'\" t .\" Title: gitformat-chunk .\" Author: [FIXME: author] [see http://www.docbook.org/tdg5/en/html/author] .\" Generator: DocBook XSL Stylesheets vsnapshot .\" Date: 02/23/2024 .\" Manual: Git Manual .\" Source: Git 2.44.0 .\" Language: English .\" .TH "GITFORMAT\-CHUNK" "5" "02/23/2024" "Git 2\&.44\&.0" "Git Manual" .\" ----------------------------------------------------------------- .\" * Define some portability stuff .\" ----------------------------------------------------------------- .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .\" http://bugs.debian.org/507673 .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .ie \n(.g .ds Aq \(aq .el .ds Aq ' .\" ----------------------------------------------------------------- .\" * set default formatting .\" ----------------------------------------------------------------- .\" disable hyphenation .nh .\" disable justification (adjust text to left margin only) .ad l .\" ----------------------------------------------------------------- .\" * MAIN CONTENT STARTS HERE * .\" ----------------------------------------------------------------- .SH "NAME" gitformat-chunk \- Chunk\-based file formats .SH "SYNOPSIS" .sp Used by \fBgitformat-commit-graph\fR(5) and the "MIDX" format (see the pack format documentation in \fBgitformat-pack\fR(5))\&. .SH "DESCRIPTION" .sp 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 \fBcommit\-graph\fR and \fBmulti\-pack\-index\fR files\&. See the \fBmulti\-pack\-index\fR format in \fBgitformat-pack\fR(5) and the \fBcommit\-graph\fR format in \fBgitformat-commit-graph\fR(5) for how they use the chunks to describe structured data\&. .sp 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\&. .sp 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: .sp .if n \{\ .RS 4 .\} .nf | Chunk ID (4 bytes) | Chunk Offset (8 bytes) | |\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-|\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-| | ID[0] | OFFSET[0] | | \&.\&.\&. | \&.\&.\&. | | ID[C] | OFFSET[C] | | 0x0000 | OFFSET[C+1] | .fi .if n \{\ .RE .\} .sp Each row consists of a 4\-byte chunk identifier (ID) and an 8\-byte offset\&. Each integer is stored in network\-byte order\&. .sp The chunk identifier \fBID[i]\fR is a label for the data stored within this file from \fBOFFSET[i]\fR (inclusive) to \fBOFFSET[i+1]\fR (exclusive)\&. Thus, the size of the \fBi`th chunk is equal to the difference between `OFFSET[i+1]\fR and \fBOFFSET[i]\fR\&. This requires that the chunk data appears contiguously in the same order as the table of contents\&. .sp 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\&. .sp Note: The chunk\-based format expects that the file contains \fIat least\fR a trailing hash after \fBOFFSET[C+1]\fR\&. .sp Functions for working with chunk\-based file formats are declared in \fBchunk\-format\&.h\fR\&. Using these methods provide extra checks that assist developers when creating new file formats\&. .SH "WRITING CHUNK\-BASED FILE FORMATS" .sp To write a chunk\-based file format, create a \fBstruct chunkfile\fR by calling \fBinit_chunkfile()\fR and pass a \fBstruct hashfile\fR pointer\&. The caller is responsible for opening the \fBhashfile\fR and writing header information so the file format is identifiable before the chunk\-based format begins\&. .sp Then, call \fBadd_chunk()\fR for each chunk that is intended for writing\&. This populates the \fBchunkfile\fR with information about the order and size of each chunk to write\&. Provide a \fBchunk_write_fn\fR function pointer to perform the write of the chunk data upon request\&. .sp Call \fBwrite_chunkfile()\fR to write the table of contents to the \fBhashfile\fR 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\&. .sp Finally, call \fBfree_chunkfile()\fR to clear the \fBstruct chunkfile\fR data\&. The caller is responsible for finalizing the \fBhashfile\fR by writing the trailing hash and closing the file\&. .SH "READING CHUNK\-BASED FILE FORMATS" .sp 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\&. .sp Initialize a \fBstruct chunkfile\fR pointer with \fBinit_chunkfile(NULL)\fR\&. .sp After reading the header information from the beginning of the file, including the chunk count, call \fBread_table_of_contents()\fR to populate the \fBstruct chunkfile\fR with the list of chunks, their offsets, and their sizes\&. .sp Extract the data information for each chunk using \fBpair_chunk()\fR or \fBread_chunk()\fR: .sp .RS 4 .ie n \{\ \h'-04'\(bu\h'+03'\c .\} .el \{\ .sp -1 .IP \(bu 2.3 .\} \fBpair_chunk()\fR assigns a given pointer with the location inside the memory\-mapped file corresponding to that chunk\(cqs offset\&. If the chunk does not exist, then the pointer is not modified\&. .RE .sp .RS 4 .ie n \{\ \h'-04'\(bu\h'+03'\c .\} .el \{\ .sp -1 .IP \(bu 2.3 .\} \fBread_chunk()\fR takes a \fBchunk_read_fn\fR 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\&. .RE .sp After calling these methods, call \fBfree_chunkfile()\fR to clear the \fBstruct chunkfile\fR 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\&. .SH "EXAMPLES" .sp These file formats use the chunk\-format API, and can be used as examples for future formats: .sp .RS 4 .ie n \{\ \h'-04'\(bu\h'+03'\c .\} .el \{\ .sp -1 .IP \(bu 2.3 .\} \fBcommit\-graph:\fR see \fBwrite_commit_graph_file()\fR and \fBparse_commit_graph()\fR in \fBcommit\-graph\&.c\fR 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 \fBgitformat-commit-graph\fR(5)\&. .RE .sp .RS 4 .ie n \{\ \h'-04'\(bu\h'+03'\c .\} .el \{\ .sp -1 .IP \(bu 2.3 .\} \fBmulti\-pack\-index:\fR see \fBwrite_midx_internal()\fR and \fBload_multi_pack_index()\fR in \fBmidx\&.c\fR 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 \fBgitformat-pack\fR(5)\&. .RE .SH "GIT" .sp Part of the \fBgit\fR(1) suite