 2)

    2-bit stage (during merge)

    12-bit name length if the length is less than 0xFFF; otherwise 0xFFF
    is stored in this field.

    (Version 3 or later) A 16-bit field, only applicable if the
    "extended flag" above is 1, split into (high to low bits).

    1-bit reserved for future

    1-bit skip-worktree flag (used by sparse checkout)

    1-bit intent-to-add flag (used by "git add -N")

    13-bit unused, must be zero

    Entry path name (variable length) relative to top level directory
      (without leading slash). '/' is used as path separator. The special
      path components ".", ".." and ".git" (without quotes) are disallowed.
      Trailing slash is also disallowed.

    The exact encoding is undefined, but the '.' and '/' characters
    are encoded in 7-bit ASCII and the encoding cannot contain a NUL
    byte (iow, this is a UNIX pathname).

    (Version 4) In version 4, the entry path name is prefix-compressed
      relative to the path name for the previous entry (the very first
      entry is encoded as if the path name for the previous entry is an
      empty string).  At the beginning of an entry, an integer N in the
      variable width encoding (the same encoding as the offset is encoded
      for OFS_DELTA pack entries; see pack-format.txt) is stored, followed
      by a NUL-terminated string S.  Removing N bytes from the end of the
      path name for the previous entry, and replacing it with the string S
      yields the path name for this entry.

    1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes
    while keeping the name NUL-terminated.

    (Version 4) In version 4, the padding after the pathname does not
    exist.

## Extensions

### Cached tree

    Cached tree extension contains pre-computed hashes for trees that can
    be derived from the index. It helps speed up tree object generation
    from index for a new commit.

    When a path is updated in index, the path must be invalidated and
    removed from tree cache.

    The signature for this extension is { 'T', 'R', 'E', 'E' }.

    A series of entries fill the entire extension; each of which
    consists of:

- NUL-terminated path component (relative to its parent directory);

- ASCII decimal number of entries in the index that is covered by the tree this entry represents (entry_count);

- A space (ASCII 32);

- ASCII decimal number that represents the number of subtrees this tree has;

- A newline (ASCII 10); and

- 160-bit object name for the object that would result from writing this span of index as a tree.

      An entry can be in an invalidated state and is represented by having
      a negative number in the entry_count field. In this case, there is no
      object name and the next entry starts immediately after the newline.
      When writing an invalid entry, -1 should always be used as entry_count.

      The entries are written out in the top-down, depth-first order.  The
      first entry represents the root level of the repository, followed by the
      first subtree---let's call this A---of the root level (with its name
      relative to the root level), followed by the first subtree of A (with
      its name relative to A), ...

### Resolve undo

    A conflict is represented in the index as a set of higher stage entries.
    When a conflict is resolved (e.g. with "git add path"), these higher
    stage entries will be removed and a stage-0 entry with proper resoluton
    is added.

    When these higher stage entries are removed, they are saved in the
    resolve undo extension, so that conflicts can be recreated (e.g. with
    "git checkout -m"), in case users want to redo a conflict resolution
    from scratch.

    The signature for this extension is { 'R', 'E', 'U', 'C' }.

    A series of entries fill the entire extension; each of which
    consists of:

- NUL-terminated pathname the entry describes (relative to the root of the repository, i.e. full pathname);

- Three NUL-terminated ASCII octal numbers, entry mode of entries in stage 1 to 3 (a missing stage is represented by "0" in this field); and

- At most three 160-bit object names of the entry in stages from 1 to 3 (nothing is written for a missing stage).