pack-*.pack files have the following format:

  • A header appears at the beginning and consists of the following: 

    4-byte signature:
    The signature is: {'P', 'A', 'C', 'K'}
    4-byte version number (network byte order):
    Git currently accepts version number 2 or 3 but
    generates version 2 only.
    4-byte number of objects contained in the pack (network byte order)
    Observation: we cannot have more than 4G versions ;-) and
more than 4G objects in a pack.

  • The header is followed by number of object entries, each of which looks like this: 

    (undeltified representation)
n-byte type and length (3-bit type, (n-1)*7+4-bit length)
compressed data
    (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
    Observation: length of each object is encoded in a variable
length format and is not constrained to 32-bit or anything.

  • The trailer records 20-byte SHA-1 checksum of all of the above.

Original (version 1) pack-*.idx files have the following format:

  • 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 first-level fan-out table.

  • The header is followed by sorted 24-byte entries, one entry per object in the pack. Each entry is: 

    4-byte network byte order integer, recording where the
object is stored in the packfile as the offset from the
beginning.
    20-byte object name.

  • The file is concluded with a trailer: 

    A copy of the 20-byte SHA-1 checksum at the end of
corresponding packfile.
    20-byte SHA-1-checksum of all of the above.

Pack Idx file:

        --  +--------------------------------+
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: <+
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.
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.

Version 2 pack-*.idx files support packs larger than 4 GiB, and

have some other reorganizations.  They have the format:

  • A 4-byte magic number \377tOc which is an unreasonable fanout[0] value.

  • A 4-byte version number (= 2)

  • A 256-entry fan-out table just like v1.

  • 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.

  • 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.

  • 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.

  • 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.

  • The same trailer as a v1 pack file: 

    A copy of the 20-byte SHA-1 checksum at the end of
corresponding packfile.
    20-byte SHA-1-checksum of all of the above.