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gitformat-chunk(5)                Git Manual                gitformat-chunk(5)


NAME

       gitformat-chunk - Chunk-based file formats


SYNOPSIS

       Used by gitformat-commit-graph(5) and the "MIDX" format (see the pack
       format documentation in gitformat-pack(5)).


DESCRIPTION

       Some file formats in Git use a common concept of "chunks" to describe
       sections of the file. This allows structured access to a large file by
       scanning a small "table of contents" for the remaining data. This
       common format is used by the commit-graph and multi-pack-index files.
       See the multi-pack-index format in gitformat-pack(5) and the
       commit-graph format in gitformat-commit-graph(5) for how they use the
       chunks to describe structured data.

       A chunk-based file format begins with some header information custom to
       that format. That header should include enough information to identify
       the file type, format version, and number of chunks in the file. From
       this information, that file can determine the start of the chunk-based
       region.

       The chunk-based region starts with a table of contents describing where
       each chunk starts and ends. This consists of (C+1) rows of 12 bytes
       each, where C is the number of chunks. Consider the following table:

           | Chunk ID (4 bytes) | Chunk Offset (8 bytes) |
           |--------------------|------------------------|
           | ID[0]              | OFFSET[0]              |
           | ...                | ...                    |
           | ID[C]              | OFFSET[C]              |
           | 0x0000             | OFFSET[C+1]            |

       Each row consists of a 4-byte chunk identifier (ID) and an 8-byte
       offset. Each integer is stored in network-byte order.

       The chunk identifier ID[i] is a label for the data stored within this
       file from OFFSET[i] (inclusive) to OFFSET[i+1] (exclusive). Thus, the
       size of the i`th chunk is equal to the difference between `OFFSET[i+1]
       and OFFSET[i]. This requires that the chunk data appears contiguously
       in the same order as the table of contents.

       The final entry in the table of contents must be four zero bytes. This
       confirms that the table of contents is ending and provides the offset
       for the end of the chunk-based data.

       Note: The chunk-based format expects that the file contains at least a
       trailing hash after OFFSET[C+1].

       Functions for working with chunk-based file formats are declared in
       chunk-format.h. Using these methods provide extra checks that assist
       developers when creating new file formats.


WRITING CHUNK-BASED FILE FORMATS

       To write a chunk-based file format, create a struct chunkfile by
       calling init_chunkfile() and pass a struct hashfile pointer. The caller
       is responsible for opening the hashfile and writing header information
       so the file format is identifiable before the chunk-based format
       begins.

       Then, call add_chunk() for each chunk that is intended for writing.
       This populates the chunkfile with information about the order and size
       of each chunk to write. Provide a chunk_write_fn function pointer to
       perform the write of the chunk data upon request.

       Call write_chunkfile() to write the table of contents to the hashfile
       followed by each of the chunks. This will verify that each chunk wrote
       the expected amount of data so the table of contents is correct.

       Finally, call free_chunkfile() to clear the struct chunkfile data. The
       caller is responsible for finalizing the hashfile by writing the
       trailing hash and closing the file.


READING CHUNK-BASED FILE FORMATS

       To read a chunk-based file format, the file must be opened as a
       memory-mapped region. The chunk-format API expects that the entire file
       is mapped as a contiguous memory region.

       Initialize a struct chunkfile pointer with init_chunkfile(NULL).

       After reading the header information from the beginning of the file,
       including the chunk count, call read_table_of_contents() to populate
       the struct chunkfile with the list of chunks, their offsets, and their
       sizes.

       Extract the data information for each chunk using pair_chunk() or
       read_chunk():

       o   pair_chunk() assigns a given pointer with the location inside the
           memory-mapped file corresponding to that chunk's offset. If the
           chunk does not exist, then the pointer is not modified.

       o   read_chunk() takes a chunk_read_fn function pointer and calls it
           with the appropriate initial pointer and size information. The
           function is not called if the chunk does not exist. Use this method
           to read chunks if you need to perform immediate parsing or if you
           need to execute logic based on the size of the chunk.

       After calling these methods, call free_chunkfile() to clear the struct
       chunkfile data. This will not close the memory-mapped region. Callers
       are expected to own that data for the timeframe the pointers into the
       region are needed.


EXAMPLES

       These file formats use the chunk-format API, and can be used as
       examples for future formats:

       o   commit-graph: see write_commit_graph_file() and
           parse_commit_graph() in commit-graph.c for how the chunk-format API
           is used to write and parse the commit-graph file format documented
           in the commit-graph file format in gitformat-commit-graph(5).

       o   multi-pack-index: see write_midx_internal() and
           load_multi_pack_index() in midx.c for how the chunk-format API is
           used to write and parse the multi-pack-index file format documented
           in the multi-pack-index file format section of gitformat-pack(5).


GIT

       Part of the git(1) suite

Git 2.43.0                        2023-11-20                gitformat-chunk(5)

git 2.43.0 - Generated Sun Nov 26 15:36:26 CST 2023
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