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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>16.5 Phantom Byte Strings</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" 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class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewselflink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running Racket</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">16.1 </td><td><a href="weakbox.html" class="tocviewlink" data-pltdoc="x">Weak Boxes</a></td></tr><tr><td align="right">16.2 </td><td><a href="ephemerons.html" class="tocviewlink" data-pltdoc="x">Ephemerons</a></td></tr><tr><td align="right">16.3 </td><td><a href="willexecutor.html" class="tocviewlink" data-pltdoc="x">Wills and Executors</a></td></tr><tr><td align="right">16.4 </td><td><a href="garbagecollection.html" class="tocviewlink" data-pltdoc="x">Garbage Collection</a></td></tr><tr><td align="right">16.5 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Phantom Byte Strings</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._phantom-bytes~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">phantom-<wbr></wbr>bytes?</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._make-phantom-bytes%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">make-<wbr></wbr>phantom-<wbr></wbr>bytes</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._set-phantom-bytes%21%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">set-<wbr></wbr>phantom-<wbr></wbr>bytes!</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="garbagecollection.html" title="backward to "16.4 Garbage Collection"" data-pltdoc="x">← prev</a> <a href="memory.html" title="up to "16 Memory Management"" data-pltdoc="x">up</a> <a href="unsafe.html" title="forward to "17 Unsafe Operations"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""phantom-bytes"">16.5<tt> </tt><a name="(part._phantom-bytes)"></a>Phantom Byte Strings</h4><p>A <a name="(tech._phantom._byte._string)"></a><span style="font-style: italic">phantom byte string</span> is a small Racket value that is
treated by the Racket memory manager as having an arbitrary size,
which is specified when the <a href="#%28tech._phantom._byte._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">phantom byte string</span></a> is created or
when it is changed via <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._set-phantom-bytes%21%29%29" class="RktValLink" data-pltdoc="x">set-phantom-bytes!</a></span>.</p><p>A <a href="#%28tech._phantom._byte._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">phantom byte string</span></a> acts as a hint to Racket’s memory
manager that memory is allocated within the process but through a
separate allocator, such as through a foreign library that is accessed
via <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=foreign&rel=index.html&version=6.7" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">ffi/unsafe</span></a>. This hint is used to trigger
<a href="eval-model.html#%28tech._garbage._collection%29" class="techoutside" data-pltdoc="x"><span class="techinside">garbage collections</span></a> or to compute the result of
<span class="RktSym"><a href="garbagecollection.html#%28def._%28%28quote._~23~25kernel%29._current-memory-use%29%29" class="RktValLink" data-pltdoc="x">current-memory-use</a></span>. Currently, the hint is used only in
Racket 3m (the main variant of Racket).</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._phantom-bytes~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._phantom-bytes~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">phantom-bytes?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is a <a href="#%28tech._phantom._byte._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">phantom byte string</span></a>,
<span class="RktVal">#f</span> otherwise.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._make-phantom-bytes))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-phantom-bytes%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-phantom-bytes</a></span></span><span class="hspace"> </span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._phantom-bytes~3f%29%29" class="RktValLink" data-pltdoc="x">phantom-bytes?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-nonnegative-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-nonnegative-integer?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Creates a <a href="#%28tech._phantom._byte._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">phantom byte string</span></a> that is treated by the Racket
memory manager as being <span class="RktVar">k</span> bytes in size. For a large enough
<span class="RktVar">k</span>, the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3aout-of-memory%29%29" class="RktValLink" data-pltdoc="x">exn:fail:out-of-memory</a></span> exception is raised—<wbr></wbr>either because the
size is implausibly large, or because a memory limit has been
installed with <span class="RktSym"><a href="custodians.html#%28def._%28%28quote._~23~25kernel%29._custodian-limit-memory%29%29" class="RktValLink" data-pltdoc="x">custodian-limit-memory</a></span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._set-phantom-bytes!))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._set-phantom-bytes%21%29%29" class="RktValDef RktValLink" data-pltdoc="x">set-phantom-bytes!</a></span></span><span class="hspace"> </span><span class="RktVar">phantom-bstr</span><span class="hspace"> </span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._phantom-bytes~3f%29%29" class="RktValLink" data-pltdoc="x">phantom-bytes?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">phantom-bstr</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._phantom-bytes~3f%29%29" class="RktValLink" data-pltdoc="x">phantom-bytes?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-nonnegative-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-nonnegative-integer?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Adjusts the size of a <a href="#%28tech._phantom._byte._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">phantom byte string</span></a> as it is treated by
the Racket memory manager.</div></p><p>For example, if the memory that <span class="RktVar">phantom-bstr</span> represents is
released through a foreign library, then <span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._set-phantom-bytes%21%29%29" class="RktValLink" data-pltdoc="x">set-phantom-bytes!</a></span><span class="stt"> </span><span class="RktVar">phantom-bstr</span><span class="stt"> </span><span class="RktVal">0</span><span class="RktPn">)</span> can reflect the change in memory use.</p><p>When <span class="RktVar">k</span> is larger than the current size of
<span class="RktVar">phantom-bstr</span>, then this function can raise
<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3aout-of-memory%29%29" class="RktValLink" data-pltdoc="x">exn:fail:out-of-memory</a></span>, like <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-phantom-bytes%29%29" class="RktValLink" data-pltdoc="x">make-phantom-bytes</a></span>.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="garbagecollection.html" title="backward to "16.4 Garbage Collection"" data-pltdoc="x">← prev</a> <a href="memory.html" title="up to "16 Memory Management"" data-pltdoc="x">up</a> <a href="unsafe.html" title="forward to "17 Unsafe Operations"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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