| gitformat-chunk(5) |
| ================== |
| |
| NAME |
| ---- |
| gitformat-chunk - Chunk-based file formats |
| |
| SYNOPSIS |
| -------- |
| |
| Used by linkgit:gitformat-commit-graph[5] and the "MIDX" format (see |
| the pack format documentation in linkgit: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 linkgit:gitformat-pack[5] and |
| the `commit-graph` format in linkgit: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 |
| fill 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 write. 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()`: |
| |
| * `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. |
| |
| * `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: |
| |
| * *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 linkgit:gitformat-commit-graph[5]. |
| |
| * *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 linkgit:gitformat-pack[5]. |
| |
| GIT |
| --- |
| Part of the linkgit:git[1] suite |