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<body class="article">
<div id="header">
<h1>Git pack format</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="_pack_pack_files_have_the_following_format">pack-*.pack files have the following format:</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
A header appears at the beginning and consists of the following:
</p>
<div class="literalblock">
<div class="content">
<pre><code>4-byte signature:
The signature is: {'P', 'A', 'C', 'K'}</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>4-byte version number (network byte order):
Git currently accepts version number 2 or 3 but
generates version 2 only.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>4-byte number of objects contained in the pack (network byte order)</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Observation: we cannot have more than 4G versions ;-) and
more than 4G objects in a pack.</code></pre>
</div></div>
</li>
<li>
<p>
The header is followed by number of object entries, each of
which looks like this:
</p>
<div class="literalblock">
<div class="content">
<pre><code>(undeltified representation)
n-byte type and length (3-bit type, (n-1)*7+4-bit length)
compressed data</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>(deltified representation)
n-byte type and length (3-bit type, (n-1)*7+4-bit length)
20-byte base object name if OBJ_REF_DELTA or a negative relative
offset from the delta object's position in the pack if this
is an OBJ_OFS_DELTA object
compressed delta data</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Observation: length of each object is encoded in a variable
length format and is not constrained to 32-bit or anything.</code></pre>
</div></div>
</li>
<li>
<p>
The trailer records 20-byte SHA-1 checksum of all of the above.
</p>
</li>
</ul></div>
<div class="sect2">
<h3 id="_object_types">Object types</h3>
<div class="paragraph"><p>Valid object types are:</p></div>
<div class="ulist"><ul>
<li>
<p>
OBJ_COMMIT (1)
</p>
</li>
<li>
<p>
OBJ_TREE (2)
</p>
</li>
<li>
<p>
OBJ_BLOB (3)
</p>
</li>
<li>
<p>
OBJ_TAG (4)
</p>
</li>
<li>
<p>
OBJ_OFS_DELTA (6)
</p>
</li>
<li>
<p>
OBJ_REF_DELTA (7)
</p>
</li>
</ul></div>
<div class="paragraph"><p>Type 5 is reserved for future expansion. Type 0 is invalid.</p></div>
</div>
<div class="sect2">
<h3 id="_deltified_representation">Deltified representation</h3>
<div class="paragraph"><p>Conceptually there are only four object types: commit, tree, tag and
blob. However to save space, an object could be stored as a "delta" of
another "base" object. These representations are assigned new types
ofs-delta and ref-delta, which is only valid in a pack file.</p></div>
<div class="paragraph"><p>Both ofs-delta and ref-delta store the "delta" to be applied to
another object (called <em>base object</em>) to reconstruct the object. The
difference between them is, ref-delta directly encodes 20-byte base
object name. If the base object is in the same pack, ofs-delta encodes
the offset of the base object in the pack instead.</p></div>
<div class="paragraph"><p>The base object could also be deltified if it’s in the same pack.
Ref-delta can also refer to an object outside the pack (i.e. the
so-called "thin pack"). When stored on disk however, the pack should
be self contained to avoid cyclic dependency.</p></div>
<div class="paragraph"><p>The delta data is a sequence of instructions to reconstruct an object
from the base object. If the base object is deltified, it must be
converted to canonical form first. Each instruction appends more and
more data to the target object until it’s complete. There are two
supported instructions so far: one for copy a byte range from the
source object and one for inserting new data embedded in the
instruction itself.</p></div>
<div class="paragraph"><p>Each instruction has variable length. Instruction type is determined
by the seventh bit of the first octet. The following diagrams follow
the convention in RFC 1951 (Deflate compressed data format).</p></div>
<div class="sect3">
<h4 id="_instruction_to_copy_from_base_object">Instruction to copy from base object</h4>
<div class="literalblock">
<div class="content">
<pre><code>+----------+---------+---------+---------+---------+-------+-------+-------+
| 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
+----------+---------+---------+---------+---------+-------+-------+-------+</code></pre>
</div></div>
<div class="paragraph"><p>This is the instruction format to copy a byte range from the source
object. It encodes the offset to copy from and the number of bytes to
copy. Offset and size are in little-endian order.</p></div>
<div class="paragraph"><p>All offset and size bytes are optional. This is to reduce the
instruction size when encoding small offsets or sizes. The first seven
bits in the first octet determines which of the next seven octets is
present. If bit zero is set, offset1 is present. If bit one is set
offset2 is present and so on.</p></div>
<div class="paragraph"><p>Note that a more compact instruction does not change offset and size
encoding. For example, if only offset2 is omitted like below, offset3
still contains bits 16-23. It does not become offset2 and contains
bits 8-15 even if it’s right next to offset1.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>+----------+---------+---------+
| 10000101 | offset1 | offset3 |
+----------+---------+---------+</code></pre>
</div></div>
<div class="paragraph"><p>In its most compact form, this instruction only takes up one byte
(0x80) with both offset and size omitted, which will have default
values zero. There is another exception: size zero is automatically
converted to 0x10000.</p></div>
</div>
<div class="sect3">
<h4 id="_instruction_to_add_new_data">Instruction to add new data</h4>
<div class="literalblock">
<div class="content">
<pre><code>+----------+============+
| 0xxxxxxx | data |
+----------+============+</code></pre>
</div></div>
<div class="paragraph"><p>This is the instruction to construct target object without the base
object. The following data is appended to the target object. The first
seven bits of the first octet determines the size of data in
bytes. The size must be non-zero.</p></div>
</div>
<div class="sect3">
<h4 id="_reserved_instruction">Reserved instruction</h4>
<div class="literalblock">
<div class="content">
<pre><code>+----------+============
| 00000000 |
+----------+============</code></pre>
</div></div>
<div class="paragraph"><p>This is the instruction reserved for future expansion.</p></div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_original_version_1_pack_idx_files_have_the_following_format">Original (version 1) pack-*.idx files have the following format:</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
The header consists of 256 4-byte network byte order
integers. N-th entry of this table records the number of
objects in the corresponding pack, the first byte of whose
object name is less than or equal to N. This is called the
<em>first-level fan-out</em> table.
</p>
</li>
<li>
<p>
The header is followed by sorted 24-byte entries, one entry
per object in the pack. Each entry is:
</p>
<div class="literalblock">
<div class="content">
<pre><code>4-byte network byte order integer, recording where the
object is stored in the packfile as the offset from the
beginning.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>20-byte object name.</code></pre>
</div></div>
</li>
<li>
<p>
The file is concluded with a trailer:
</p>
<div class="literalblock">
<div class="content">
<pre><code>A copy of the 20-byte SHA-1 checksum at the end of
corresponding packfile.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>20-byte SHA-1-checksum of all of the above.</code></pre>
</div></div>
</li>
</ul></div>
<div class="paragraph"><p>Pack Idx file:</p></div>
<div class="literalblock">
<div class="content">
<pre><code> -- +--------------------------------+
fanout | fanout[0] = 2 (for example) |-.
table +--------------------------------+ |
| fanout[1] | |
+--------------------------------+ |
| fanout[2] | |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| fanout[255] = total objects |---.
-- +--------------------------------+ | |
main | offset | | |
index | object name 00XXXXXXXXXXXXXXXX | | |
table +--------------------------------+ | |
| offset | | |
| object name 00XXXXXXXXXXXXXXXX | | |
+--------------------------------+<+ |
.-| offset | |
| | object name 01XXXXXXXXXXXXXXXX | |
| +--------------------------------+ |
| | offset | |
| | object name 01XXXXXXXXXXXXXXXX | |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| | offset | |
| | object name FFXXXXXXXXXXXXXXXX | |
--| +--------------------------------+<--+
trailer | | packfile checksum |
| +--------------------------------+
| | idxfile checksum |
| +--------------------------------+
.-------.
|
Pack file entry: <+</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>packed object header:
1-byte size extension bit (MSB)
type (next 3 bit)
size0 (lower 4-bit)
n-byte sizeN (as long as MSB is set, each 7-bit)
size0..sizeN form 4+7+7+..+7 bit integer, size0
is the least significant part, and sizeN is the
most significant part.
packed object data:
If it is not DELTA, then deflated bytes (the size above
is the size before compression).
If it is REF_DELTA, then
20-byte base object name SHA-1 (the size above is the
size of the delta data that follows).
delta data, deflated.
If it is OFS_DELTA, then
n-byte offset (see below) interpreted as a negative
offset from the type-byte of the header of the
ofs-delta entry (the size above is the size of
the delta data that follows).
delta data, deflated.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>offset encoding:
n bytes with MSB set in all but the last one.
The offset is then the number constructed by
concatenating the lower 7 bit of each byte, and
for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
to the result.</code></pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_version_2_pack_idx_files_support_packs_larger_than_4_gib_and">Version 2 pack-*.idx files support packs larger than 4 GiB, and</h2>
<div class="sectionbody">
<div class="literalblock">
<div class="content">
<pre><code>have some other reorganizations. They have the format:</code></pre>
</div></div>
<div class="ulist"><ul>
<li>
<p>
A 4-byte magic number <em>\377tOc</em> which is an unreasonable
fanout[0] value.
</p>
</li>
<li>
<p>
A 4-byte version number (= 2)
</p>
</li>
<li>
<p>
A 256-entry fan-out table just like v1.
</p>
</li>
<li>
<p>
A table of sorted 20-byte SHA-1 object names. These are
packed together without offset values to reduce the cache
footprint of the binary search for a specific object name.
</p>
</li>
<li>
<p>
A table of 4-byte CRC32 values of the packed object data.
This is new in v2 so compressed data can be copied directly
from pack to pack during repacking without undetected
data corruption.
</p>
</li>
<li>
<p>
A table of 4-byte offset values (in network byte order).
These are usually 31-bit pack file offsets, but large
offsets are encoded as an index into the next table with
the msbit set.
</p>
</li>
<li>
<p>
A table of 8-byte offset entries (empty for pack files less
than 2 GiB). Pack files are organized with heavily used
objects toward the front, so most object references should
not need to refer to this table.
</p>
</li>
<li>
<p>
The same trailer as a v1 pack file:
</p>
<div class="literalblock">
<div class="content">
<pre><code>A copy of the 20-byte SHA-1 checksum at the end of
corresponding packfile.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>20-byte SHA-1-checksum of all of the above.</code></pre>
</div></div>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_multi_pack_index_midx_files_have_the_following_format">multi-pack-index (MIDX) files have the following format:</h2>
<div class="sectionbody">
<div class="paragraph"><p>The multi-pack-index files refer to multiple pack-files and loose objects.</p></div>
<div class="paragraph"><p>In order to allow extensions that add extra data to the MIDX, we organize
the body into "chunks" and provide a lookup table at the beginning of the
body. The header includes certain length values, such as the number of packs,
the number of base MIDX files, hash lengths and types.</p></div>
<div class="paragraph"><p>All 4-byte numbers are in network order.</p></div>
<div class="paragraph"><p>HEADER:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>4-byte signature:
The signature is: {'M', 'I', 'D', 'X'}</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>1-byte version number:
Git only writes or recognizes version 1.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>1-byte Object Id Version
Git only writes or recognizes version 1 (SHA1).</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>1-byte number of "chunks"</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>1-byte number of base multi-pack-index files:
This value is currently always zero.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>4-byte number of pack files</code></pre>
</div></div>
<div class="paragraph"><p>CHUNK LOOKUP:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>(C + 1) * 12 bytes providing the chunk offsets:
First 4 bytes describe chunk id. Value 0 is a terminating label.
Other 8 bytes provide offset in current file for chunk to start.
(Chunks are provided in file-order, so you can infer the length
using the next chunk position if necessary.)</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>The remaining data in the body is described one chunk at a time, and
these chunks may be given in any order. Chunks are required unless
otherwise specified.</code></pre>
</div></div>
<div class="paragraph"><p>CHUNK DATA:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>Packfile Names (ID: {'P', 'N', 'A', 'M'})
Stores the packfile names as concatenated, null-terminated strings.
Packfiles must be listed in lexicographic order for fast lookups by
name. This is the only chunk not guaranteed to be a multiple of four
bytes in length, so should be the last chunk for alignment reasons.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>OID Fanout (ID: {'O', 'I', 'D', 'F'})
The ith entry, F[i], stores the number of OIDs with first
byte at most i. Thus F[255] stores the total
number of objects.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>OID Lookup (ID: {'O', 'I', 'D', 'L'})
The OIDs for all objects in the MIDX are stored in lexicographic
order in this chunk.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Object Offsets (ID: {'O', 'O', 'F', 'F'})
Stores two 4-byte values for every object.
1: The pack-int-id for the pack storing this object.
2: The offset within the pack.
If all offsets are less than 2^31, then the large offset chunk
will not exist and offsets are stored as in IDX v1.
If there is at least one offset value larger than 2^32-1, then
the large offset chunk must exist. If the large offset chunk
exists and the 31st bit is on, then removing that bit reveals
the row in the large offsets containing the 8-byte offset of
this object.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
8-byte offsets into large packfiles.</code></pre>
</div></div>
<div class="paragraph"><p>TRAILER:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>20-byte SHA1-checksum of the above contents.</code></pre>
</div></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated 2019-12-10 07:18:07 UTC
</div>
</div>
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