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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>18 Concurrency and Synchronization</title><link rel="stylesheet" type="text/css" href="../scribble.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" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" 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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>18 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Concurrency and Synchronization</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">18.1 </td><td><a href="#%28part._.Threads%29" class="tocviewlink" data-pltdoc="x">Threads</a></td></tr><tr><td align="right">18.2 </td><td><a href="#%28part._.Thread_.Mailboxes%29" class="tocviewlink" data-pltdoc="x">Thread Mailboxes</a></td></tr><tr><td align="right">18.3 </td><td><a href="#%28part._.Semaphores%29" class="tocviewlink" data-pltdoc="x">Semaphores</a></td></tr><tr><td align="right">18.4 </td><td><a href="#%28part._.Channels%29" class="tocviewlink" data-pltdoc="x">Channels</a></td></tr><tr><td align="right">18.5 </td><td><a href="#%28part._.Buffered_.Asynchronous_.Channels%29" class="tocviewlink" data-pltdoc="x">Buffered Asynchronous Channels</a></td></tr><tr><td align="right">18.6 </td><td><a href="#%28part._.Synchronizable_.Events_and_sync%29" class="tocviewlink" data-pltdoc="x">Synchronizable Events and <span class="RktSym"><span class="RktValLink">sync</span></span></a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><span class="tocsublinknumber">18.1<tt> </tt></span><a href="#%28part._.Threads%29" class="tocsubseclink" data-pltdoc="x">Threads</a></td></tr><tr><td><span class="tocsublinknumber">18.2<tt> </tt></span><a href="#%28part._.Thread_.Mailboxes%29" class="tocsubseclink" data-pltdoc="x">Thread Mailboxes</a></td></tr><tr><td><span class="tocsublinknumber">18.3<tt> </tt></span><a href="#%28part._.Semaphores%29" class="tocsubseclink" data-pltdoc="x">Semaphores</a></td></tr><tr><td><span class="tocsublinknumber">18.4<tt> </tt></span><a href="#%28part._.Channels%29" class="tocsubseclink" data-pltdoc="x">Channels</a></td></tr><tr><td><span class="tocsublinknumber">18.5<tt> </tt></span><a href="#%28part._.Buffered_.Asynchronous_.Channels%29" class="tocsubseclink" data-pltdoc="x">Buffered Asynchronous Channels</a></td></tr><tr><td><span class="tocsublinknumber">18.6<tt> </tt></span><a href="#%28part._.Synchronizable_.Events_and_sync%29" class="tocsubseclink" data-pltdoc="x">Synchronizable Events and <span class="RktSym"><span class="RktValLink">sync</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="module-runtime-config.html" title="backward to "17.3.6 Module-Handling Configuration"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Guide"" data-pltdoc="x">up</a> <a href="performance.html" title="forward to "19 Performance"" data-pltdoc="x">next →</a></span> </div><h3 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""concurrency"">18<tt> </tt><a name="(part._concurrency)"></a>Concurrency and Synchronization</h3><p>Racket provides <a name="(tech._concurrency)"></a><span style="font-style: italic">concurrency</span> in the form of
<a name="(tech._thread)"></a><span style="font-style: italic">threads</span>, and it provides a general <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span> function
that can be used to synchronize both threads and other implicit forms of
concurrency, such as <a href="i_o.html#%28tech._port%29" class="techoutside" data-pltdoc="x"><span class="techinside">ports</span></a>.</p><p>Threads run concurrently in the sense that one thread can preempt
another without its cooperation, but threads do not run in parallel in
the sense of using multiple hardware processors. See
<a href="parallelism.html" data-pltdoc="x">Parallelism</a> for information on parallelism in Racket.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Threads"">18.1<tt> </tt><a name="(part._.Threads)"></a>Threads</h4><p>To execute a procedure concurrently, use <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span>. The
following example creates two new threads from the main thread:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"This is the original thread"</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"This is a new thread."</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"This is another new thread."</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The next example creates a new thread that would otherwise loop forever, but
the main thread uses <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sleep%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sleep</a></span> to pause itself for 2.5 seconds, then
uses <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._kill-thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">kill-thread</a></span> to terminate the worker thread:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">worker</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"Working..."</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sleep%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sleep</a></span><span class="hspace"> </span><span class="RktVal">0.2</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sleep%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sleep</a></span><span class="hspace"> </span><span class="RktVal">2.5</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._kill-thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">kill-thread</a></span><span class="hspace"> </span><span class="RktSym">worker</span><span class="RktPn">)</span></td></tr></table></blockquote><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>In DrRacket, the main thread keeps going until the Stop button is
clicked, so in DrRacket the <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span> is not necessary.</p></blockquote></blockquote></blockquote><p>If the main thread finishes or is killed, the application exits, even if
other threads are still running. A thread can use <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span> to
wait for another thread to finish. Here, the main thread uses
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span> to make sure the worker thread finishes before the main
thread exits:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">worker</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">100</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Working hard... ~a~n"</span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span><span class="hspace"> </span><span class="RktSym">worker</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"Worker finished"</span><span class="RktPn">)</span></td></tr></table></blockquote><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Thread_Mailboxes"">18.2<tt> </tt><a name="(part._.Thread_.Mailboxes)"></a>Thread Mailboxes</h4><p>Each thread has a mailbox for receiving messages. The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span> function
asynchronously sends a message to another thread’s mailbox, while
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span> returns the oldest message from the current
thread’s mailbox, blocking to wait for a message if necessary. In the
following example, the main thread sends data to the worker thread to be
processed, then sends a <span class="RktVal">'</span><span class="RktVal">done</span> message when there is no more data and
waits for the worker thread to finish.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">worker-thread</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">?</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=number-types.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._number%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">number?</a></span><span class="hspace"> </span><span class="RktSym">num</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Processing ~a~n"</span><span class="hspace"> </span><span class="RktSym">num</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">'</span><span class="RktVal">done</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Done~n"</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">20</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">worker-thread</span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">worker-thread</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">done</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span><span class="hspace"> </span><span class="RktSym">worker-thread</span><span class="RktPn">)</span></td></tr></table></blockquote><p>In the next example, the main thread delegates work to multiple arithmetic
threads, then waits to receive the results. The arithmetic threads process work
items then send the results to the main thread.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-arithmetic-thread</span><span class="hspace"> </span><span class="RktSym">operation</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">oper1</span><span class="hspace"> </span><span class="RktSym">oper2</span><span class="hspace"> </span><span class="RktSym">result-thread</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">result-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._format%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"~a + ~a = ~a"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">oper1</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">oper2</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">operation</span><span class="hspace"> </span><span class="RktSym">oper1</span><span class="hspace"> </span><span class="RktSym">oper2</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">addition-thread</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-arithmetic-thread</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">subtraction-thread</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-arithmetic-thread</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._-%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x"><span class="nobreak">-</span></a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">worklist</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">(</span><span class="RktVal">+</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktVal">)</span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal">+</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal"><span class="nobreak">-</span></span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal"><span class="nobreak">-</span></span><span class="hspace"> </span><span class="RktVal">4</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktVal">)</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">item</span><span class="hspace"> </span><span class="RktSym">worklist</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktSym">item</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">+</span><span class="hspace"> </span><span class="RktSym">o1</span><span class="hspace"> </span><span class="RktSym">o2</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">addition-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">o1</span><span class="hspace"> </span><span class="RktSym">o2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">current-thread</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal"><span class="nobreak">-</span></span><span class="hspace"> </span><span class="RktSym">o1</span><span class="hspace"> </span><span class="RktSym">o2</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">subtraction-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">o1</span><span class="hspace"> </span><span class="RktSym">o2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">current-thread</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._length%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">length</a></span><span class="hspace"> </span><span class="RktSym">worklist</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Semaphores"">18.3<tt> </tt><a name="(part._.Semaphores)"></a>Semaphores</h4><p>Semaphores facilitate synchronized access to an arbitrary shared resource.
Use semaphores when multiple threads must perform non-atomic operations on a
single resource.</p><p>In the following example, multiple threads print to standard output
concurrently. Without synchronization, a line printed by one thread might
appear in the middle of a line printed by another thread. By using a semaphore
initialized with a count of <span class="RktVal">1</span>, only one thread will print at a time.
The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._semaphore-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">semaphore-wait</a></span> function blocks until the semaphore’s internal counter is
non-zero, then decrements the counter and returns. The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._semaphore-post%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">semaphore-post</a></span> function
increments the counter so that another thread can unblock and then print.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">output-semaphore</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-semaphore%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-semaphore</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-thread</span><span class="hspace"> </span><span class="RktSym">name</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">10</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._semaphore-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">semaphore-wait</a></span><span class="hspace"> </span><span class="RktSym">output-semaphore</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"thread ~a: ~a~n"</span><span class="hspace"> </span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._semaphore-post%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">semaphore-post</a></span><span class="hspace"> </span><span class="RktSym">output-semaphore</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">threads</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._map%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktSym">make-thread</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">A</span><span class="hspace"> </span><span class="RktVal">B</span><span class="hspace"> </span><span class="RktVal">C</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._for-each%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">for-each</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span><span class="hspace"> </span><span class="RktSym">threads</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The pattern of waiting on a semaphore, working, and posting to the
semaphore can also be expressed using
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._call-with-semaphore%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">call-with-semaphore</a></span>,which has the advantage of posting to the
semaphore if control escapes (e.g., due to an exception):</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">output-semaphore</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-semaphore%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-semaphore</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-thread</span><span class="hspace"> </span><span class="RktSym">name</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">10</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=semaphore.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._call-with-semaphore%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">call-with-semaphore</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">output-semaphore</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"thread ~a: ~a~n"</span><span class="hspace"> </span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">threads</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._map%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktSym">make-thread</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">A</span><span class="hspace"> </span><span class="RktVal">B</span><span class="hspace"> </span><span class="RktVal">C</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._for-each%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">for-each</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span><span class="hspace"> </span><span class="RktSym">threads</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Semaphores are a low-level technique. Often, a better solution is to restrict
resource access to a single thread. For example, synchronizing access to
standard output might be better accomplished by having a dedicated thread for
printing output.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Channels"">18.4<tt> </tt><a name="(part._.Channels)"></a>Channels</h4><p>Channels synchronize two threads while a value is passed from one thread to the
other. Unlike a thread mailbox, multiple threads can get items from a single
channel, so channels should be used when multiple threads need to consume items
from a single work queue.</p><p>In the following example, the main thread adds items to a channel using
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span>, while multiple worker threads consume those items using
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-get%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-get</a></span>. Each call to either procedure blocks until another
thread calls the other procedure with the same channel. The workers process
the items and then pass their results to the result thread via the <span class="RktSym">result-channel</span>.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">result-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">result-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-get%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-get</a></span><span class="hspace"> </span><span class="RktSym">result-channel</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">work-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-worker</span><span class="hspace"> </span><span class="RktSym">thread-id</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">item</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-get%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-get</a></span><span class="hspace"> </span><span class="RktSym">work-channel</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">case</a></span><span class="hspace"> </span><span class="RktSym">item</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">DONE</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">result-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._format%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"Thread ~a done"</span><span class="hspace"> </span><span class="RktSym">thread-id</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._else%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">else</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">result-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._format%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"Thread ~a processed ~a"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">thread-id</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">item</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">work-threads</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._map%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktSym">make-worker</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">item</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">A</span><span class="hspace"> </span><span class="RktVal">B</span><span class="hspace"> </span><span class="RktVal">C</span><span class="hspace"> </span><span class="RktVal">D</span><span class="hspace"> </span><span class="RktVal">E</span><span class="hspace"> </span><span class="RktVal">F</span><span class="hspace"> </span><span class="RktVal">G</span><span class="hspace"> </span><span class="RktVal">H</span><span class="hspace"> </span><span class="RktVal">DONE</span><span class="hspace"> </span><span class="RktVal">DONE</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">work-channel</span><span class="hspace"> </span><span class="RktSym">item</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._for-each%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">for-each</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span><span class="hspace"> </span><span class="RktSym">work-threads</span><span class="RktPn">)</span></td></tr></table></blockquote><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Buffered_Asynchronous_Channels"">18.5<tt> </tt><a name="(part._.Buffered_.Asynchronous_.Channels)"></a>Buffered Asynchronous Channels</h4><p>Buffered asynchronous channels are similar to the channels described above, but
the “put” operation of asynchronous channels does not block—<wbr></wbr>unless the given
channel was created with a buffer limit and the limit has been reached. The
asynchronous-put operation is therefore somewhat similar to
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span>, but unlike thread mailboxes, asynchronous channels allow
multiple threads to consume items from a single channel.</p><p>In the following
example, the main thread adds items to the work channel, which holds a maximum
of three items at a time. The worker threads process items from this channel and
then send results to the print thread.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">racket/async-channel</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">print-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">safer-printf</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym">items</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">print-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=procedures.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._apply%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">apply</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._format%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktSym">items</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">work-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-async-channel</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-worker-thread</span><span class="hspace"> </span><span class="RktSym">thread-id</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">item</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">async-channel-get</span><span class="hspace"> </span><span class="RktSym">work-channel</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">safer-printf</span><span class="hspace"> </span><span class="RktVal">"Thread ~a processing item: ~a"</span><span class="hspace"> </span><span class="RktSym">thread-id</span><span class="hspace"> </span><span class="RktSym">item</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._for-each%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">for-each</a></span><span class="hspace"> </span><span class="RktSym">make-worker-thread</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">item</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">a</span><span class="hspace"> </span><span class="RktVal">b</span><span class="hspace"> </span><span class="RktVal">c</span><span class="hspace"> </span><span class="RktVal">d</span><span class="hspace"> </span><span class="RktVal">e</span><span class="hspace"> </span><span class="RktVal">f</span><span class="hspace"> </span><span class="RktVal">g</span><span class="hspace"> </span><span class="RktVal">h</span><span class="hspace"> </span><span class="RktVal">i</span><span class="hspace"> </span><span class="RktVal">j</span><span class="hspace"> </span><span class="RktVal">k</span><span class="hspace"> </span><span class="RktVal">l</span><span class="hspace"> </span><span class="RktVal">m</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">async-channel-put</span><span class="hspace"> </span><span class="RktSym">work-channel</span><span class="hspace"> </span><span class="RktSym">item</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Note the above example lacks any synchronization to verify that all items were
processed. If the main thread were to exit without such synchronization, it is
possible that the worker threads will not finish processing some items or the
print thread will not print all items.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Synchronizable_Events_and_sync"">18.6<tt> </tt><a name="(part._.Synchronizable_.Events_and_sync)"></a>Synchronizable Events and <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span></h4><p>There are other ways to synchronize threads. The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span> function allows
threads to coordinate via <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528tech._synchronizable._event%2529&version=6.7" class="techoutside Sq" data-pltdoc="x"><span class="techinside">synchronizable events</span></a>.
Many values double as events, allowing a uniform way to synchronize threads
using different types. Examples of events include channels, ports, threads,
and alarms.</p><p>In the next example, a channel and an alarm are used as synchronizable events.
The workers <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span> on both so that they can process channel items until the
alarm is activated. The channel items are processed, and then results are sent back
to the main thread.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">main-thread</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">current-thread</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">alarm</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._alarm-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">alarm-evt</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">3000</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=time.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-inexact-milliseconds%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">current-inexact-milliseconds</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-worker-thread</span><span class="hspace"> </span><span class="RktSym">thread-id</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">evt</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span><span class="hspace"> </span><span class="RktSym">channel</span><span class="hspace"> </span><span class="RktSym">alarm</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._cond%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">cond</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">evt</span><span class="hspace"> </span><span class="RktSym">alarm</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">main-thread</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">alarm</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._else%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">else</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-send%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-send</a></span><span class="hspace"> </span><span class="RktSym">main-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._format%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"Thread ~a received ~a"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">thread-id</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">evt</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym">make-worker-thread</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym">make-worker-thread</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym">make-worker-thread</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">A</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">B</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">'</span><span class="RktVal">alarm</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"Done"</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">result</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktSym">result</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The next example shows a function for use in a simple TCP echo server. The
function uses <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%252Ftimeout%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync/timeout</a></span> to synchronize on input from the given port
or a message in the thread’s mailbox. The first argument to <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%252Ftimeout%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync/timeout</a></span>
specifies the maximum number of seconds it should wait on the given events. The
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-lib.html%23%2528def._%2528%2528lib._racket%252Fport..rkt%2529._read-line-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">read-line-evt</a></span> function returns an event that is ready when a line of input is
available in the given input port. The result of <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive-evt</a></span> is ready when
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span> would not block. In a real application, the messages
received in the thread mailbox could be used for control messages, etc.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">serve</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">in-port</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">evt</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%252Ftimeout%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync/timeout</a></span><span class="hspace"> </span><span class="RktVal">2</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-lib.html%23%2528def._%2528%2528lib._racket%252Fport..rkt%2529._read-line-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">read-line-evt</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">in-port</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">any</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive-evt</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._cond%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">cond</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktSym">evt</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._displayln%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"Timed out, exiting"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=tcp.html%23%2528def._%2528%2528lib._racket%252Ftcp..rkt%2529._tcp-abandon-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">tcp-abandon-port</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">in-port</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=tcp.html%23%2528def._%2528%2528lib._racket%252Ftcp..rkt%2529._tcp-abandon-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">tcp-abandon-port</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">string?</a></span><span class="hspace"> </span><span class="RktSym">evt</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._fprintf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">fprintf</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="hspace"> </span><span class="RktVal">"~a~n"</span><span class="hspace"> </span><span class="RktSym">evt</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-buffers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._flush-output%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">flush-output</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._else%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">else</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Received a message in mailbox: ~a~n"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-receive%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-receive</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The <span class="RktSym">serve</span> function is used in the following example, which
starts a server thread and a client thread that communicate over TCP. The
client prints three lines to the server, which echoes them back. The client’s
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-lib.html%23%2528def._%2528%2528lib._racket%252Fport..rkt%2529._copy-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">copy-port</a></span> call blocks until EOF is received. The server times out after
two seconds, closing the ports, which allows <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-lib.html%23%2528def._%2528%2528lib._racket%252Fport..rkt%2529._copy-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">copy-port</a></span> to finish and the
client to exit. The main thread uses <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span> to wait for the
client thread to exit (since, without <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span>, the main thread might
exit before the other threads are finished).</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">port-num</span><span class="hspace"> </span><span class="RktVal">4321</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">start-server</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">listener</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=tcp.html%23%2528def._%2528%2528lib._racket%252Ftcp..rkt%2529._tcp-listen%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">tcp-listen</a></span><span class="hspace"> </span><span class="RktSym">port-num</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._define-values%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define-values</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">in-port</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=tcp.html%23%2528def._%2528%2528lib._racket%252Ftcp..rkt%2529._tcp-accept%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">tcp-accept</a></span><span class="hspace"> </span><span class="RktSym">listener</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">serve</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">in-port</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym">start-server</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">client-thread</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._define-values%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define-values</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">in-port</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=tcp.html%23%2528def._%2528%2528lib._racket%252Ftcp..rkt%2529._tcp-connect%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">tcp-connect</a></span><span class="hspace"> </span><span class="RktVal">"localhost"</span><span class="hspace"> </span><span class="RktSym">port-num</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktVal">"first\nsecond\nthird\n"</span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-buffers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._flush-output%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">flush-output</a></span><span class="hspace"> </span><span class="RktSym">out-port</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">copy-port will block until EOF is read from in-port</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-lib.html%23%2528def._%2528%2528lib._racket%252Fport..rkt%2529._copy-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">copy-port</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">in-port</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=port-ops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-output-port%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">current-output-port</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span><span class="hspace"> </span><span class="RktSym">client-thread</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Sometimes, you want to attach result behavior directly to the event passed to
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span>. In the following example, the worker thread synchronizes on three
channels, but each channel must be handled differently. Using
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span> associates a callback with the given event. When
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span> selects the given event, it calls the callback to generate the
synchronization result, rather than using the event’s normal synchronization
result. Since the event is handled in the callback, there is no need to
dispatch on the return value of <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span>.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">multiply-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">append-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">work</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktSym">loop</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">list-of-numbers</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Sum of ~a is ~a~n"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">list-of-numbers</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=procedures.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._apply%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">apply</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">list-of-numbers</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span><span class="hspace"> </span><span class="RktSym">multiply-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">list-of-numbers</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Product of ~a is ~a~n"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">list-of-numbers</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=procedures.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._apply%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">apply</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252A%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">*</a></span><span class="hspace"> </span><span class="RktSym">list-of-numbers</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span><span class="hspace"> </span><span class="RktSym">append-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">list-of-strings</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Concatenation of ~s is ~s~n"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">list-of-strings</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=procedures.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._apply%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">apply</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-append%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">string-append</a></span><span class="hspace"> </span><span class="RktSym">list-of-strings</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">loop</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">worker</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktSym">work</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">multiply-channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">multiply-channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">5</span><span class="hspace"> </span><span class="RktVal">6</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">7</span><span class="hspace"> </span><span class="RktVal">8</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">append-channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"a"</span><span class="hspace"> </span><span class="RktVal">"b"</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The result of <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span> invokes its callback in tail position
with respect to <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span>, so it is safe to
use recursion as in the following example.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">control-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">subtract-channel</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-channel%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-channel</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">work</span><span class="hspace"> </span><span class="RktSym">state</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Current state: ~a~n"</span><span class="hspace"> </span><span class="RktSym">state</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">number</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Adding: ~a~n"</span><span class="hspace"> </span><span class="RktSym">number</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">work</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">state</span><span class="hspace"> </span><span class="RktSym">number</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span><span class="hspace"> </span><span class="RktSym">subtract-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">number</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Subtracting: ~a~n"</span><span class="hspace"> </span><span class="RktSym">number</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">work</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._-%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x"><span class="nobreak">-</span></a></span><span class="hspace"> </span><span class="RktSym">state</span><span class="hspace"> </span><span class="RktSym">number</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span><span class="hspace"> </span><span class="RktSym">control-channel</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">kill-message</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Done~n"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">worker</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">work</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">subtract-channel</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">add-channel</span><span class="hspace"> </span><span class="RktVal">5</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">subtract-channel</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=channel.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmisc..rkt%2529._channel-put%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">channel-put</a></span><span class="hspace"> </span><span class="RktSym">control-channel</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">done</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._thread-wait%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">thread-wait</a></span><span class="hspace"> </span><span class="RktSym">worker</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._wrap-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">wrap-evt</a></span> function is like <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._handle-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">handle-evt</a></span>, except
that its handler is not called in tail position with respect to
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._sync%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">sync</a></span>. At the same time, <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=sync.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._wrap-evt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">wrap-evt</a></span> disables break
exceptions during its handler’s invocation.</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="module-runtime-config.html" title="backward to "17.3.6 Module-Handling Configuration"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Guide"" data-pltdoc="x">up</a> <a href="performance.html" title="forward to "19 Performance"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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