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| <head> | |
| <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" /> | |
| <meta name="generator" content="AsciiDoc 10.2.0" /> | |
| <title>gitformat-chunk(5)</title> | |
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| /* Default font. */ | |
| body { | |
| font-family: Georgia,serif; | |
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| h1, h2, h3, h4, h5, h6, | |
| div.title, caption.title, | |
| thead, p.table.header, | |
| #toctitle, | |
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| color: fuchsia; | |
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| em { | |
| font-style: italic; | |
| color: navy; | |
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| font-weight: bold; | |
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| h5 { | |
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| margin-top: 0.5em; | |
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| color: #527bbd; | |
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| font-size: small; | |
| border-top: 2px solid silver; | |
| padding-top: 0.5em; | |
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| margin-top: 1.5em; | |
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| margin-top: 1.0em; | |
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| margin-bottom: 2.0em; | |
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| padding: 0; | |
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| margin-top: 1.0em; | |
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| margin-top: 0; | |
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| td div.title:first-child { | |
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| margin-top: 0.0em; | |
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| background: #ffffee; | |
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| border-left: 4px solid #f0f0f0; | |
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| border: 1px solid #dddddd; | |
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| background: #f8f8f8; | |
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| padding-left: 1.0em; | |
| margin-left: 1.0em; | |
| margin-right: 10%; | |
| border-left: 5px solid #f0f0f0; | |
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| padding-top: 0.5em; | |
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| div.verseblock > div.attribution { | |
| padding-top: 0.75em; | |
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| /* DEPRECATED: Pre version 8.2.7 verse style literal block. */ | |
| div.verseblock + div.attribution { | |
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| font-size: 1.1em; | |
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| padding-right: 0.5em; | |
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| border-left: 3px solid #dddddd; | |
| padding-left: 0.5em; | |
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| div.imageblock div.content { padding-left: 0; } | |
| span.image img { border-style: none; vertical-align: text-bottom; } | |
| a.image:visited { color: white; } | |
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| margin-top: 0.8em; | |
| margin-bottom: 0.8em; | |
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| margin-top: 0.5em; | |
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| ol.arabic { | |
| list-style-type: decimal; | |
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| div.compact ul, div.compact ol, | |
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| div.compact div, div.compact div { | |
| margin-top: 0.1em; | |
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| tfoot { | |
| font-weight: bold; | |
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| div.hdlist tr { | |
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| padding-right: 0.8em; | |
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| margin: 0; | |
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| .comment { | |
| background: yellow; | |
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| #footnotes { | |
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| #footnotes hr { | |
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| border-top: 1px solid silver; | |
| height: 1px; | |
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| width: 20%; | |
| min-width: 100px; | |
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| vertical-align: top; | |
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| div.colist td img { | |
| margin-top: 0.3em; | |
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| @media print { | |
| #footer-badges { display: none; } | |
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| #toc { | |
| margin-bottom: 2.5em; | |
| } | |
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| font-size: 1.1em; | |
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| font-size: 0.9em; | |
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| span.blue { color: blue; } | |
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| span.lime { color: lime; } | |
| span.maroon { color: maroon; } | |
| span.navy { color: navy; } | |
| span.olive { color: olive; } | |
| span.purple { color: purple; } | |
| span.red { color: red; } | |
| span.silver { color: silver; } | |
| span.teal { color: teal; } | |
| span.white { color: white; } | |
| span.yellow { color: yellow; } | |
| span.aqua-background { background: aqua; } | |
| span.black-background { background: black; } | |
| span.blue-background { background: blue; } | |
| span.fuchsia-background { background: fuchsia; } | |
| span.gray-background { background: gray; } | |
| span.green-background { background: green; } | |
| span.lime-background { background: lime; } | |
| span.maroon-background { background: maroon; } | |
| span.navy-background { background: navy; } | |
| span.olive-background { background: olive; } | |
| span.purple-background { background: purple; } | |
| span.red-background { background: red; } | |
| span.silver-background { background: silver; } | |
| span.teal-background { background: teal; } | |
| span.white-background { background: white; } | |
| span.yellow-background { background: yellow; } | |
| span.big { font-size: 2em; } | |
| span.small { font-size: 0.6em; } | |
| span.underline { text-decoration: underline; } | |
| span.overline { text-decoration: overline; } | |
| span.line-through { text-decoration: line-through; } | |
| div.unbreakable { page-break-inside: avoid; } | |
| /* | |
| * xhtml11 specific | |
| * | |
| * */ | |
| div.tableblock { | |
| margin-top: 1.0em; | |
| margin-bottom: 1.5em; | |
| } | |
| div.tableblock > table { | |
| border: 3px solid #527bbd; | |
| } | |
| thead, p.table.header { | |
| font-weight: bold; | |
| color: #527bbd; | |
| } | |
| p.table { | |
| margin-top: 0; | |
| } | |
| /* Because the table frame attribute is overridden by CSS in most browsers. */ | |
| div.tableblock > table[frame="void"] { | |
| border-style: none; | |
| } | |
| div.tableblock > table[frame="hsides"] { | |
| border-left-style: none; | |
| border-right-style: none; | |
| } | |
| div.tableblock > table[frame="vsides"] { | |
| border-top-style: none; | |
| border-bottom-style: none; | |
| } | |
| /* | |
| * html5 specific | |
| * | |
| * */ | |
| table.tableblock { | |
| margin-top: 1.0em; | |
| margin-bottom: 1.5em; | |
| } | |
| thead, p.tableblock.header { | |
| font-weight: bold; | |
| color: #527bbd; | |
| } | |
| p.tableblock { | |
| margin-top: 0; | |
| } | |
| table.tableblock { | |
| border-width: 3px; | |
| border-spacing: 0px; | |
| border-style: solid; | |
| border-color: #527bbd; | |
| border-collapse: collapse; | |
| } | |
| th.tableblock, td.tableblock { | |
| border-width: 1px; | |
| padding: 4px; | |
| border-style: solid; | |
| border-color: #527bbd; | |
| } | |
| table.tableblock.frame-topbot { | |
| border-left-style: hidden; | |
| border-right-style: hidden; | |
| } | |
| table.tableblock.frame-sides { | |
| border-top-style: hidden; | |
| border-bottom-style: hidden; | |
| } | |
| table.tableblock.frame-none { | |
| border-style: hidden; | |
| } | |
| th.tableblock.halign-left, td.tableblock.halign-left { | |
| text-align: left; | |
| } | |
| th.tableblock.halign-center, td.tableblock.halign-center { | |
| text-align: center; | |
| } | |
| th.tableblock.halign-right, td.tableblock.halign-right { | |
| text-align: right; | |
| } | |
| th.tableblock.valign-top, td.tableblock.valign-top { | |
| vertical-align: top; | |
| } | |
| th.tableblock.valign-middle, td.tableblock.valign-middle { | |
| vertical-align: middle; | |
| } | |
| th.tableblock.valign-bottom, td.tableblock.valign-bottom { | |
| vertical-align: bottom; | |
| } | |
| /* | |
| * manpage specific | |
| * | |
| * */ | |
| body.manpage h1 { | |
| padding-top: 0.5em; | |
| padding-bottom: 0.5em; | |
| border-top: 2px solid silver; | |
| border-bottom: 2px solid silver; | |
| } | |
| body.manpage h2 { | |
| border-style: none; | |
| } | |
| body.manpage div.sectionbody { | |
| margin-left: 3em; | |
| } | |
| @media print { | |
| body.manpage div#toc { display: none; } | |
| } | |
| </style> | |
| <script type="text/javascript"> | |
| /*<+'])'); | |
| // Function that scans the DOM tree for header elements (the DOM2 | |
| // nodeIterator API would be a better technique but not supported by all | |
| // browsers). | |
| var iterate = function (el) { | |
| for (var i = el.firstChild; i != null; i = i.nextSibling) { | |
| if (i.nodeType == 1 /* Node.ELEMENT_NODE */) { | |
| var mo = re.exec(i.tagName); | |
| if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") { | |
| result[result.length] = new TocEntry(i, getText(i), mo[1]-1); | |
| } | |
| iterate(i); | |
| } | |
| } | |
| } | |
| iterate(el); | |
| return result; | |
| } | |
| var toc = document.getElementById("toc"); | |
| if (!toc) { | |
| return; | |
| } | |
| // Delete existing TOC entries in case we're reloading the TOC. | |
| var tocEntriesToRemove = []; | |
| var i; | |
| for (i = 0; i < toc.childNodes.length; i++) { | |
| var entry = toc.childNodes[i]; | |
| if (entry.nodeName.toLowerCase() == 'div' | |
| && entry.getAttribute("class") | |
| && entry.getAttribute("class").match(/^toclevel/)) | |
| tocEntriesToRemove.push(entry); | |
| } | |
| for (i = 0; i < tocEntriesToRemove.length; i++) { | |
| toc.removeChild(tocEntriesToRemove[i]); | |
| } | |
| // Rebuild TOC entries. | |
| var entries = tocEntries(document.getElementById("content"), toclevels); | |
| for (var i = 0; i < entries.length; ++i) { | |
| var entry = entries[i]; | |
| if (entry.element.id == "") | |
| entry.element.id = "_toc_" + i; | |
| var a = document.createElement("a"); | |
| a.href = "#" + entry.element.id; | |
| a.appendChild(document.createTextNode(entry.text)); | |
| var div = document.createElement("div"); | |
| div.appendChild(a); | |
| div.className = "toclevel" + entry.toclevel; | |
| toc.appendChild(div); | |
| } | |
| if (entries.length == 0) | |
| toc.parentNode.removeChild(toc); | |
| }, | |
| ///////////////////////////////////////////////////////////////////// | |
| // Footnotes generator | |
| ///////////////////////////////////////////////////////////////////// | |
| /* Based on footnote generation code from: | |
| * http://www.brandspankingnew.net/archive/2005/07/format_footnote.html | |
| */ | |
| footnotes: function () { | |
| // Delete existing footnote entries in case we're reloading the footnodes. | |
| var i; | |
| var noteholder = document.getElementById("footnotes"); | |
| if (!noteholder) { | |
| return; | |
| } | |
| var entriesToRemove = []; | |
| for (i = 0; i < noteholder.childNodes.length; i++) { | |
| var entry = noteholder.childNodes[i]; | |
| if (entry.nodeName.toLowerCase() == 'div' && entry.getAttribute("class") == "footnote") | |
| entriesToRemove.push(entry); | |
| } | |
| for (i = 0; i < entriesToRemove.length; i++) { | |
| noteholder.removeChild(entriesToRemove[i]); | |
| } | |
| // Rebuild footnote entries. | |
| var cont = document.getElementById("content"); | |
| var spans = cont.getElementsByTagName("span"); | |
| var refs = {}; | |
| var n = 0; | |
| for (i=0; i<spans.length; i++) { | |
| if (spans[i].className == "footnote") { | |
| n++; | |
| var note = spans[i].getAttribute("data-note"); | |
| if (!note) { | |
| // Use [\s\S] in place of . so multi-line matches work. | |
| // Because JavaScript has no s (dotall) regex flag. | |
| note = spans[i].innerHTML.match(/\s*\[([\s\S]*)]\s*/)[1]; | |
| spans[i].innerHTML = | |
| "[<a id='_footnoteref_" + n + "' href='#_footnote_" + n + | |
| "' title='View footnote' class='footnote'>" + n + "</a>]"; | |
| spans[i].setAttribute("data-note", note); | |
| } | |
| noteholder.innerHTML += | |
| "<div class='footnote' id='_footnote_" + n + "'>" + | |
| "<a href='#_footnoteref_" + n + "' title='Return to text'>" + | |
| n + "</a>. " + note + "</div>"; | |
| var id =spans[i].getAttribute("id"); | |
| if (id != null) refs["#"+id] = n; | |
| } | |
| } | |
| if (n == 0) | |
| noteholder.parentNode.removeChild(noteholder); | |
| else { | |
| // Process footnoterefs. | |
| for (i=0; i<spans.length; i++) { | |
| if (spans[i].className == "footnoteref") { | |
| var href = spans[i].getElementsByTagName("a")[0].getAttribute("href"); | |
| href = href.match(/#.*/)[0]; // Because IE return full URL. | |
| n = refs[href]; | |
| spans[i].innerHTML = | |
| "[<a href='#_footnote_" + n + | |
| "' title='View footnote' class='footnote'>" + n + "</a>]"; | |
| } | |
| } | |
| } | |
| }, | |
| install: function(toclevels) { | |
| var timerId; | |
| function reinstall() { | |
| asciidoc.footnotes(); | |
| if (toclevels) { | |
| asciidoc.toc(toclevels); | |
| } | |
| } | |
| function reinstallAndRemoveTimer() { | |
| clearInterval(timerId); | |
| reinstall(); | |
| } | |
| timerId = setInterval(reinstall, 500); | |
| if (document.addEventListener) | |
| document.addEventListener("DOMContentLoaded", reinstallAndRemoveTimer, false); | |
| else | |
| window.onload = reinstallAndRemoveTimer; | |
| } | |
| } | |
| asciidoc.install(); | |
| /*]]>*/ | |
| </script> | |
| </head> | |
| <body class="manpage"> | |
| <div id="header"> | |
| <h1> | |
| gitformat-chunk(5) Manual Page | |
| </h1> | |
| <h2>NAME</h2> | |
| <div class="sectionbody"> | |
| <p>gitformat-chunk - | |
| Chunk-based file formats | |
| </p> | |
| </div> | |
| </div> | |
| <div id="content"> | |
| <div class="sect1"> | |
| <h2 id="_synopsis">SYNOPSIS</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>Used by <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a> and the "MIDX" format (see | |
| the pack format documentation in <a href="gitformat-pack.html">gitformat-pack(5)</a>).</p></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_description">DESCRIPTION</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>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 <code>commit-graph</code> and <code>multi-pack-index</code> files. See | |
| the <code>multi-pack-index</code> format in <a href="gitformat-pack.html">gitformat-pack(5)</a> and | |
| the <code>commit-graph</code> format in <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a> for | |
| how they use the chunks to describe structured data.</p></div> | |
| <div class="paragraph"><p>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.</p></div> | |
| <div class="paragraph"><p>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:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>| Chunk ID (4 bytes) | Chunk Offset (8 bytes) | | |
| |--------------------|------------------------| | |
| | ID[0] | OFFSET[0] | | |
| | ... | ... | | |
| | ID[C] | OFFSET[C] | | |
| | 0x0000 | OFFSET[C+1] |</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>Each row consists of a 4-byte chunk identifier (ID) and an 8-byte offset. | |
| Each integer is stored in network-byte order.</p></div> | |
| <div class="paragraph"><p>The chunk identifier <code>ID[i]</code> is a label for the data stored within this | |
| file from <code>OFFSET[i]</code> (inclusive) to <code>OFFSET[i+1]</code> (exclusive). Thus, the | |
| size of the <code>i`th chunk is equal to the difference between `OFFSET[i+1]</code> | |
| and <code>OFFSET[i]</code>. This requires that the chunk data appears contiguously | |
| in the same order as the table of contents.</p></div> | |
| <div class="paragraph"><p>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.</p></div> | |
| <div class="paragraph"><p>Note: The chunk-based format expects that the file contains <em>at least</em> a | |
| trailing hash after <code>OFFSET[C+1]</code>.</p></div> | |
| <div class="paragraph"><p>Functions for working with chunk-based file formats are declared in | |
| <code>chunk-format.h</code>. Using these methods provide extra checks that assist | |
| developers when creating new file formats.</p></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_writing_chunk_based_file_formats">Writing chunk-based file formats</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>To write a chunk-based file format, create a <code>struct chunkfile</code> by | |
| calling <code>init_chunkfile()</code> and pass a <code>struct hashfile</code> pointer. The | |
| caller is responsible for opening the <code>hashfile</code> and writing header | |
| information so the file format is identifiable before the chunk-based | |
| format begins.</p></div> | |
| <div class="paragraph"><p>Then, call <code>add_chunk()</code> for each chunk that is intended for writing. This | |
| populates the <code>chunkfile</code> with information about the order and size of | |
| each chunk to write. Provide a <code>chunk_write_fn</code> function pointer to | |
| perform the write of the chunk data upon request.</p></div> | |
| <div class="paragraph"><p>Call <code>write_chunkfile()</code> to write the table of contents to the <code>hashfile</code> | |
| 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.</p></div> | |
| <div class="paragraph"><p>Finally, call <code>free_chunkfile()</code> to clear the <code>struct chunkfile</code> data. The | |
| caller is responsible for finalizing the <code>hashfile</code> by writing the trailing | |
| hash and closing the file.</p></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_reading_chunk_based_file_formats">Reading chunk-based file formats</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>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.</p></div> | |
| <div class="paragraph"><p>Initialize a <code>struct chunkfile</code> pointer with <code>init_chunkfile(NULL)</code>.</p></div> | |
| <div class="paragraph"><p>After reading the header information from the beginning of the file, | |
| including the chunk count, call <code>read_table_of_contents()</code> to populate | |
| the <code>struct chunkfile</code> with the list of chunks, their offsets, and their | |
| sizes.</p></div> | |
| <div class="paragraph"><p>Extract the data information for each chunk using <code>pair_chunk()</code> or | |
| <code>read_chunk()</code>:</p></div> | |
| <div class="ulist"><ul> | |
| <li> | |
| <p> | |
| <code>pair_chunk()</code> 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. | |
| </p> | |
| </li> | |
| <li> | |
| <p> | |
| <code>read_chunk()</code> takes a <code>chunk_read_fn</code> 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. | |
| </p> | |
| </li> | |
| </ul></div> | |
| <div class="paragraph"><p>After calling these methods, call <code>free_chunkfile()</code> to clear the | |
| <code>struct chunkfile</code> 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.</p></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_examples">Examples</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>These file formats use the chunk-format API, and can be used as examples | |
| for future formats:</p></div> | |
| <div class="ulist"><ul> | |
| <li> | |
| <p> | |
| <strong>commit-graph:</strong> see <code>write_commit_graph_file()</code> and <code>parse_commit_graph()</code> | |
| in <code>commit-graph.c</code> 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 <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a>. | |
| </p> | |
| </li> | |
| <li> | |
| <p> | |
| <strong>multi-pack-index:</strong> see <code>write_midx_internal()</code> and <code>load_multi_pack_index()</code> | |
| in <code>midx.c</code> 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 <a href="gitformat-pack.html">gitformat-pack(5)</a>. | |
| </p> | |
| </li> | |
| </ul></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_git">GIT</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div> | |
| </div> | |
| </div> | |
| </div> | |
| <div id="footnotes"><hr /></div> | |
| <div id="footer"> | |
| <div id="footer-text"> | |
| Last updated | |
| 2023-10-23 14:43:46 PDT | |
| </div> | |
| </div> | |
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