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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>11.2.3 Semaphores</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Reference</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewselflink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running Racket</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">►</a></td><td>11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr></table><div class="tocviewsublist" style="display: none;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">11.1 </td><td><a href="threads.html" class="tocviewlink" data-pltdoc="x">Threads</a></td></tr><tr><td align="right">11.2 </td><td><a href="all-sync.html" class="tocviewselflink" data-pltdoc="x">Synchronization</a></td></tr><tr><td align="right">11.3 </td><td><a href="thread-local-storage.html" class="tocviewlink" data-pltdoc="x">Thread-<wbr></wbr>Local Storage</a></td></tr><tr><td align="right">11.4 </td><td><a href="futures.html" class="tocviewlink" data-pltdoc="x">Futures</a></td></tr><tr><td align="right">11.5 </td><td><a href="places.html" class="tocviewlink" data-pltdoc="x">Places</a></td></tr><tr><td align="right">11.6 </td><td><a href="engine.html" class="tocviewlink" data-pltdoc="x">Engines</a></td></tr><tr><td align="right">11.7 </td><td><a href="memory-order.html" class="tocviewlink" data-pltdoc="x">Machine Memory Order</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_2");">▼</a></td><td>11.2 </td><td><a href="all-sync.html" class="tocviewlink" data-pltdoc="x">Synchronization</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">11.2.1 </td><td><a href="sync.html" class="tocviewlink" data-pltdoc="x">Events</a></td></tr><tr><td align="right">11.2.2 </td><td><a href="channel.html" class="tocviewlink" data-pltdoc="x">Channels</a></td></tr><tr><td align="right">11.2.3 </td><td><a href="semaphore.html" class="tocviewselflink" data-pltdoc="x">Semaphores</a></td></tr><tr><td align="right">11.2.4 </td><td><a href="async-channel.html" class="tocviewlink" data-pltdoc="x">Buffered Asynchronous Channels</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">semaphore?</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._make-semaphore%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">make-<wbr></wbr>semaphore</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-post%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">semaphore-<wbr></wbr>post</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">semaphore-<wbr></wbr>wait</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-try-wait~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">semaphore-<wbr></wbr>try-<wbr></wbr>wait?</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">semaphore-<wbr></wbr>wait/<span class="mywbr"> </span>enable-<wbr></wbr>break</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-peek-evt%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">semaphore-<wbr></wbr>peek-<wbr></wbr>evt</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-peek-evt~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">semaphore-<wbr></wbr>peek-<wbr></wbr>evt?</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._call-with-semaphore%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>semaphore</span></span></a></td></tr><tr><td><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._call-with-semaphore%2Fenable-break%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>semaphore/<span class="mywbr"> </span>enable-<wbr></wbr>break</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" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="channel.html" title="backward to "11.2.2 Channels"" data-pltdoc="x">← prev</a> <a href="all-sync.html" title="up to "11.2 Synchronization"" data-pltdoc="x">up</a> <a href="async-channel.html" title="forward to "11.2.4 Buffered Asynchronous Channels"" data-pltdoc="x">next →</a></span> </div><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""semaphore"">11.2.3<tt> </tt><a name="(part._semaphore)"></a>Semaphores</h5><p>A <a name="(tech._semaphore)"></a><span style="font-style: italic">semaphore</span> has an internal counter; when this counter is
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zero, the semaphore can block a thread’s execution (through
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<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>) until another thread increments the counter
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(using <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-post%29%29" class="RktValLink" data-pltdoc="x">semaphore-post</a></span>). The maximum value for a semaphore’s
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internal counter is platform-specific, but always at least
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<span class="RktVal">10000</span>.</p><p>A semaphore’s counter is updated in a single-threaded manner, so that
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semaphores can be used for reliable synchronization. Semaphore waiting
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is <span style="font-style: italic">fair</span>: if a thread is blocked on a semaphore and the
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semaphore’s internal value is non-zero infinitely often, then the
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thread is eventually unblocked.</p><p>In addition to its use with semaphore-specific procedures, a semaphore
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can be used as a <a href="sync.html#%28tech._synchronizable._event%29" class="techoutside" data-pltdoc="x"><span class="techinside">synchronizable event</span></a> (see <a href="sync.html" data-pltdoc="x">Events</a>).
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A semaphore is <a href="sync.html#%28tech._ready._for._synchronization%29" class="techoutside" data-pltdoc="x"><span class="techinside">ready for synchronization</span></a> when
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<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span> would not block. Upon synchronization, the
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semaphore’s counter is decremented, and the <a href="sync.html#%28tech._synchronization._result%29" class="techoutside" data-pltdoc="x"><span class="techinside">synchronization result</span></a> of a semaphore is the semaphore itself.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._semaphore~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">semaphore?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is a <a href="semaphore.html#%28tech._semaphore%29" class="techoutside" data-pltdoc="x"><span class="techinside">semaphore</span></a>,
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<span class="RktVal">#f</span> otherwise.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._make-semaphore))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._make-semaphore%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-semaphore</a></span></span><span class="hspace"> </span><span class="RktOpt">[</span><span class="RktVar">init</span><span class="RktOpt">]</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">init</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-nonnegative-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-nonnegative-integer?</a></span><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">0</span></td></tr></table></blockquote></div><div class="SIntrapara">Creates and returns a new semaphore with the counter initially set to
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<span class="RktVar">init</span>. If <span class="RktVar">init</span> is larger than a semaphore’s maximum
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internal counter value, the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail%29%29" class="RktValLink" data-pltdoc="x">exn:fail</a></span> exception is raised.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._semaphore-post))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-post%29%29" class="RktValDef RktValLink" data-pltdoc="x">semaphore-post</a></span></span><span class="hspace"> </span><span class="RktVar">sema</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void~3f%29%29" class="RktValLink" data-pltdoc="x">void?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Increments the
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semaphore’s internal counter and returns <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=void_undefined.html&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<void></span></span></a>. If the
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semaphore’s internal counter has already reached its maximum value,
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the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail%29%29" class="RktValLink" data-pltdoc="x">exn:fail</a></span> exception is raised.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._semaphore-wait))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValDef RktValLink" data-pltdoc="x">semaphore-wait</a></span></span><span class="hspace"> </span><span class="RktVar">sema</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void~3f%29%29" class="RktValLink" data-pltdoc="x">void?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Blocks until the
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internal counter for semaphore <span class="RktVar">sema</span> is non-zero. When the
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counter is non-zero, it is decremented and <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>
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returns <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=void_undefined.html&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<void></span></span></a>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._semaphore-try-wait~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-try-wait~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">semaphore-try-wait?</a></span></span><span class="hspace"> </span><span class="RktVar">sema</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Like
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<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>, but <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-try-wait~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore-try-wait?</a></span> never blocks
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execution. If <span class="RktVar">sema</span>’s internal counter is zero,
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<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-try-wait~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore-try-wait?</a></span> returns <span class="RktVal">#f</span> immediately without
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decrementing the counter. If <span class="RktVar">sema</span>’s counter is positive, it
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is decremented and <span class="RktVal">#t</span> is returned.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._semaphore-wait/enable-break))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="RktValDef RktValLink" data-pltdoc="x">semaphore-wait/enable-break</a></span></span><span class="hspace"> </span><span class="RktVar">sema</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void~3f%29%29" class="RktValLink" data-pltdoc="x">void?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Like
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<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>, but breaking is enabled (see
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<a href="breakhandler.html" data-pltdoc="x">Breaks</a>) while waiting on <span class="RktVar">sema</span>. If
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breaking is disabled when <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait/enable-break</a></span> is
|
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called, then either the semaphore’s counter is decremented or the
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<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak%29%29" class="RktValLink" data-pltdoc="x">exn:break</a></span> exception is raised, but not both.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._semaphore-peek-evt))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-peek-evt%29%29" class="RktValDef RktValLink" data-pltdoc="x">semaphore-peek-evt</a></span></span><span class="hspace"> </span><span class="RktVar">sema</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-peek-evt~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore-peek-evt?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Creates and
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returns a new <a href="sync.html#%28tech._synchronizable._event%29" class="techoutside" data-pltdoc="x"><span class="techinside">synchronizable event</span></a> (for use with <span class="RktSym"><a href="sync.html#%28def._%28%28quote._~23~25kernel%29._sync%29%29" class="RktValLink" data-pltdoc="x">sync</a></span>, for
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example) that is <a href="sync.html#%28tech._ready._for._synchronization%29" class="techoutside" data-pltdoc="x"><span class="techinside">ready for synchronization</span></a> when <span class="RktVar">sema</span> is ready,
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but synchronizing
|
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the event does not decrement <span class="RktVar">sema</span>’s internal count.
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The <a href="sync.html#%28tech._synchronization._result%29" class="techoutside" data-pltdoc="x"><span class="techinside">synchronization result</span></a> of a semaphore-peek event is the semaphore-peek event itself.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._semaphore-peek-evt~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-peek-evt~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">semaphore-peek-evt?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is a semaphore wrapper produced by
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<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-peek-evt%29%29" class="RktValLink" data-pltdoc="x">semaphore-peek-evt</a></span>, <span class="RktVal">#f</span> otherwise.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-with-semaphore))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._call-with-semaphore%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-semaphore</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">sema</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">proc</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span><span class="RktOpt">[</span></td><td valign="top"><span class="RktVar">try-fail-thunk</span><span class="RktOpt">]</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">arg</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">try-fail-thunk</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">arg</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Waits on <span class="RktVar">sema</span> using <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>, calls
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<span class="RktVar">proc</span> with all <span class="RktVar">arg</span>s, and then posts to
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<span class="RktVar">sema</span>. A <a href="eval-model.html#%28tech._continuation._barrier%29" class="techoutside" data-pltdoc="x"><span class="techinside">continuation barrier</span></a> blocks full continuation jumps
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into or out of <span class="RktVar">proc</span> (see <a href="eval-model.html#%28part._prompt-model%29" data-pltdoc="x">Prompts, Delimited Continuations, and Barriers</a>), but
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escape jumps are allowed, and <span class="RktVar">sema</span> is posted on escape. If
|
|
<span class="RktVar">try-fail-thunk</span> is provided and is not <span class="RktVal">#f</span>, then
|
|
<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-try-wait~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore-try-wait?</a></span> is called on <span class="RktVar">sema</span> instead of
|
|
<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>, and <span class="RktVar">try-fail-thunk</span> is called if the
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|
wait fails.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-with-semaphore/enable-break))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._call-with-semaphore%2Fenable-break%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-semaphore/enable-break</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">sema</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">proc</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span><span class="RktOpt">[</span></td><td valign="top"><span class="RktVar">try-fail-thunk</span><span class="RktOpt">]</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">arg</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">try-fail-thunk</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">arg</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._call-with-semaphore%29%29" class="RktValLink" data-pltdoc="x">call-with-semaphore</a></span>, except that
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<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait/enable-break</a></span> is used with <span class="RktVar">sema</span> in
|
|
non-try mode. When <span class="RktVar">try-fail-thunk</span> is provided and not
|
|
<span class="RktVal">#f</span>, then breaks are enabled around the use of
|
|
<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-try-wait~3f%29%29" class="RktValLink" data-pltdoc="x">semaphore-try-wait?</a></span> on <span class="RktVar">sema</span>.</div></p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="channel.html" title="backward to "11.2.2 Channels"" data-pltdoc="x">← prev</a> <a href="all-sync.html" title="up to "11.2 Synchronization"" data-pltdoc="x">up</a> <a href="async-channel.html" title="forward to "11.2.4 Buffered Asynchronous Channels"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |