140 lines
No EOL
64 KiB
HTML
140 lines
No EOL
64 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<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>11.4 Futures</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="icons.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: block;" 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="tocviewlink" 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="tocviewselflink" 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>11.4 </td><td><a href="futures.html" class="tocviewselflink" data-pltdoc="x">Futures</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">11.4.1 </td><td><a href="futures.html#%28part._.Creating_and_.Touching_.Futures%29" class="tocviewlink" data-pltdoc="x">Creating and Touching Futures</a></td></tr><tr><td align="right">11.4.2 </td><td><a href="futures.html#%28part._.Future_.Semaphores%29" class="tocviewlink" data-pltdoc="x">Future Semaphores</a></td></tr><tr><td align="right">11.4.3 </td><td><a href="futures.html#%28part._future-logging%29" class="tocviewlink" data-pltdoc="x">Future Performance Logging</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">11.4.1<tt> </tt></span><a href="futures.html#%28part._.Creating_and_.Touching_.Futures%29" class="tocsubseclink" data-pltdoc="x">Creating and Touching Futures</a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">future</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">touch</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._futures-enabled~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">futures-<wbr></wbr>enabled?</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._current-future%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">current-<wbr></wbr>future</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">future?</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._would-be-future%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">would-<wbr></wbr>be-<wbr></wbr>future</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._processor-count%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">processor-<wbr></wbr>count</span></span></a></td></tr><tr><td><a href="futures.html#%28form._%28%28lib._racket%2Ffuture..rkt%29._for%2Fasync%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">for/<span class="mywbr"> </span>async</span></span></a></td></tr><tr><td><a href="futures.html#%28form._%28%28lib._racket%2Ffuture..rkt%29._for%2A%2Fasync%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">for*/<span class="mywbr"> </span>async</span></span></a></td></tr><tr><td><span class="tocsublinknumber">11.4.2<tt> </tt></span><a href="futures.html#%28part._.Future_.Semaphores%29" class="tocsubseclink" data-pltdoc="x">Future Semaphores</a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._make-fsemaphore%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">make-<wbr></wbr>fsemaphore</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">fsemaphore?</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-post%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">fsemaphore-<wbr></wbr>post</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-wait%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">fsemaphore-<wbr></wbr>wait</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-try-wait~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">fsemaphore-<wbr></wbr>try-<wbr></wbr>wait?</span></span></a></td></tr><tr><td><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-count%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">fsemaphore-<wbr></wbr>count</span></span></a></td></tr><tr><td><span class="tocsublinknumber">11.4.3<tt> </tt></span><a href="futures.html#%28part._future-logging%29" class="tocsubseclink" data-pltdoc="x">Future Performance Logging</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="parameters.html" title="backward to "11.3.2 Parameters"" data-pltdoc="x">← prev</a> <a href="concurrency.html" title="up to "11 Concurrency and Parallelism"" data-pltdoc="x">up</a> <a href="places.html" title="forward to "11.5 Places"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""futures"">11.4<tt> </tt><a name="(part._futures)"></a><a name="(mod-path._racket/future)"></a>Futures</h4><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="finger.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=parallelism.html%23%2528part._effective-futures%2529&version=8.6" class="Sq" data-pltdoc="x">Parallelism with Futures</a> in <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=index.html&version=8.6" class="Sq" data-pltdoc="x">The Racket Guide</a> introduces futures.</p></blockquote></blockquote></blockquote><p><div class="SIntrapara"><table cellspacing="0" cellpadding="0" class="defmodule"><tr><td align="left"><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="stt"> </span><a href="futures.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/future</span></a><span class="RktPn">)</span></td><td align="right"><span class="RpackageSpec"><span class="Smaller"> package:</span> <a href="https://pkgs.racket-lang.org/package/base" title="Install this package using `raco pkg install base`"><span class="stt">base</span></a></span></td></tr></table></div><div class="SIntrapara">The bindings documented in this section are provided by the <a href="futures.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/future</span></a> and <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket</span></a> libraries, but not <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/base</span></a>.</div></p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>Currently, parallel support for <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span> is enabled
|
|
by default for Windows, Linux x86/x86_64, and Mac OS x86/x86_64. To
|
|
enable support for other platforms, use <span class="nobreak"><span class="stt">--enable-futures</span></span> with
|
|
<span class="stt">configure</span> when building Racket.</p></blockquote></blockquote></blockquote><p>The <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span> and <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> functions from
|
|
<a href="futures.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/future</span></a> provide access to parallelism as supported
|
|
by the hardware and operating system. In contrast to <span class="RktSym"><a href="threads.html#%28def._%28%28quote._~23~25kernel%29._thread%29%29" class="RktValLink" data-pltdoc="x">thread</a></span>,
|
|
which provides concurrency for arbitrary computations without
|
|
parallelism, <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span> provides parallelism for limited
|
|
computations. A <a name="(tech._future)"></a><span style="font-style: italic">future</span> executes its work in parallel (assuming that
|
|
support for parallelism is available) until it detects an attempt to
|
|
perform an operation that is too complex for the system to run safely in
|
|
parallel. Similarly, work in a future is suspended if it depends in some
|
|
way on the current continuation, such as raising an exception. A
|
|
suspended computation for a future is resumed when <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> is
|
|
applied to the future.</p><p>“Safe” parallel execution of a future means that all operations
|
|
provided by the system must be able to enforce contracts and produce
|
|
results as documented. “Safe” does not preclude concurrent access to
|
|
mutable data that is visible in the program. For example, a computation
|
|
in a future might use <span class="RktSym"><a href="set_.html#%28form._%28%28quote._~23~25kernel%29._set%21%29%29" class="RktStxLink" data-pltdoc="x">set!</a></span> to modify a shared variable, in
|
|
which case concurrent assignment to the variable can be visible in other
|
|
futures and threads. Furthermore, guarantees about the visibility of
|
|
effects and ordering are determined by the operating system and
|
|
hardware—<wbr></wbr>which rarely support, for example, the guarantee of
|
|
sequential consistency that is provided for <span class="RktSym"><a href="threads.html#%28def._%28%28quote._~23~25kernel%29._thread%29%29" class="RktValLink" data-pltdoc="x">thread</a></span>-based
|
|
concurrency; see also <a href="memory-order.html" data-pltdoc="x">Machine Memory Order</a>. At the same time, operations
|
|
that seem obviously safe may
|
|
have a complex enough implementation internally that they cannot run in
|
|
parallel. See also <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=parallelism.html%23%2528part._effective-futures%2529&version=8.6" class="Sq" data-pltdoc="x">Parallelism with Futures</a> in <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=index.html&version=8.6" class="Sq" data-pltdoc="x">The Racket Guide</a>.</p><p>A future never runs in parallel if all of the <a href="eval-model.html#%28tech._custodian%29" class="techoutside" data-pltdoc="x"><span class="techinside">custodians</span></a> that
|
|
allow its creating thread to run are shut down. Such futures can
|
|
execute through a call to <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span>, however.</p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Creating_and_Touching_Futures"">11.4.1<tt> </tt><a name="(part._.Creating_and_.Touching_.Futures)"></a>Creating and Touching Futures</h5><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><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._((lib._racket/future..rkt)._future))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValDef RktValLink" data-pltdoc="x">future</a></span></span><span class="hspace"> </span><span class="RktVar">thunk</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future~3f%29%29" class="RktValLink" data-pltdoc="x">future?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">thunk</span><span class="hspace"> </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></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><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._((lib._racket/future..rkt)._touch))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValDef RktValLink" data-pltdoc="x">touch</a></span></span><span class="hspace"> </span><span class="RktVar">f</span><span class="RktPn">)</span><span class="hspace"> </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></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">f</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future~3f%29%29" class="RktValLink" data-pltdoc="x">future?</a></span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span> procedure returns a future value that encapsulates
|
|
<span class="RktVar">thunk</span>. The <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> function forces the evaluation of
|
|
the <span class="RktVar">thunk</span> inside the given future, returning the values
|
|
produced by <span class="RktVar">thunk</span>. After <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> forces the evaluation
|
|
of a <span class="RktVar">thunk</span>, the resulting values are retained by the future
|
|
in place of <span class="RktVar">thunk</span>, and additional <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span>es of the
|
|
future return those values.</div></p><p>Between a call to <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span> and <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> for a given
|
|
future, the given <span class="RktVar">thunk</span> may run speculatively in parallel to
|
|
other computations, as described above.</p><p><div class="SIntrapara">Example:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktVar">f</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</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="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._list%29%29" class="RktValLink" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span><span class="hspace"> </span><span class="RktVar">f</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'(7 3)</span></p></td></tr></table></blockquote></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._((lib._racket/future..rkt)._futures-enabled~3f))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._futures-enabled~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">futures-enabled?</a></span></span><span class="RktPn"></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></table></blockquote></div><div class="SIntrapara">Returns whether parallel support for futures is enabled
|
|
in the current Racket configuration.</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._((lib._racket/future..rkt)._current-future))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._current-future%29%29" class="RktValDef RktValLink" data-pltdoc="x">current-future</a></span></span><span class="RktPn"></span><span class="RktPn">)</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="RktVal">#f</span><span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future~3f%29%29" class="RktValLink" data-pltdoc="x">future?</a></span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Returns the descriptor of the future whose thunk execution is the
|
|
current continuation; that is, if a future descriptor <span class="RktSym">f</span> is returned,
|
|
<span class="RktPn">(</span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span><span class="stt"> </span><span class="RktSym">f</span><span class="RktPn">)</span> will produce the result of the current continuation.
|
|
If a future thunk itself uses <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span>,
|
|
future-thunk executions can be nested, in which case the descriptor of
|
|
the most immediately executing future is returned. If the current
|
|
continuation does not return to the <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> of any future, the result is
|
|
<span class="RktVal">#f</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/future..rkt)._future~3f))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">future?</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 future value, <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._((lib._racket/future..rkt)._would-be-future))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._would-be-future%29%29" class="RktValDef RktValLink" data-pltdoc="x">would-be-future</a></span></span><span class="hspace"> </span><span class="RktVar">thunk</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future~3f%29%29" class="RktValLink" data-pltdoc="x">future?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">thunk</span><span class="hspace"> </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></td></tr></table></blockquote></div><div class="SIntrapara">Returns a future that never runs in parallel, but that consistently
|
|
logs all potentially “unsafe” operations during the execution of
|
|
the future’s thunk (i.e., operations that interfere with parallel
|
|
execution).</div></p><p>With a normal future, certain circumstances might prevent the logging
|
|
of unsafe operations. For example, when executed with debug-level logging,</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"hello1"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"hello2"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"hello3"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>might log three messages, one for each <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span>
|
|
invocation. However, if the <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> is performed before the future
|
|
has a chance to start running in parallel, the future thunk evaluates
|
|
in the same manner as any ordinary thunk, and no unsafe operations
|
|
are logged. Replacing <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span> with <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._would-be-future%29%29" class="RktValLink" data-pltdoc="x">would-be-future</a></span>
|
|
ensures the logging of all three calls to <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span>.</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._((lib._racket/future..rkt)._processor-count))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._processor-count%29%29" class="RktValDef RktValLink" data-pltdoc="x">processor-count</a></span></span><span class="RktPn"></span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-positive-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-positive-integer?</a></span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Returns the number of parallel computation units (e.g., processors or
|
|
cores) that are available on the current machine.</div></p><p>This is the same binding as available from <a href="places.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/place</span></a>.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/future..rkt)._for/async))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28form._%28%28lib._racket%2Ffuture..rkt%29._for%2Fasync%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">for/async</a></span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">for-clause</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/future..rkt)._for*/async))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28form._%28%28lib._racket%2Ffuture..rkt%29._for%2A%2Fasync%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">for*/async</a></span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">for-clause</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="for.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for%29%29" class="RktStxLink" data-pltdoc="x">for</a></span> and <span class="RktSym"><a href="for.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for%2A%29%29" class="RktStxLink" data-pltdoc="x">for*</a></span>, but each iteration of the
|
|
<span class="RktVar">body</span> is executed in a separate <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._future%29%29" class="RktValLink" data-pltdoc="x">future</a></span>, and
|
|
the futures may be <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span>ed in any order.</div></p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Future_Semaphores"">11.4.2<tt> </tt><a name="(part._.Future_.Semaphores)"></a>Future Semaphores</h5><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._((lib._racket/future..rkt)._make-fsemaphore))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._make-fsemaphore%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-fsemaphore</a></span></span><span class="hspace"> </span><span class="RktVar">init</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore~3f%29%29" class="RktValLink" data-pltdoc="x">fsemaphore?</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></td></tr></table></blockquote></div><div class="SIntrapara">Creates and returns a new <a name="(tech._future._semaphore)"></a><span style="font-style: italic">future semaphore</span> with the
|
|
counter initially set to <span class="RktVar">init</span>.</div></p><p>A future semaphore is similar to a plain <a href="semaphore.html#%28tech._semaphore%29" class="techoutside" data-pltdoc="x"><span class="techinside">semaphore</span></a>, but
|
|
future-semaphore operations can be performed safely in parallel (to synchronize
|
|
parallel computations). In contrast, operations on plain <a href="semaphore.html#%28tech._semaphore%29" class="techoutside" data-pltdoc="x"><span class="techinside">semaphores</span></a>
|
|
are not safe to perform in parallel, and they therefore prevent
|
|
a computation from continuing in parallel.</p><p>Beware of trying to use an fsemaphore to implement a lock. A future
|
|
may run concurrently and in parallel to other futures, but a future
|
|
that is not demanded by a Racket thread can be suspended at any
|
|
time—<wbr></wbr>such as just after it takes a lock and before it releases the
|
|
lock. If you must share mutable data among futures, lock-free data
|
|
structures are generally a better fit.</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._((lib._racket/future..rkt)._fsemaphore~3f))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">fsemaphore?</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 an <a href="futures.html#%28tech._future._semaphore%29" class="techoutside" data-pltdoc="x"><span class="techinside">future semaphore</span></a>
|
|
value, <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._((lib._racket/future..rkt)._fsemaphore-post))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-post%29%29" class="RktValDef RktValLink" data-pltdoc="x">fsemaphore-post</a></span></span><span class="hspace"> </span><span class="RktVar">fsema</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">fsema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore~3f%29%29" class="RktValLink" data-pltdoc="x">fsemaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Increments the <a href="futures.html#%28tech._future._semaphore%29" class="techoutside" data-pltdoc="x"><span class="techinside">future semaphore</span></a>’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>.</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._((lib._racket/future..rkt)._fsemaphore-wait))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-wait%29%29" class="RktValDef RktValLink" data-pltdoc="x">fsemaphore-wait</a></span></span><span class="hspace"> </span><span class="RktVar">fsema</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">fsema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore~3f%29%29" class="RktValLink" data-pltdoc="x">fsemaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Blocks until the internal counter for <span class="RktVar">fsema</span> is non-zero.
|
|
When the counter is non-zero, it is decremented and
|
|
<span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-wait%29%29" class="RktValLink" data-pltdoc="x">fsemaphore-wait</a></span> 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._((lib._racket/future..rkt)._fsemaphore-try-wait~3f))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-try-wait~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">fsemaphore-try-wait?</a></span></span><span class="hspace"> </span><span class="RktVar">fsema</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">fsema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore~3f%29%29" class="RktValLink" data-pltdoc="x">fsemaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-wait%29%29" class="RktValLink" data-pltdoc="x">fsemaphore-wait</a></span>, but <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-try-wait~3f%29%29" class="RktValLink" data-pltdoc="x">fsemaphore-try-wait?</a></span>
|
|
never blocks execution. If <span class="RktVar">fsema</span>’s internal counter is zero,
|
|
<span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-try-wait~3f%29%29" class="RktValLink" data-pltdoc="x">fsemaphore-try-wait?</a></span> returns <span class="RktVal">#f</span> immediately without
|
|
decrementing the counter. If <span class="RktVar">fsema</span>’s counter is positive, it
|
|
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._((lib._racket/future..rkt)._fsemaphore-count))"></a><span title="Provided from: racket/future, racket | Package: base"><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore-count%29%29" class="RktValDef RktValLink" data-pltdoc="x">fsemaphore-count</a></span></span><span class="hspace"> </span><span class="RktVar">fsema</span><span class="RktPn">)</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></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">fsema</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._fsemaphore~3f%29%29" class="RktValLink" data-pltdoc="x">fsemaphore?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVar">fsema</span>’s current internal counter value.</div></p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""future-logging"">11.4.3<tt> </tt><a name="(part._future-logging)"></a>Future Performance Logging</h5><p>Racket traces use logging (see <a href="logging.html" data-pltdoc="x">Logging</a>) extensively to
|
|
report information about how futures are evaluated. Logging output is
|
|
useful for debugging the performance of programs that use futures.</p><p>Though textual log output can be viewed directly (or retrieved in
|
|
code via <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=future-visualizer&rel=futures-trace.html%23%2528form._%2528%2528lib._future-visualizer%252Ftrace..rkt%2529._trace-futures%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">trace-futures</a></span>), it is much
|
|
easier to use the graphical profiler tool provided by
|
|
<a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=future-visualizer&rel=index.html&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">future-visualizer</span></a>.</p><p>Future events are logged with the topic <span class="RktVal">'</span><span class="RktVal">future</span>.
|
|
In addition to its string message, each event logged for a future has
|
|
a data value that is an instance of a <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=future-visualizer&rel=futures-trace.html%23%2528def._%2528%2528lib._future-visualizer%252Ftrace..rkt%2529._future-event%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">future-event</a></span>
|
|
<a href="structures.html#%28tech._prefab%29" class="techoutside" data-pltdoc="x"><span class="techinside">prefab</span></a> structure:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=future-visualizer&rel=futures-trace.html%23%2528def._%2528%2528lib._future-visualizer%252Ftrace..rkt%2529._future-event%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">future-event</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">future-id</span><span class="hspace"> </span><span class="RktSym">proc-id</span><span class="hspace"> </span><span class="RktSym">action</span><span class="hspace"> </span><span class="RktSym"><a href="time.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._time%29%29" class="RktStxLink" data-pltdoc="x">time</a></span><span class="hspace"> </span><span class="RktSym">prim-name</span><span class="hspace"> </span><span class="RktSym">user-data</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:prefab</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The <span class="RktSym">future-id</span> field is an exact integer that identifies a
|
|
future, or it is <span class="RktVal">#f</span> when <span class="RktSym">action</span> is
|
|
<span class="RktVal">'</span><span class="RktVal">missing</span>. The <span class="RktSym">future-id</span> field is particularly useful
|
|
for correlating logged events.</p><p>The <span class="RktSym">proc-id</span> fields is an exact, non-negative integer that
|
|
identifies a parallel process. Process 0 is the main Racket process,
|
|
where all expressions other than future thunks evaluate.</p><p>The <span class="RktSym"><a href="time.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._time%29%29" class="RktStxLink" data-pltdoc="x">time</a></span> field is an inexact number that represents time in
|
|
the same way as <span class="RktSym"><a href="time.html#%28def._%28%28quote._~23~25kernel%29._current-inexact-milliseconds%29%29" class="RktValLink" data-pltdoc="x">current-inexact-milliseconds</a></span>.</p><p>The <span class="RktSym">action</span> field is a symbol:</p><ul><li><p><span class="RktVal">'</span><span class="RktVal">create</span>: a future was created.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">complete</span>: a future’s thunk evaluated successfully, so
|
|
that <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> will produce a value for the future
|
|
immediately.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">start-work</span> and <span class="RktVal">'</span><span class="RktVal">end-work</span>: a particular
|
|
process started and ended working on a particular future.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">start-0-work</span>: like <span class="RktVal">'</span><span class="RktVal">start-work</span>, but for a
|
|
future thunk that for some structural reason could not be
|
|
started in a process other than 0 (e.g., the thunk requires too
|
|
much local storage to start).</p></li><li><p><span class="RktVal">'</span><span class="RktVal">start-overflow-work</span>: like <span class="RktVal">'</span><span class="RktVal">start-work</span>, where
|
|
the future thunk’s work was previously stopped due to an
|
|
internal stack overflow.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">sync</span>: blocking (processes other than 0) or initiation
|
|
of handing (process 0) for an “unsafe” operation in a future
|
|
thunk’s evaluation; the operation must run in process 0.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">block</span>: like <span class="RktVal">'</span><span class="RktVal">sync</span>, but for a part of
|
|
evaluation that must be delayed until the future is
|
|
<span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span>ed, because the evaluation may depend on the
|
|
current continuation.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">touch</span> (never in process 0): like <span class="RktVal">'</span><span class="RktVal">sync</span> or
|
|
<span class="RktVal">'</span><span class="RktVal">block</span>, but for a <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> operation within a
|
|
future thunk.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">overflow</span> (never in process 0): like <span class="RktVal">'</span><span class="RktVal">sync</span> or
|
|
<span class="RktVal">'</span><span class="RktVal">block</span>, but for the case that a process encountered an
|
|
internal stack overflow while evaluating a future thunk.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">result</span> or <span class="RktVal">'</span><span class="RktVal">abort</span>: waiting or handling for
|
|
<span class="RktVal">'</span><span class="RktVal">sync</span>, <span class="RktVal">'</span><span class="RktVal">block</span>, or <span class="RktVal">'</span><span class="RktVal">touch</span> ended with
|
|
a value or an error, respectively.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">suspend</span> (never in process 0): a process blocked by
|
|
<span class="RktVal">'</span><span class="RktVal">sync</span>, <span class="RktVal">'</span><span class="RktVal">block</span>, or <span class="RktVal">'</span><span class="RktVal">touch</span> abandoned
|
|
evaluation of a future; some other process may pick up the
|
|
future later.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">touch-pause</span> and <span class="RktVal">'</span><span class="RktVal">touch-resume</span> (in process 0,
|
|
only): waiting in <span class="RktSym"><a href="futures.html#%28def._%28%28lib._racket%2Ffuture..rkt%29._touch%29%29" class="RktValLink" data-pltdoc="x">touch</a></span> for a future whose thunk is
|
|
being evaluated in another process.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">missing</span>: one or more events for the process were lost
|
|
due to internal buffer limits before they could be reported,
|
|
and the <span class="RktSym">time-id</span> field reports an upper limit on the time
|
|
of the missing events; this kind of event is rare.</p></li></ul><p>Assuming no <span class="RktVal">'</span><span class="RktVal">missing</span> events, then <span class="RktVal">'</span><span class="RktVal">start-work</span>,
|
|
<span class="RktVal">'</span><span class="RktVal">start-0-work</span>, <span class="RktVal">'</span><span class="RktVal">start-overflow-work</span> is always paired with <span class="RktVal">'</span><span class="RktVal">end-work</span>;
|
|
<span class="RktVal">'</span><span class="RktVal">sync</span>, <span class="RktVal">'</span><span class="RktVal">block</span>, and <span class="RktVal">'</span><span class="RktVal">touch</span> are always paired
|
|
with <span class="RktVal">'</span><span class="RktVal">result</span>, <span class="RktVal">'</span><span class="RktVal">abort</span>, or <span class="RktVal">'</span><span class="RktVal">suspend</span>; and
|
|
<span class="RktVal">'</span><span class="RktVal">touch-pause</span> is always paired with <span class="RktVal">'</span><span class="RktVal">touch-resume</span>.</p><p>In process 0, some event pairs can be nested within other event pairs:
|
|
<span class="RktVal">'</span><span class="RktVal">sync</span>, <span class="RktVal">'</span><span class="RktVal">block</span>, or <span class="RktVal">'</span><span class="RktVal">touch</span> with
|
|
<span class="RktVal">'</span><span class="RktVal">result</span> or <span class="RktVal">'</span><span class="RktVal">abort</span>; <span class="RktVal">'</span><span class="RktVal">touch-pause</span> with
|
|
<span class="RktVal">'</span><span class="RktVal">touch-resume</span>; and <span class="RktVal">'</span><span class="RktVal">start-work</span> with <span class="RktVal">'</span><span class="RktVal">end-work</span>.</p><p>A <span class="RktVal">'</span><span class="RktVal">block</span> in process 0 is generated when an unsafe operation
|
|
is handled. This type of event will contain a symbol in the
|
|
<span class="RktSym">unsafe-op-name</span> field that is the name of the operation. In all
|
|
other cases, this field contains <span class="RktVal">#f</span>.</p><p>The <span class="RktSym">prim-name</span> field will always be <span class="RktVal">#f</span> unless the event occurred
|
|
on process 0 and its <span class="RktSym">action</span> is either <span class="RktVal">'</span><span class="RktVal">block</span> or <span class="RktVal">'</span><span class="RktVal">sync</span>. If
|
|
these conditions are met, <span class="RktSym">prim-name</span> will contain the name
|
|
of the Racket primitive which required the future to synchronize with the runtime
|
|
thread (represented as a symbol).</p><p>The <span class="RktSym">user-data</span> field may take on a number of different
|
|
values depending on both the <span class="RktSym">action</span> and <span class="RktSym">prim-name</span> fields:</p><ul><li><p><span class="RktVal">'</span><span class="RktVal">touch</span> on process 0: contains the integer ID of the future
|
|
being touched.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">sync</span> and <span class="RktSym">prim-name</span> is <span class="RktVal">'</span><span class="RktVal">|allocate memory|</span>:
|
|
The size (in bytes) of the requested allocation.</p></li><li><p><span class="RktVal">'</span><span class="RktVal">sync</span> and <span class="RktSym">prim-name</span> is <span class="RktVal">'</span><span class="RktVal">jit_on_demand</span>:
|
|
The runtime thread is performing a JIT compilation on behalf of the
|
|
future <span class="RktSym">future-id</span>. The field contains the name of the function
|
|
being JIT compiled (as a symbol).</p></li><li><p><span class="RktVal">'</span><span class="RktVal">create</span>: A new future was created. The field contains the integer ID
|
|
of the newly created future.</p></li></ul><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="parameters.html" title="backward to "11.3.2 Parameters"" data-pltdoc="x">← prev</a> <a href="concurrency.html" title="up to "11 Concurrency and Parallelism"" data-pltdoc="x">up</a> <a href="places.html" title="forward to "11.5 Places"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |