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 Interpretation of index entries in split index mode is completely 133 different. See below for details. 134 135== Extensions 136 137=== Cached tree 138 139 Cached tree extension contains pre-computed hashes for trees that can 140 be derived from the index. It helps speed up tree object generation 141 from index for a new commit. 142 143 When a path is updated in index, the path must be invalidated and 144 removed from tree cache. 145 146 The signature for this extension is { 'T', 'R', 'E', 'E' }. 147 148 A series of entries fill the entire extension; each of which 149 consists of: 150 151 - NUL-terminated path component (relative to its parent directory); 152 153 - ASCII decimal number of entries in the index that is covered by the 154 tree this entry represents (entry_count); 155 156 - A space (ASCII 32); 157 158 - ASCII decimal number that represents the number of subtrees this 159 tree has; 160 161 - A newline (ASCII 10); and 162 163 - 160-bit object name for the object that would result from writing 164 this span of index as a tree. 165 166 An entry can be in an invalidated state and is represented by having 167 a negative number in the entry_count field. In this case, there is no 168 object name and the next entry starts immediately after the newline. 169 When writing an invalid entry, -1 should always be used as entry_count. 170 171 The entries are written out in the top-down, depth-first order. The 172 first entry represents the root level of the repository, followed by the 173 first subtree---let's call this A---of the root level (with its name 174 relative to the root level), followed by the first subtree of A (with 175 its name relative to A), ... 176 177=== Resolve undo 178 179 A conflict is represented in the index as a set of higher stage entries. 180 When a conflict is resolved (e.g. with "git add path"), these higher 181 stage entries will be removed and a stage-0 entry with proper resolution 182 is added. 183 184 When these higher stage entries are removed, they are saved in the 185 resolve undo extension, so that conflicts can be recreated (e.g. with 186 "git checkout -m"), in case users want to redo a conflict resolution 187 from scratch. 188 189 The signature for this extension is { 'R', 'E', 'U', 'C' }. 190 191 A series of entries fill the entire extension; each of which 192 consists of: 193 194 - NUL-terminated pathname the entry describes (relative to the root of 195 the repository, i.e. full pathname); 196 197 - Three NUL-terminated ASCII octal numbers, entry mode of entries in 198 stage 1 to 3 (a missing stage is represented by "0" in this field); 199 and 200 201 - At most three 160-bit object names of the entry in stages from 1 to 3 202 (nothing is written for a missing stage). 203 204=== Split index 205 206 In split index mode, the majority of index entries could be stored 207 in a separate file. This extension records the changes to be made on 208 top of that to produce the final index. 209 210 The signature for this extension is { 'l', 'i, 'n', 'k' }. 211 212 The extension consists of: 213 214 - 160-bit SHA-1 of the shared index file. The shared index file path 215 is $GIT_DIR/sharedindex.<SHA-1>. If all 160 bits are zero, the 216 index does not require a shared index file. 217 218 - An ewah-encoded delete bitmap, each bit represents an entry in the 219 shared index. If a bit is set, its corresponding entry in the 220 shared index will be removed from the final index. Note, because 221 a delete operation changes index entry positions, but we do need 222 original positions in replace phase, it's best to just mark 223 entries for removal, then do a mass deletion after replacement. 224 225 - An ewah-encoded replace bitmap, each bit represents an entry in 226 the shared index. If a bit is set, its corresponding entry in the 227 shared index will be replaced with an entry in this index 228 file. All replaced entries are stored in sorted order in this 229 index. The first "1" bit in the replace bitmap corresponds to the 230 first index entry, the second "1" bit to the second entry and so 231 on. Replaced entries may have empty path names to save space. 232 233 The remaining index entries after replaced ones will be added to the 234 final index. These added entries are also sorted by entry name then 235 stage.