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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.
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