gitformat-chunk.html

   1 <!DOCTYPE html>
   2 <html xmlns="http://www.w3.org/1999/xhtml" lang="en">
   3 <head>
   4 <meta charset="UTF-8"/>
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   7 <meta name="generator" content="Asciidoctor 2.0.20"/>
   8 <title>gitformat-chunk(5)</title>
   9 <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700"/>
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  12 /* Uncomment the following line when using as a custom stylesheet */
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 440 </head>
 441 <body class="manpage">
 442 <div id="header">
 443 <h1>gitformat-chunk(5) Manual Page</h1>
 444 <h2 id="_name">NAME</h2>
 445 <div class="sectionbody">
 446 <p>gitformat-chunk - Chunk-based file formats</p>
 447 </div>
 448 </div>
 449 <div id="content">
 450 <div class="sect1">
 451 <h2 id="_synopsis">SYNOPSIS</h2>
 452 <div class="sectionbody">
 453 <div class="paragraph">
 454 <p>Used by <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a> and the "MIDX" format (see
 455 the pack format documentation in <a href="gitformat-pack.html">gitformat-pack(5)</a>).</p>
 456 </div>
 457 </div>
 458 </div>
 459 <div class="sect1">
 460 <h2 id="_description">DESCRIPTION</h2>
 461 <div class="sectionbody">
 462 <div class="paragraph">
 463 <p>Some file formats in Git use a common concept of "chunks" to describe
 464 sections of the file. This allows structured access to a large file by
 465 scanning a small "table of contents" for the remaining data. This common
 466 format is used by the <code>commit-graph</code> and <code>multi-pack-index</code> files. See
 467 the <code>multi-pack-index</code> format in <a href="gitformat-pack.html">gitformat-pack(5)</a> and
 468 the <code>commit-graph</code> format in <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a> for
 469 how they use the chunks to describe structured data.</p>
 470 </div>
 471 <div class="paragraph">
 472 <p>A chunk-based file format begins with some header information custom to
 473 that format. That header should include enough information to identify
 474 the file type, format version, and number of chunks in the file. From this
 475 information, that file can determine the start of the chunk-based region.</p>
 476 </div>
 477 <div class="paragraph">
 478 <p>The chunk-based region starts with a table of contents describing where
 479 each chunk starts and ends. This consists of (C+1) rows of 12 bytes each,
 480 where C is the number of chunks. Consider the following table:</p>
 481 </div>
 482 <div class="literalblock">
 483 <div class="content">
 484 <pre>| Chunk ID (4 bytes) | Chunk Offset (8 bytes) |
 485 |--------------------|------------------------|
 486 | ID[0]              | OFFSET[0]              |
 487 | ...                | ...                    |
 488 | ID[C]              | OFFSET[C]              |
 489 | 0x0000             | OFFSET[C+1]            |</pre>
 490 </div>
 491 </div>
 492 <div class="paragraph">
 493 <p>Each row consists of a 4-byte chunk identifier (ID) and an 8-byte offset.
 494 Each integer is stored in network-byte order.</p>
 495 </div>
 496 <div class="paragraph">
 497 <p>The chunk identifier <code>ID[i]</code> is a label for the data stored within this
 498 file from <code>OFFSET[i]</code> (inclusive) to <code>OFFSET[i+1]</code> (exclusive). Thus, the
 499 size of the <code>i`th chunk is equal to the difference between `OFFSET[i+1]</code>
 500 and <code>OFFSET[i]</code>. This requires that the chunk data appears contiguously
 501 in the same order as the table of contents.</p>
 502 </div>
 503 <div class="paragraph">
 504 <p>The final entry in the table of contents must be four zero bytes. This
 505 confirms that the table of contents is ending and provides the offset for
 506 the end of the chunk-based data.</p>
 507 </div>
 508 <div class="paragraph">
 509 <p>Note: The chunk-based format expects that the file contains <em>at least</em> a
 510 trailing hash after <code>OFFSET[C+1]</code>.</p>
 511 </div>
 512 <div class="paragraph">
 513 <p>Functions for working with chunk-based file formats are declared in
 514 <code>chunk-format.h</code>. Using these methods provide extra checks that assist
 515 developers when creating new file formats.</p>
 516 </div>
 517 </div>
 518 </div>
 519 <div class="sect1">
 520 <h2 id="_writing_chunk_based_file_formats">Writing chunk-based file formats</h2>
 521 <div class="sectionbody">
 522 <div class="paragraph">
 523 <p>To write a chunk-based file format, create a <code>struct chunkfile</code> by
 524 calling <code>init_chunkfile()</code> and pass a <code>struct hashfile</code> pointer. The
 525 caller is responsible for opening the <code>hashfile</code> and writing header
 526 information so the file format is identifiable before the chunk-based
 527 format begins.</p>
 528 </div>
 529 <div class="paragraph">
 530 <p>Then, call <code>add_chunk()</code> for each chunk that is intended for writing. This
 531 populates the <code>chunkfile</code> with information about the order and size of
 532 each chunk to write. Provide a <code>chunk_write_fn</code> function pointer to
 533 perform the write of the chunk data upon request.</p>
 534 </div>
 535 <div class="paragraph">
 536 <p>Call <code>write_chunkfile()</code> to write the table of contents to the <code>hashfile</code>
 537 followed by each of the chunks. This will verify that each chunk wrote
 538 the expected amount of data so the table of contents is correct.</p>
 539 </div>
 540 <div class="paragraph">
 541 <p>Finally, call <code>free_chunkfile()</code> to clear the <code>struct chunkfile</code> data. The
 542 caller is responsible for finalizing the <code>hashfile</code> by writing the trailing
 543 hash and closing the file.</p>
 544 </div>
 545 </div>
 546 </div>
 547 <div class="sect1">
 548 <h2 id="_reading_chunk_based_file_formats">Reading chunk-based file formats</h2>
 549 <div class="sectionbody">
 550 <div class="paragraph">
 551 <p>To read a chunk-based file format, the file must be opened as a
 552 memory-mapped region. The chunk-format API expects that the entire file
 553 is mapped as a contiguous memory region.</p>
 554 </div>
 555 <div class="paragraph">
 556 <p>Initialize a <code>struct chunkfile</code> pointer with <code>init_chunkfile(NULL)</code>.</p>
 557 </div>
 558 <div class="paragraph">
 559 <p>After reading the header information from the beginning of the file,
 560 including the chunk count, call <code>read_table_of_contents()</code> to populate
 561 the <code>struct chunkfile</code> with the list of chunks, their offsets, and their
 562 sizes.</p>
 563 </div>
 564 <div class="paragraph">
 565 <p>Extract the data information for each chunk using <code>pair_chunk()</code> or
 566 <code>read_chunk()</code>:</p>
 567 </div>
 568 <div class="ulist">
 569 <ul>
 570 <li>
 571 <p><code>pair_chunk()</code> assigns a given pointer with the location inside the
 572 memory-mapped file corresponding to that chunk&#8217;s offset. If the chunk
 573 does not exist, then the pointer is not modified.</p>
 574 </li>
 575 <li>
 576 <p><code>read_chunk()</code> takes a <code>chunk_read_fn</code> function pointer and calls it
 577 with the appropriate initial pointer and size information. The function
 578 is not called if the chunk does not exist. Use this method to read chunks
 579 if you need to perform immediate parsing or if you need to execute logic
 580 based on the size of the chunk.</p>
 581 </li>
 582 </ul>
 583 </div>
 584 <div class="paragraph">
 585 <p>After calling these methods, call <code>free_chunkfile()</code> to clear the
 586 <code>struct chunkfile</code> data. This will not close the memory-mapped region.
 587 Callers are expected to own that data for the timeframe the pointers into
 588 the region are needed.</p>
 589 </div>
 590 </div>
 591 </div>
 592 <div class="sect1">
 593 <h2 id="_examples">Examples</h2>
 594 <div class="sectionbody">
 595 <div class="paragraph">
 596 <p>These file formats use the chunk-format API, and can be used as examples
 597 for future formats:</p>
 598 </div>
 599 <div class="ulist">
 600 <ul>
 601 <li>
 602 <p><strong>commit-graph:</strong> see <code>write_commit_graph_file()</code> and <code>parse_commit_graph()</code>
 603 in <code>commit-graph.c</code> for how the chunk-format API is used to write and
 604 parse the commit-graph file format documented in
 605 the commit-graph file format in <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a>.</p>
 606 </li>
 607 <li>
 608 <p><strong>multi-pack-index:</strong> see <code>write_midx_internal()</code> and <code>load_multi_pack_index()</code>
 609 in <code>midx.c</code> for how the chunk-format API is used to write and
 610 parse the multi-pack-index file format documented in
 611 the multi-pack-index file format section of <a href="gitformat-pack.html">gitformat-pack(5)</a>.</p>
 612 </li>
 613 </ul>
 614 </div>
 615 </div>
 616 </div>
 617 <div class="sect1">
 618 <h2 id="_git">GIT</h2>
 619 <div class="sectionbody">
 620 <div class="paragraph">
 621 <p>Part of the <a href="git.html">git(1)</a> suite</p>
 622 </div>
 623 </div>
 624 </div>
 625 </div>
 626 <div id="footer">
 627 <div id="footer-text">
 628 Last updated 2023-10-23 14:43:46 -0700
 629 </div>
 630 </div>
 631 </body>
 632 </html>