1Git 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== Extensions 133 134=== Cached tree 135 136 Cached tree extension contains pre-computed hashes for trees that can 137 be derived from the index. It helps speed up tree object generation 138 from index for a new commit. 139 140 When a path is updated in index, the path must be invalidated and 141 removed from tree cache. 142 143 The signature for this extension is { 'T', 'R', 'E', 'E' }. 144 145 A series of entries fill the entire extension; each of which 146 consists of: 147 148 - NUL-terminated path component (relative to its parent directory); 149 150 - ASCII decimal number of entries in the index that is covered by the 151 tree this entry represents (entry_count); 152 153 - A space (ASCII 32); 154 155 - ASCII decimal number that represents the number of subtrees this 156 tree has; 157 158 - A newline (ASCII 10); and 159 160 - 160-bit object name for the object that would result from writing 161 this span of index as a tree. 162 163 An entry can be in an invalidated state and is represented by having 164 a negative number in the entry_count field. In this case, there is no 165 object name and the next entry starts immediately after the newline. 166 When writing an invalid entry, -1 should always be used as entry_count. 167 168 The entries are written out in the top-down, depth-first order. The 169 first entry represents the root level of the repository, followed by the 170 first subtree---let's call this A---of the root level (with its name 171 relative to the root level), followed by the first subtree of A (with 172 its name relative to A), ... 173 174=== Resolve undo 175 176 A conflict is represented in the index as a set of higher stage entries. 177 When a conflict is resolved (e.g. with "git add path"), these higher 178 stage entries will be removed and a stage-0 entry with proper resolution 179 is added. 180 181 When these higher stage entries are removed, they are saved in the 182 resolve undo extension, so that conflicts can be recreated (e.g. with 183 "git checkout -m"), in case users want to redo a conflict resolution 184 from scratch. 185 186 The signature for this extension is { 'R', 'E', 'U', 'C' }. 187 188 A series of entries fill the entire extension; each of which 189 consists of: 190 191 - NUL-terminated pathname the entry describes (relative to the root of 192 the repository, i.e. full pathname); 193 194 - Three NUL-terminated ASCII octal numbers, entry mode of entries in 195 stage 1 to 3 (a missing stage is represented by "0" in this field); 196 and 197 198 - At most three 160-bit object names of the entry in stages from 1 to 3 199 (nothing is written for a missing stage). 200