# gitformat-index

## NAME



           gitformat-index - Git index format
    


## SYNOPSIS



           $GIT_DIR/index
    


## DESCRIPTION



           Git index format
    


## THE GIT INDEX FILE HAS THE FOLLOWING FORMAT



               All binary numbers are in network byte order.
               In a repository using the traditional SHA-1, checksums and object IDs
               (object names) mentioned below are all computed using SHA-1.  Similarly,
               in SHA-256 repositories, these values are computed using SHA-256.
               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.
    
                   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
    
           •   Hash checksum 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.
    
               An index entry typically represents a file. However, if sparse-checkout
               is enabled in cone mode (`core.sparseCheckoutCone` is enabled) and the
               `extensions.sparseIndex` extension is enabled, then the index may
               contain entries for directories outside of the sparse-checkout definition.
               These entries have mode `040000`, include the `SKIP_WORKTREE` bit, and
               the path ends in a directory separator.
    
               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)
    
               16-bit unused, must be zero
    
               4-bit object type
                 valid values in binary are 1000 (regular file), 1010 (symbolic link)
                 and 1110 (gitlink)
    
               3-bit unused, must be zero
    
               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.
    
               Object name 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 linkgit:gitformat-pack[5]) 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.
    
               Interpretation of index entries in split index mode is completely
               different. See below for details.
    


## EXTENSIONS



       **Cache tree**
               Since the index does not record entries for directories, the cache
               entries cannot describe tree objects that already exist in the object
               database for regions of the index that are unchanged from an existing
               commit. The cache tree extension stores a recursive tree structure that
               describes the trees that already exist and completely match sections of
               the cache entries. This speeds up tree object generation from the index
               for a new commit by only computing the trees that are "new" to that
               commit. It also assists when comparing the index to another tree, such
               as `HEAD^{tree}`, since sections of the index can be skipped when a tree
               comparison demonstrates equality.
    
               The recursive tree structure uses nodes that store a number of cache
               entries, a list of subnodes, and an object ID (OID). The OID references
               the existing tree for that node, if it is known to exist. The subnodes
               correspond to subdirectories that themselves have cache tree nodes. The
               number of cache entries corresponds to the number of cache entries in
               the index that describe paths within that tree's directory.
    
               The extension tracks the full directory structure in the cache tree
               extension, but this is generally smaller than the full cache entry list.
    
               When a path is updated in index, Git invalidates all nodes of the
               recursive cache tree corresponding to the parent directories of that
               path. We store these tree nodes as being "invalid" by using "-1" as the
               number of cache entries. Invalid nodes still store a span of index
               entries, allowing Git to focus its efforts when reconstructing a full
               cache tree.
    
               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
    
           •   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), and so on. The specified number of subtrees
                   indicates when the current level of the recursive stack is complete.
    
       **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 resolution
               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 object names of the entry in stages from 1 to 3
               (nothing is written for a missing stage).
    
       **Split index**
               In split index mode, the majority of index entries could be stored
               in a separate file. This extension records the changes to be made on
               top of that to produce the final index.
    
               The signature for this extension is { 'l', 'i', 'n', 'k' }.
    
               The extension consists of:
    
           •   Hash of the shared index file. The shared index file path is
               $GIT_DIR/sharedindex.<hash>. If all bits are zero, the index
               does not require a shared index file.
    
           •   An ewah-encoded delete bitmap, each bit represents an entry in
               the shared index. If a bit is set, its corresponding entry in
               the shared index will be removed from the final index. Note,
               because a delete operation changes index entry positions, but
               we do need original positions in replace phase, it’s best to
               just mark entries for removal, then do a mass deletion after
               replacement.
    
           •   An ewah-encoded replace bitmap, each bit represents an entry
               in the shared index. If a bit is set, its corresponding entry
               in the shared index will be replaced with an entry in this
               index file. All replaced entries are stored in sorted order in
               this index. The first "1" bit in the replace bitmap
               corresponds to the first index entry, the second "1" bit to
               the second entry and so on. Replaced entries may have empty
               path names to save space.
    
                   The remaining index entries after replaced ones will be added to the
                   final index. These added entries are also sorted by entry name then
                   stage.
    


## UNTRACKED CACHE



               Untracked cache saves the untracked file list and necessary data to
               verify the cache. The signature for this extension is { 'U', 'N',
               'T', 'R' }.
    
               The extension starts with
    
           •   A sequence of NUL-terminated strings, preceded by the size of
               the sequence in variable width encoding. Each string describes
               the environment where the cache can be used.
    
           •   Stat data of $GIT_DIR/info/exclude. See "Index entry" section
               from ctime field until "file size".
    
           •   Stat data of core.excludesFile
    
           •   32-bit dir_flags (see struct dir_struct)
    
           •   Hash of $GIT_DIR/info/exclude. A null hash means the file does
               not exist.
    
           •   Hash of core.excludesFile. A null hash means the file does not
               exist.
    
           •   NUL-terminated string of per-dir exclude file name. This
               usually is ".gitignore".
    
           •   The number of following directory blocks, variable width
               encoding. If this number is zero, the extension ends here with
               a following NUL.
    
           •   A number of directory blocks in depth-first-search order, each
               consists of
    
           •   The number of untracked entries, variable width encoding.
    
           •   The number of sub-directory blocks, variable width encoding.
    
           •   The directory name terminated by NUL.
    
           •   A number of untracked file/dir names terminated by NUL.
    
           The remaining data of each directory block is grouped by type:
    
           •   An ewah bitmap, the n-th bit marks whether the n-th directory
               has valid untracked cache entries.
    
           •   An ewah bitmap, the n-th bit records "check-only" bit of
               read_directory_recursive() for the n-th directory.
    
           •   An ewah bitmap, the n-th bit indicates whether hash and stat
               data is valid for the n-th directory and exists in the next
               data.
    
           •   An array of stat data. The n-th data corresponds with the n-th
               "one" bit in the previous ewah bitmap.
    
           •   An array of hashes. The n-th hash corresponds with the n-th
               "one" bit in the previous ewah bitmap.
    
           •   One NUL.
    


## FILE SYSTEM MONITOR CACHE



               The file system monitor cache tracks files for which the core.fsmonitor
               hook has told us about changes.  The signature for this extension is
               { 'F', 'S', 'M', 'N' }.
    
               The extension starts with
    
           •   32-bit version number: the current supported versions are 1
               and 2.
    
           •   (Version 1) 64-bit time: the extension data reflects all
               changes through the given time which is stored as the
               nanoseconds elapsed since midnight, January 1, 1970.
    
           •   (Version 2) A null terminated string: an opaque token defined
               by the file system monitor application. The extension data
               reflects all changes relative to that token.
    
           •   32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap.
    
           •   An ewah bitmap, the n-th bit indicates whether the n-th index
               entry is not CE_FSMONITOR_VALID.
    


## END OF INDEX ENTRY



               The End of Index Entry (EOIE) is used to locate the end of the variable
               length index entries and the beginning of the extensions. Code can take
               advantage of this to quickly locate the index extensions without having
               to parse through all of the index entries.
    
               Because it must be able to be loaded before the variable length cache
               entries and other index extensions, this extension must be written last.
               The signature for this extension is { 'E', 'O', 'I', 'E' }.
    
               The extension consists of:
    
           •   32-bit offset to the end of the index entries
    
           •   Hash over the extension types and their sizes (but not their
               contents). E.g. if we have "TREE" extension that is N-bytes
               long, "REUC" extension that is M-bytes long, followed by
               "EOIE", then the hash would be:
    
                   Hash("TREE" + <binary-representation-of-N> +
                           "REUC" + <binary-representation-of-M>)
    


## INDEX ENTRY OFFSET TABLE



               The Index Entry Offset Table (IEOT) is used to help address the CPU
               cost of loading the index by enabling multi-threading the process of
               converting cache entries from the on-disk format to the in-memory format.
               The signature for this extension is { 'I', 'E', 'O', 'T' }.
    
               The extension consists of:
    
           •   32-bit version (currently 1)
    
           •   A number of index offset entries each consisting of:
    
           •   32-bit offset from the beginning of the file to the first
               cache entry in this block of entries.
    
           •   32-bit count of cache entries in this block
    


## SPARSE DIRECTORY ENTRIES



               When using sparse-checkout in cone mode, some entire directories within
               the index can be summarized by pointing to a tree object instead of the
               entire expanded list of paths within that tree. An index containing such
               entries is a "sparse index". Index format versions 4 and less were not
               implemented with such entries in mind. Thus, for these versions, an
               index containing sparse directory entries will include this extension
               with signature { 's', 'd', 'i', 'r' }. Like the split-index extension,
               tools should avoid interacting with a sparse index unless they understand
               this extension.
    


## GIT



           Part of the git(1) suite
    


* * *