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authorNguyễn Thái Ngọc Duy <pclouds@gmail.com>2010-09-06 10:37:10 (GMT)
committerJunio C Hamano <gitster@pobox.com>2011-02-27 09:21:27 (GMT)
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doc: technical details about the index file format
This bases on the original work by Robin Rosenberg. Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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+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' }
+
+ 4-byte version number:
+ The current supported versions are 2 and 3.
+
+ 32-bit number of index entries.
+
+ - A number of sorted index entries
+
+ - Extensions
+
+ Extensions are identified by signature. Optional extensions can
+ be ignored if GIT does not understand them.
+
+ GIT currently supports tree cache 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. 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 (blob), 1010 (symbolic link)
+ and 1110 (gitlink)
+
+ 3-bit unused
+
+ 9-bit unix permission (only 0755 and 0644 are valid)
+
+ 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)
+
+ 160-bit SHA-1 for the represented object
+
+ A 16-bit 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 0x0FFF
+
+ (Version 3) 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
+ paths ".", ".." 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. Generally a superset of ASCII.
+
+ 1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes
+ while keeping the name NUL-terminated.
+
+== Extensions
+
+=== Tree cache
+
+ Tree cache 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.
+
+ - Extension tag { 'T', 'R', 'E', 'E' }
+
+ - 32-bit size
+
+ - A number of entries
+
+ NUL-terminated tree name
+
+ Blank-terminated ASCII decimal number of entries in this tree
+
+ Newline-terminated position of this tree in the parent tree. 0 for
+ the root tree
+
+ 160-bit SHA-1 for this tree and it's children
+
+=== Resolve undo
+
+ A conflict is represented in 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.
+
+ Resolve undo extension saves these higher stage entries so that
+ conflicts can be recreated (e.g. with "git checkout -m"), in case
+ users want to redo a conflict resolution from scratch.
+
+ - Extension tag { 'R', 'E', 'U', 'C' }
+
+ - 32-bit size
+
+ - A number of conflict entries
+
+ NUL-terminated conflict path
+
+ Three NUL-terminated ASCII octal numbers, entry mode of entries in
+ stage 1 to 3.
+
+ At most three 160-bit SHA-1s of the entry in three stages from 1
+ to 3. SHA-1 is not saved for any stage with entry mode zero.