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<span class="normal">683</span></pre></div></td><td class="code"><div><pre><div id="l1" class="code_block"><span class="k">\chapter</span><span class="nb">{</span>Multi-Threaded Programming in XSB<span class="nb">}</span> <span class="k">\label</span><span class="nb">{</span>chap:threads<span class="nb">}</span>
</div><div id="l2" class="code_block">
</div><div id="l3" class="code_block">Starting with Version 3.0, XSB supports the use of Posix threads to
</div><div id="l4" class="code_block">perform separable computations, and in certain cases to parallelize
</div><div id="l5" class="code_block">them.  Posix threads have a simple and clear API, and are available on
</div><div id="l6" class="code_block">all Unixes as well as on Windows via ???.  This chapter introduces how
</div><div id="l7" class="code_block">to program with threads in XSB through a series of examples.  Later
</div><div id="l8" class="code_block">sections discuss performance aspects of our implementation as well as
</div><div id="l9" class="code_block">describing relevant predicates.  A general knowledge of multi-threaded
</div><div id="l10" class="code_block">programming is assumed, such as can be found in <span class="k">\cite</span><span class="nb">{</span>LewB98,Bute97<span class="nb">}</span>.
</div><div id="l11" class="code_block">
</div><div id="l12" class="code_block"><span class="k">\section</span><span class="nb">{</span>Getting Started with Multi-Threading<span class="nb">}</span>
</div><div id="l13" class="code_block"><span class="c">%</span>
</div><div id="l14" class="code_block">In Version 3.0 the default configuration of XSB does not include
</div><div id="l15" class="code_block">multi-threading.  This is partly because multi-threading is new, and
</div><div id="l16" class="code_block">despite our efforts, the multi-threaded engine may contain bugs not
</div><div id="l17" class="code_block">present in the single-
</div><div id="l18" class="code_block">threaded engine.  In addition, in Version 3.0,
</div><div id="l19" class="code_block">not all packages have yet been made thread-safe so that not all
</div><div id="l20" class="code_block">configurations are supported with multi-threading.  Currently ODBC is
</div><div id="l21" class="code_block">supported with the multi-threaded configuration, but <span class="nb">{</span><span class="k">\tt</span> dbdrivers<span class="nb">}</span>,
</div><div id="l22" class="code_block"><span class="nb">{</span><span class="k">\tt</span> xpath<span class="nb">}</span>, <span class="nb">{</span><span class="k">\tt</span> iodbc<span class="nb">}</span>, <span class="nb">{</span><span class="k">\tt</span> interprolog<span class="nb">}</span> and <span class="nb">{</span><span class="k">\tt</span> smodels<span class="nb">}</span> have not
</div><div id="l23" class="code_block">been. <span class="nb">{</span><span class="k">\sc</span> What about others?<span class="nb">}</span>
</div><div id="l24" class="code_block">
</div><div id="l25" class="code_block">With this in mind, making the multi-threaded engine is simple:
</div><div id="l26" class="code_block">configure and make XSB as in Chapter~<span class="k">\ref</span><span class="nb">{</span>chap:system<span class="nb">}</span>, but include
</div><div id="l27" class="code_block">the command <span class="nb">{</span><span class="k">\tt</span> --enable-mt<span class="nb">}</span>.  When you invoke the newly made
</div><div id="l28" class="code_block">configuration of XSB you should see <span class="nb">{</span><span class="k">\tt</span> engine: multi-threading<span class="nb">}</span> in
</div><div id="l29" class="code_block">the configuration list below the banner rather than <span class="nb">{</span><span class="k">\tt</span> engine:
</div><div id="l30" class="code_block">  slg-wam<span class="nb">}</span>.
</div><div id="l31" class="code_block">
</div><div id="l32" class="code_block"><span class="k">\paragraph</span><span class="nb">{</span>Hello World for Beginners<span class="nb">}</span>
</div><div id="l33" class="code_block"><span class="c">%</span>
</div><div id="l34" class="code_block">We naturally start with a program to print ``hello world&#39;&#39;.  Within
</div><div id="l35" class="code_block">the multi-threaded engine, import <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_create</span>/2<span class="nb">}</span> from the
</div><div id="l36" class="code_block">module <span class="nb">{</span><span class="k">\tt</span> thread<span class="nb">}</span>, and type the command
</div><div id="l37" class="code_block"><span class="k">\begin</span><span class="nb">{</span>center<span class="nb">}</span> 
</div><div id="l38" class="code_block"><span class="nb">{</span><span class="k">\tt</span> ?- xsb<span class="k">\_thread\_create</span>(writeln(&#39;hello world&#39;),Id)<span class="nb">}</span> 
</div><div id="l39" class="code_block"><span class="k">\end</span><span class="nb">{</span>center<span class="nb">}</span> 
</div><div id="l40" class="code_block">you should see something like 
</div><div id="l41" class="code_block"><span class="k">\begin</span><span class="nb">{</span>center<span class="nb">}</span> 
</div><div id="l42" class="code_block"><span class="nb">{</span><span class="k">\tt</span> Id = 1hello world<span class="nb">}</span> 
</div><div id="l43" class="code_block"><span class="k">\end</span><span class="nb">{</span>center<span class="nb">}</span> 
</div><div id="l44" class="code_block"><span class="c">%</span>
</div><div id="l45" class="code_block">while the output is a little ugly, the simple ``hello world&#39;&#39; program
</div><div id="l46" class="code_block">does illustrate simple multi-threading at work.  The calling thread
</div><div id="l47" class="code_block">(i.e. the thread controlling the command-line interpreter which we
</div><div id="l48" class="code_block">call <span class="s">$</span><span class="nb">T_{prompt}</span><span class="s">$</span>) executes the predicate <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_create</span>/2<span class="nb">}</span>
</div><div id="l49" class="code_block">which creates a thread <span class="s">$</span><span class="nb">T_{child}</span><span class="s">$</span> and immediately returns with the
</div><div id="l50" class="code_block"><span class="nb">{</span><span class="k">\em</span> XSB thread id<span class="nb">}</span> of the created thread.  Meanwhile, <span class="s">$</span><span class="nb">T_{child}</span><span class="s">$</span>
</div><div id="l51" class="code_block">initializes its stacks and other memory areas and executes the goal
</div><div id="l52" class="code_block"><span class="nb">{</span><span class="k">\tt</span> writeln(&#39;hello world&#39;)<span class="nb">}</span>.  <span class="s">$</span><span class="nb">T_{child}</span><span class="s">$</span> and <span class="s">$</span><span class="nb">T_{prompt}</span><span class="s">$</span> share most
</div><div id="l53" class="code_block">of their process-level information: in particular they share a common
</div><div id="l54" class="code_block">I/O stream for standard output, leading to the output above.  What is
</div><div id="l55" class="code_block">happening may be seen a little more easily by executing the command
</div><div id="l56" class="code_block"><span class="c">%</span>
</div><div id="l57" class="code_block"><span class="k">\begin</span><span class="nb">{</span>center<span class="nb">}</span> 
</div><div id="l58" class="code_block"><span class="nb">{</span><span class="k">\tt</span> ?- xsb<span class="k">\_thread\_create</span>((sleep(1),writeln(&#39;hello world&#39;)),Id)<span class="nb">}</span> 
</div><div id="l59" class="code_block"><span class="k">\end</span><span class="nb">{</span>center<span class="nb">}</span> 
</div><div id="l60" class="code_block"><span class="c">%</span>
</div><div id="l61" class="code_block">In this case the interpreter reports that <span class="nb">{</span><span class="k">\tt</span> F<span class="nb">}</span> is bound to a thread
</div><div id="l62" class="code_block">id, then about a second later <span class="nb">{</span><span class="k">\tt</span> writeln/1<span class="nb">}</span> is executed.  
</div><div id="l63" class="code_block">
</div><div id="l64" class="code_block">The simple ``hello world&#39;&#39; program illustrates a couple of points.
</div><div id="l65" class="code_block">First, it is easy to create a thread in XSB and have that thread do
</div><div id="l66" class="code_block">work.  Second, it can be tricky to coordinate actions among threads.
</div><div id="l67" class="code_block">We&#39;ll explore these two themes in more detail, but first suppose we
</div><div id="l68" class="code_block">are determined to extend out multi-threaded program so that it
</div><div id="l69" class="code_block">produces good output.  One way to do this is to <span class="nb">{</span><span class="k">\em</span> join<span class="nb">}</span>
</div><div id="l70" class="code_block"><span class="s">$</span><span class="nb">T_{prompt}</span><span class="s">$</span> and <span class="s">$</span><span class="nb">T_{child}</span><span class="s">$</span> as follows 
</div><div id="l71" class="code_block"><span class="c">%</span>
</div><div id="l72" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l73" class="code_block">?- xsb<span class="nb">_</span>thread<span class="nb">_</span>create(writeln(&#39;hello world&#39;),Id),
</div><div id="l74" class="code_block">   xsb<span class="nb">_</span>thread<span class="nb">_</span>join(Id,ExitCode).
</div><div id="l75" class="code_block">hello world
</div><div id="l76" class="code_block">
</div><div id="l77" class="code_block">Id = 1
</div><div id="l78" class="code_block">ExitCode = no<span class="nb">_</span>error<span class="nb">_</span>ball - 0
</div><div id="l79" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l80" class="code_block"><span class="c">%</span>
</div><div id="l81" class="code_block">In this case, as soon as <span class="s">$</span><span class="nb">T_{prompt}</span><span class="s">$</span> has issued a command to create
</div><div id="l82" class="code_block"><span class="s">$</span><span class="nb">T_{child}</span><span class="s">$</span>, it executes <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_join</span>/2<span class="nb">}</span>.  This later
</div><div id="l83" class="code_block">predicate makes a system call to the underlying operating system to
</div><div id="l84" class="code_block">suspend <span class="s">$</span><span class="nb">T_{prompt}</span><span class="s">$</span> until <span class="s">$</span><span class="nb">T_{child}</span><span class="s">$</span> has exited.  <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l85" class="code_block">  xsb<span class="k">\_thread\_join</span>/2<span class="nb">}</span> returns both the error ball (if any) and the
</div><div id="l86" class="code_block">exit code of <span class="s">$</span><span class="nb">T_{child}</span><span class="s">$</span>.  As usual, a value of <span class="nb">{</span><span class="k">\tt</span> 0<span class="nb">}</span> indicates
</div><div id="l87" class="code_block">normal termination, as does the fact that no error ball was found.
</div><div id="l88" class="code_block">
</div><div id="l89" class="code_block">So far, we&#39;ve introduced a few concepts that have not been fully
</div><div id="l90" class="code_block">discussed.  First is the concept of an <span class="nb">{</span><span class="k">\em</span> XSB thread id<span class="nb">}</span>: XSB
</div><div id="l91" class="code_block">manages up to <span class="nb">{</span><span class="k">\tt</span> MAXTHREAD<span class="nb">}</span> active threads using XSB thread
</div><div id="l92" class="code_block">ids~<span class="k">\footnote</span><span class="nb">{</span>This number can be reset via a command line option to
</div><div id="l93" class="code_block">  XSB, see Section~<span class="k">\ref</span><span class="nb">{</span>sec:EmuOptions<span class="nb">}}</span>.  It should be noted that the
</div><div id="l94" class="code_block">XSB thread id of a thread is different from the identifier of the
</div><div id="l95" class="code_block">underlying pthread.  An XSB thread id is a Prolog term, and unlike
</div><div id="l96" class="code_block">Posix thread ids, XSB thread ids can be compared for equality using
</div><div id="l97" class="code_block">unification.  The actual form of an XSB thread id, however, is subject
</div><div id="l98" class="code_block">to change between versions, so programs should not make use of the
</div><div id="l99" class="code_block">exact form of an XSB thread id.  In the multi-threaded engine, the XSB
</div><div id="l100" class="code_block">thread id of any thread can be queried using the predicate <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l101" class="code_block">  xsb<span class="k">\_thread\_self</span>/1<span class="nb">}</span>.
</div><div id="l102" class="code_block">
</div><div id="l103" class="code_block"><span class="k">\section</span><span class="nb">{</span>Communication among Threads<span class="nb">}</span>
</div><div id="l104" class="code_block">
</div><div id="l105" class="code_block"><span class="k">\begin</span><span class="nb">{</span>example<span class="nb">}</span>
</div><div id="l106" class="code_block">Consider the program fragment
</div><div id="l107" class="code_block"><span class="c">%</span>
</div><div id="l108" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l109" class="code_block">:- dynamic p/1.
</div><div id="l110" class="code_block">
</div><div id="l111" class="code_block">test:- xsb<span class="nb">_</span>thread<span class="nb">_</span>create(assert(p(1)),<span class="nb">_</span>X).
</div><div id="l112" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l113" class="code_block">If you type the goal <span class="nb">{</span><span class="k">\tt</span> ?- test<span class="nb">}</span> and then the goal <span class="nb">{</span><span class="k">\tt</span> ?- p(X)<span class="nb">}</span>,
</div><div id="l114" class="code_block">the call <span class="nb">{</span><span class="k">\tt</span> p(X)<span class="nb">}</span> will fail.  
</div><div id="l115" class="code_block"><span class="k">\end</span><span class="nb">{</span>example<span class="nb">}</span>
</div><div id="l116" class="code_block">
</div><div id="l117" class="code_block">This illustrates an important point about dynamic and tabled
</div><div id="l118" class="code_block">predicates in the multi-threaded engine: by default such predicate
</div><div id="l119" class="code_block"><span class="nb">{</span><span class="k">\tt</span> p/n<span class="nb">}</span> are private to the thread that asserts clauses to <span class="nb">{</span><span class="k">\tt</span> p/n<span class="nb">}</span>
</div><div id="l120" class="code_block">or that creates the table for <span class="nb">{</span><span class="k">\tt</span> p/n<span class="nb">}</span>.  This contrasts to static
</div><div id="l121" class="code_block">code is always shared between threads.  To allow <span class="nb">{</span><span class="k">\tt</span> p(1)<span class="nb">}</span> to be
</div><div id="l122" class="code_block">visible to various threads, it must be declared to be shared with the
</div><div id="l123" class="code_block">following declaration.
</div><div id="l124" class="code_block"><span class="c">%</span>
</div><div id="l125" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l126" class="code_block">n:- thread<span class="nb">_</span>shared(p(<span class="nb">_</span>)).
</div><div id="l127" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l128" class="code_block">
</div><div id="l129" class="code_block">The ability to share dynamic code between predicates provides an
</div><div id="l130" class="code_block">extremely powerful mechanism for threads to communicate.  So why does
</div><div id="l131" class="code_block">XSB make dynamic predicates thread-private by default?  The main
</div><div id="l132" class="code_block">reason for this is that if dozens or hundreds of threads are running
</div><div id="l133" class="code_block">concurrently, shared dynamic code becomes an expensive synchronization
</div><div id="l134" class="code_block">point.  Code for shared predicates must be more heavily mutexed than
</div><div id="l135" class="code_block">code for private predicates.  In particular, XSB may not immediately
</div><div id="l136" class="code_block">reclaim the space of retracted clause, to avoid the possibility of
</div><div id="l137" class="code_block">some computation backtracking into a clause that has been reclaimed.
</div><div id="l138" class="code_block">Rather XSB may garbage collect the space of the retracted clauses at a
</div><div id="l139" class="code_block">later time.  While clause garbage collection is simple enough to
</div><div id="l140" class="code_block">implement for a single thread, garbage collecting clauses for shared
</div><div id="l141" class="code_block">dynamic predicates is nearly impossible to do when multiple threads
</div><div id="l142" class="code_block">are active.  Thus one set of reasons for making dynamic predicates
</div><div id="l143" class="code_block">private by default are based on efficiency~<span class="k">\footnote</span><span class="nb">{</span><span class="k">\version</span><span class="nb">{}</span> of
</div><div id="l144" class="code_block">  XSB allows garbage collection of retracted clauses only when there
</div><div id="l145" class="code_block">  is a single active thread.  Future versions will likely offer more
</div><div id="l146" class="code_block">  powerful garbage collectors.<span class="nb">}</span>.  <span class="nb">{</span><span class="k">\sc</span> Clause GC is not yet
</div><div id="l147" class="code_block">  implemented; also need to talk about tries<span class="nb">}</span>.
</div><div id="l148" class="code_block">
</div><div id="l149" class="code_block">The second reason for making dynamic predicates thread-private by
</div><div id="l150" class="code_block">default is semantic.  Suppose thread <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span> starts a tabled computation
</div><div id="l151" class="code_block">that depends on the dynamic shared predicate <span class="nb">{</span><span class="k">\tt</span> p/1<span class="nb">}</span>.  While <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span>
</div><div id="l152" class="code_block">is computing the table, thread <span class="s">$</span><span class="nb">T_</span><span class="m">2</span><span class="s">$</span> asserts a clause to <span class="nb">{</span><span class="k">\tt</span> p/1<span class="nb">}</span>.
</div><div id="l153" class="code_block"><span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span>&#39;s table is likely to be inconsistent, leading to the problem of
</div><div id="l154" class="code_block"><span class="nb">{</span><span class="k">\em</span> read consistency<span class="nb">}</span> of any table that depends on thread-shared
</div><div id="l155" class="code_block">dynamic predicates.  In <span class="k">\version</span> , users are responsible for ensuring
</div><div id="l156" class="code_block">read consistency of any tables that depend on shared dynamic data.
</div><div id="l157" class="code_block">Future versions of XSB are intended to allow more sophisticated
</div><div id="l158" class="code_block">mechanisms for read consistency.
</div><div id="l159" class="code_block">
</div><div id="l160" class="code_block">Not only can tables depend on thread-shared or thread-private dynamic
</div><div id="l161" class="code_block">data, but 
</div><div id="l162" class="code_block"><span class="c">%in the default multi-threaded configuration of XSB, which</span>
</div><div id="l163" class="code_block"><span class="c">%uses local scheduling for tabled evaluation, </span>
</div><div id="l164" class="code_block">the tables themselves may be thread-shared or thread-private.  Like
</div><div id="l165" class="code_block">dynamic code, tables are private by default; the declaration <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l166" class="code_block">  thread<span class="k">\_shared</span>/1<span class="nb">}</span> allows completed tables for a predicate evaluated
</div><div id="l167" class="code_block">with call-variance to be shared among threads~<span class="k">\footnote</span><span class="nb">{</span>In <span class="k">\version</span>,
</div><div id="l168" class="code_block">  tabled predicates using call-subsumption are always private; an
</div><div id="l169" class="code_block">  attempt to make such a predicate thread-shared throws an
</div><div id="l170" class="code_block">  exception.<span class="nb">}</span>.
</div><div id="l171" class="code_block"><span class="c">%</span>
</div><div id="l172" class="code_block">To some extent, tabling considerations for making a predicate
</div><div id="l173" class="code_block">thread-shared or thread-private are like those of dynamic code.
</div><div id="l174" class="code_block">Thread-private tables require fewer synchonization points overall, and
</div><div id="l175" class="code_block">garbage collection of space for abolished thread-private tables is far
</div><div id="l176" class="code_block">easier to perform.  However the semantics of how tabling information
</div><div id="l177" class="code_block">is shared depends on whether local or batched evaluation is used with
</div><div id="l178" class="code_block">the multi-threaded engine.  Recall from
</div><div id="l179" class="code_block">Chapter~<span class="k">\ref</span><span class="nb">{</span>chap:TablingOverview<span class="nb">}</span> that local evaluation is so-named
</div><div id="l180" class="code_block">because computation always takes place in the SCC most recently
</div><div id="l181" class="code_block">created, and no answer is returned outside of an SCC until the SCC has
</div><div id="l182" class="code_block">been completely evaluated.  Within this scheduling strategy it is not
</div><div id="l183" class="code_block">often useful to share answers between tables that have not been
</div><div id="l184" class="code_block">completed -- as local evaluation would allow these answers to be
</div><div id="l185" class="code_block">returned only if the tables were in the same SCC.  This leads to a
</div><div id="l186" class="code_block">concurrency semantics called <span class="nb">{</span><span class="k">\em</span> Shared Completed Tables<span class="nb">}</span>.  Shared
</div><div id="l187" class="code_block">Completed Tables can in fact be supported by a relatively simple
</div><div id="l188" class="code_block">algorithm for optimistic concurrency control.  If goals to two
</div><div id="l189" class="code_block">mutually dependent tables <span class="s">$</span><span class="nb">Table_a</span><span class="s">$</span> and <span class="s">$</span><span class="nb">Table_b</span><span class="s">$</span> are called
</div><div id="l190" class="code_block">concurrently by two different threads, <span class="s">$</span><span class="nb">Thread_a</span><span class="s">$</span> and <span class="s">$</span><span class="nb">Thread_b</span><span class="s">$</span>,
</div><div id="l191" class="code_block">nothing is done until it is detected that <span class="s">$</span><span class="nb">Table_a</span><span class="s">$</span> and <span class="s">$</span><span class="nb">Table_b</span><span class="s">$</span> are
</div><div id="l192" class="code_block">both incomplete and are contained in a SCC of the table dependency
</div><div id="l193" class="code_block">grahp.  At that time, one of the threads (e.g. <span class="s">$</span><span class="nb">Thread_a</span><span class="s">$</span>) takes over
</div><div id="l194" class="code_block">recomputation of all tables in the SCC, and when the SCC is completed,
</div><div id="l195" class="code_block">any remaining answers are returned to other threads that had invoked
</div><div id="l196" class="code_block">goals in the SCC.  While <span class="s">$</span><span class="nb">Thread_a</span><span class="s">$</span> is completing this computation,
</div><div id="l197" class="code_block"><span class="s">$</span><span class="nb">Thread_b</span><span class="s">$</span> suspends until the SCC is complete.  Thus the semantics of
</div><div id="l198" class="code_block">Shared Completed Tables supports concurrency, but does not support any
</div><div id="l199" class="code_block">but the most coarse-grained parallelism.
</div><div id="l200" class="code_block">
</div><div id="l201" class="code_block">Batched evaluation, on the other hand, allows answers to be returned
</div><div id="l202" class="code_block">outside of an SCC before that SCC has been completed.  Concurrency
</div><div id="l203" class="code_block">control for batched evaluation is similar to that for local
</div><div id="l204" class="code_block">evaluation, except in the following case.  Assume as before that
</div><div id="l205" class="code_block"><span class="s">$</span><span class="nb">Table_a</span><span class="s">$</span>, first called by <span class="s">$</span><span class="nb">Thread_a</span><span class="s">$</span>, and <span class="s">$</span><span class="nb">Table_b</span><span class="s">$</span> first called by
</div><div id="l206" class="code_block"><span class="s">$</span><span class="nb">Thread_b</span><span class="s">$</span> are determined to be in the same SCC, and that <span class="s">$</span><span class="nb">Thread_a</span><span class="s">$</span>
</div><div id="l207" class="code_block">takes over computation of subgoals in the SCC.  Now, <span class="s">$</span><span class="nb">Thread_b</span><span class="s">$</span>,
</div><div id="l208" class="code_block">rather than suspending, may continue work.  In particular, <span class="s">$</span><span class="nb">Thread_b</span><span class="s">$</span>
</div><div id="l209" class="code_block">can return any answers in <span class="s">$</span><span class="nb">Table_b</span><span class="s">$</span> that it finds, regardless of
</div><div id="l210" class="code_block">whether they have been produced by <span class="s">$</span><span class="nb">Thread_b</span><span class="s">$</span> (before <span class="s">$</span><span class="nb">Thread_a</span><span class="s">$</span> took
</div><div id="l211" class="code_block">over the SCC) or by <span class="s">$</span><span class="nb">Thread_a</span><span class="s">$</span> (afterwards).  We call this type of
</div><div id="l212" class="code_block">concurrency semantics, <span class="nb">{</span><span class="k">\em</span> Table Parallelism<span class="nb">}</span>.  Table Parallelism can
</div><div id="l213" class="code_block">be used to program producer-consumer examples, as well as to implement
</div><div id="l214" class="code_block">Or- and And- parallelism.  Table Parallelism was first introduced
</div><div id="l215" class="code_block">in~<span class="k">\cite</span><span class="nb">{</span>FHSW95<span class="nb">}</span>, but the mechanism now used for implementing Table
</div><div id="l216" class="code_block">Parallelism differs significantly from what was described there.  In
</div><div id="l217" class="code_block"><span class="k">\version</span><span class="nb">{}</span> of XSB, the implementation of Table Parallelism is
</div><div id="l218" class="code_block">experimental: in particular, it does not yet support tabled negation.
</div><div id="l219" class="code_block">
</div><div id="l220" class="code_block">For either semantics of shared tables, in <span class="k">\version</span><span class="nb">{}</span>, users of
</div><div id="l221" class="code_block">thread-shared tables are responsible for ensuring read consistency.
</div><div id="l222" class="code_block">Note that, in principle, thread-shared tables may depend on
</div><div id="l223" class="code_block">thread-private tables and vice-versa.  Either type of table may depend
</div><div id="l224" class="code_block">on thread-private or thread-shared dynamic code.  In addition, a
</div><div id="l225" class="code_block">predicate may be <span class="nb">{</span><span class="k">\em</span> both<span class="nb">}</span> dynamic and tabled, and its clauses and
</div><div id="l226" class="code_block">tables may be either thread-private or thread-shared.
</div><div id="l227" class="code_block">
</div><div id="l228" class="code_block"><span class="k">\section</span><span class="nb">{</span>Error Conditions: Joinable and Detached Threads<span class="nb">}</span>
</div><div id="l229" class="code_block">
</div><div id="l230" class="code_block">So far we have assumed that the goal called in <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l231" class="code_block">  xsb<span class="k">\_thread\_create</span>/2<span class="nb">}</span> terminates normally --- by success or
</div><div id="l232" class="code_block">failure.  But what if a thread throws an error while executing a goal?
</div><div id="l233" class="code_block">How long should error information for a thread persist, and how can it
</div><div id="l234" class="code_block">be checked?
</div><div id="l235" class="code_block">
</div><div id="l236" class="code_block">Our approach relies on the semantics of pthreads, which can be either
</div><div id="l237" class="code_block"><span class="nb">{</span><span class="k">\em</span> joinable<span class="nb">}</span> or <span class="nb">{</span><span class="k">\em</span> detached<span class="nb">}</span>.  After a joinable pthread <span class="s">$</span><span class="nb">T_{dead}</span><span class="s">$</span>
</div><div id="l238" class="code_block">has terminated, status information about <span class="s">$</span><span class="nb">T_{dead}</span><span class="s">$</span> persists until
</div><div id="l239" class="code_block">some other thread joins it -- at which time the information is
</div><div id="l240" class="code_block">removed.  On the other hand, if <span class="s">$</span><span class="nb">T_{dead}</span><span class="s">$</span> is detached, status
</div><div id="l241" class="code_block">information is removed as soon as <span class="s">$</span><span class="nb">T_{dead}</span><span class="s">$</span> terminates.  A thread is
</div><div id="l242" class="code_block">valid if it has not yet terminated, or if it is joinable and has not
</div><div id="l243" class="code_block">yet been joined.  Reclamation of thread status information differs
</div><div id="l244" class="code_block">from that of thread-specific data structures: upon normal or
</div><div id="l245" class="code_block">exceptional termination of <span class="s">$</span><span class="nb">T_{dead}</span><span class="s">$</span>, any mutexes held by <span class="s">$</span><span class="nb">T_{dead}</span><span class="s">$</span>,
</div><div id="l246" class="code_block">are released, as well as any memory automatically allocated in the
</div><div id="l247" class="code_block">process of initializing <span class="s">$</span><span class="nb">T_{dead}</span><span class="s">$</span>&#39;s, or executing its goal --
</div><div id="l248" class="code_block">including stacks, private dynamic code, private tables, etc.
</div><div id="l249" class="code_block">XSB-specific status information about threads follows the pthread
</div><div id="l250" class="code_block">model: by default, error information is available when joining a
</div><div id="l251" class="code_block">joinable thread, but not otherwise~<span class="k">\footnote</span><span class="nb">{</span>This behavior can, of
</div><div id="l252" class="code_block">  course, be overridden by embedding goals within <span class="nb">{</span><span class="k">\tt</span> catch/3<span class="nb">}</span> and
</div><div id="l253" class="code_block">  handling errors separately, or simply by adding a default user error
</div><div id="l254" class="code_block">  handler: see Chapter~<span class="k">\ref</span><span class="nb">{</span>chap:exception<span class="nb">}</span> for details.<span class="nb">}</span>.
</div><div id="l255" class="code_block">
</div><div id="l256" class="code_block"><span class="k">\begin</span><span class="nb">{</span>example<span class="nb">}</span>
</div><div id="l257" class="code_block">Suppose the goal 
</div><div id="l258" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l259" class="code_block"> ?- xsb<span class="nb">_</span>thread<span class="nb">_</span>create(functor(X,Y,Z),F).
</div><div id="l260" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l261" class="code_block"><span class="c">%</span>
</div><div id="l262" class="code_block">is executed.  By default, this will produce the result
</div><div id="l263" class="code_block"><span class="c">%</span>
</div><div id="l264" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l265" class="code_block">X = <span class="nb">_</span>h113
</div><div id="l266" class="code_block">Y = <span class="nb">_</span>h127
</div><div id="l267" class="code_block">Z = <span class="nb">_</span>h141
</div><div id="l268" class="code_block">F = 1++Error[XSB/Runtime/P]: [Instantiation]  in arg 2 of predicate functor/3
</div><div id="l269" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l270" class="code_block"><span class="c">%</span>
</div><div id="l271" class="code_block">In fact, the variable bindings are output to <span class="nb">{</span><span class="k">\tt</span> STDOUT<span class="nb">}</span>, while the
</div><div id="l272" class="code_block">error message
</div><div id="l273" class="code_block"><span class="c">%</span>
</div><div id="l274" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l275" class="code_block">++Error[XSB/Runtime/P]: [Instantiation]  in arg 2 of predicate functor/3
</div><div id="l276" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l277" class="code_block"><span class="c">%</span>
</div><div id="l278" class="code_block">is output to <span class="nb">{</span><span class="k">\tt</span> STDERR<span class="nb">}</span>, and may be redirected.  The call
</div><div id="l279" class="code_block"><span class="c">%</span>
</div><div id="l280" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l281" class="code_block">?- xsb<span class="nb">_</span>thread<span class="nb">_</span>join(2,Error).
</div><div id="l282" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l283" class="code_block"><span class="c">%</span>
</div><div id="l284" class="code_block">returns
</div><div id="l285" class="code_block"><span class="c">%</span>
</div><div id="l286" class="code_block"><span class="k">\begin</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l287" class="code_block">Error = error(instantiation<span class="nb">_</span>error, in arg 2 of predicate functor/3,
</div><div id="l288" class="code_block">              [[Forward Continuation...,... standard:call/1,... standard:catch/3],
</div><div id="l289" class="code_block">                Backward Continuation...]) - 0
</div><div id="l290" class="code_block"><span class="k">\end</span><span class="nb">{</span>verbatim<span class="nb">}</span>
</div><div id="l291" class="code_block"><span class="c">%</span>
</div><div id="l292" class="code_block">In other words, <span class="nb">{</span><span class="k">\tt</span> Error<span class="nb">}</span> is instantiated to a <span class="nb">{</span><span class="k">\tt</span> -/2<span class="nb">}</span> structure,
</div><div id="l293" class="code_block">the first argument of which is a standard XSB error term, and the
</div><div id="l294" class="code_block">second argument of which is the exit code of the thread.  
</div><div id="l295" class="code_block"><span class="k">\end</span><span class="nb">{</span>example<span class="nb">}</span>
</div><div id="l296" class="code_block"><span class="c">%</span>
</div><div id="l297" class="code_block">In general, error terms in XSB should be more useful than C exit
</div><div id="l298" class="code_block">codes, but C exit codes may conceivably be useful if XSB is called by
</div><div id="l299" class="code_block">one of the interfaces for C calling XSB~<span class="k">\footnote</span><span class="nb">{</span>C calling XSB
</div><div id="l300" class="code_block">  interfaces are not yet supported in <span class="k">\version</span>.<span class="nb">}</span>.  By default,
</div><div id="l301" class="code_block">however, normal termination of an xsb thread will result in an exit
</div><div id="l302" class="code_block">code of <span class="nb">{</span><span class="k">\tt</span> 0<span class="nb">}</span>, a cancelled thread will have an exit code of <span class="nb">{</span><span class="k">\tt</span> 1<span class="nb">}</span>
</div><div id="l303" class="code_block">and a thread terminated with any other error will have an exit code of
</div><div id="l304" class="code_block"><span class="nb">{</span><span class="k">\tt</span> 2<span class="nb">}</span>.
</div><div id="l305" class="code_block">
</div><div id="l306" class="code_block">As with pthreads, XSB threads are created as joinable by default, but
</div><div id="l307" class="code_block">can be created as detached using an option in <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l308" class="code_block">  xsb<span class="k">\_thread\_create</span>/3<span class="nb">}</span>.  Alternatively, a thread created as joinable
</div><div id="l309" class="code_block">can be made detached by <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_set\_detached</span>/1<span class="nb">}</span>.  Both of these
</div><div id="l310" class="code_block">predicates are described in Section~<span class="k">\ref</span><span class="nb">{</span>sec:mt-threading<span class="nb">}</span>.
</div><div id="l311" class="code_block">
</div><div id="l312" class="code_block"><span class="k">\section</span><span class="nb">{</span>Thread Cancellation<span class="nb">}</span>
</div><div id="l313" class="code_block"><span class="c">%</span>
</div><div id="l314" class="code_block">There may be a number of situations in which it is useful to give one
</div><div id="l315" class="code_block">thread the ability to cancel the execution of another thread.  Within
</div><div id="l316" class="code_block">the semantics of pthreads, this is called <span class="nb">{</span><span class="k">\em</span> thread cancellation<span class="nb">}</span>.
</div><div id="l317" class="code_block">At the C level, thread cancellation can be tricky, as mutexes must be
</div><div id="l318" class="code_block">released, allocated memory freed, and so on.  Accordingly, the
</div><div id="l319" class="code_block">predicate <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_cancel</span>/1<span class="nb">}</span> cancels XSB threads by acting
</div><div id="l320" class="code_block">purely within the SLG-WAM engine.  When thread <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span> interrupts thread
</div><div id="l321" class="code_block"><span class="s">$</span><span class="nb">T_</span><span class="m">2</span><span class="s">$</span>, <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span> writes to the XSB interrupt vector in <span class="s">$</span><span class="nb">T_</span><span class="m">2</span><span class="s">$</span>.  Later, when
</div><div id="l322" class="code_block"><span class="s">$</span><span class="nb">T_</span><span class="m">2</span><span class="s">$</span> checks its interrupt vector, it throws a cancellation error,
</div><div id="l323" class="code_block">which causes it to clean up its mutexes, memory, private tables and
</div><div id="l324" class="code_block">dynamic code, and then exit.
</div><div id="l325" class="code_block">
</div><div id="l326" class="code_block">Any thread can cancel any other thread, with the exception that the
</div><div id="l327" class="code_block"><span class="nb">{</span><span class="k">\em</span> main<span class="nb">}</span> thread, which controls the console (or interface to C or
</div><div id="l328" class="code_block">interprolog) cannot be cancelled.  The main thread always has XSB
</div><div id="l329" class="code_block">thread id <span class="nb">{</span><span class="k">\tt</span> 0<span class="nb">}</span> in both the single-threaded and multi-threaded
</div><div id="l330" class="code_block">systems.
</div><div id="l331" class="code_block">
</div><div id="l332" class="code_block"><span class="k">\section</span><span class="nb">{</span>Performance and other Considerations<span class="nb">}</span>
</div><div id="l333" class="code_block"><span class="c">%</span>
</div><div id="l334" class="code_block">For running programs that do not use multiple threads, the
</div><div id="l335" class="code_block">multi-threaded engine has a minimal overhead compared to the
</div><div id="l336" class="code_block">single-threaded engine.  Times for single-threaded execution of Prolog
</div><div id="l337" class="code_block">or tabled programs range from about 10-20<span class="k">\%</span> slower to 10-20<span class="k">\%</span> <span class="nb">{</span><span class="k">\em</span>
</div><div id="l338" class="code_block">  faster<span class="nb">}</span> for the multi-threaded engine compared to the
</div><div id="l339" class="code_block">single-threaded engine.  Speedups for running multiple threads on
</div><div id="l340" class="code_block">multiple processors depends heavily on the applications run and on the
</div><div id="l341" class="code_block">underlying operating system.
</div><div id="l342" class="code_block">
</div><div id="l343" class="code_block">The size of a given thread may be a consideration for multi-threaded
</div><div id="l344" class="code_block">applications.  Each thread has an area of thread-private variables
</div><div id="l345" class="code_block">that are ``global&#39;&#39; to its own virtual machine, called the <span class="nb">{</span><span class="k">\em</span> thread
</div><div id="l346" class="code_block">  context<span class="nb">}</span>, which accounts for about 2-4Kbytes of space.  Much larger
</div><div id="l347" class="code_block">are the various stacks used by the threads for tabled and Prolog
</div><div id="l348" class="code_block">execution.  Almost all of XSB&#39;s stacks are fully expandable, and the
</div><div id="l349" class="code_block">initial size of several of these stacks may be set explicitly as
</div><div id="l350" class="code_block">options in <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_create</span>/3<span class="nb">}</span>.  Explicitly setting a default
</div><div id="l351" class="code_block">thread stack size for an XSB thread to be smaller than the default
</div><div id="l352" class="code_block">process stack size may be useful for applications that have a large
</div><div id="l353" class="code_block">number of concurrently running threads.  
</div><div id="l354" class="code_block">
</div><div id="l355" class="code_block">Other performance considerations involve the contention by threads for
</div><div id="l356" class="code_block">shared resources.  As discussed above, contention may arise when
</div><div id="l357" class="code_block">creating or abolishing tables, or when asserting or retracting dynamic
</div><div id="l358" class="code_block">code --- however in either case thread-private predicates give rise to
</div><div id="l359" class="code_block">less contention than thread-shared predicates.  In terms of I/O, each
</div><div id="l360" class="code_block">XSB stream up to the maximum number of file descriptors has its own
</div><div id="l361" class="code_block">mutex; as a result threads writing to different streams will not
</div><div id="l362" class="code_block">contend for I/O.  Thus, in multi-threaded applications, it may be more
</div><div id="l363" class="code_block">efficient to open and close streams and access these streams
</div><div id="l364" class="code_block">explicitly, than to redirect standard input or standard output through
</div><div id="l365" class="code_block"><span class="nb">{</span><span class="k">\tt</span> see/1<span class="nb">}</span> and <span class="nb">{</span><span class="k">\tt</span> tell/1<span class="nb">}</span>.
</div><div id="l366" class="code_block">
</div><div id="l367" class="code_block"><span class="k">\section</span><span class="nb">{</span>Predicates for Multi-Threading<span class="nb">}</span> <span class="k">\label</span><span class="nb">{</span>sec:mt-threading<span class="nb">}</span>
</div><div id="l368" class="code_block">
</div><div id="l369" class="code_block"><span class="k">\begin</span><span class="nb">{</span>description<span class="nb">}</span>
</div><div id="l370" class="code_block">
</div><div id="l371" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_create</span>(+Goal,ThreadId,,+OptionsList)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l372" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_create</span>/3<span class="nb">}}</span>
</div><div id="l373" class="code_block"><span class="c">%</span>
</div><div id="l374" class="code_block">When called from thread <span class="s">$</span><span class="nb">T</span><span class="s">$</span>, this predicate creates a new XSB thread
</div><div id="l375" class="code_block"><span class="s">$</span><span class="nb">T_{new}</span><span class="s">$</span> to execute <span class="nb">{</span><span class="k">\tt</span> Goal<span class="nb">}</span>.  When goal either succeeds, throws an
</div><div id="l376" class="code_block">unhandled error, or fails, <span class="s">$</span><span class="nb">T_{new}</span><span class="s">$</span> exits, but <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l377" class="code_block">  xsb<span class="k">\_thread\_create</span>/2<span class="nb">}</span> will succeed immediately, binding <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l378" class="code_block">  ThreadId<span class="nb">}</span> to the XSB thread id of <span class="s">$</span><span class="nb">T_{new}</span><span class="s">$</span>.  <span class="nb">{</span><span class="k">\tt</span> Goal<span class="nb">}</span> must be
</div><div id="l379" class="code_block">callable, but need not be fully instantiated.  No bindings from <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l380" class="code_block">  Goal<span class="nb">}</span> are passed back from <span class="s">$</span><span class="nb">T</span><span class="s">$</span> to <span class="s">$</span><span class="nb">T_{new}</span><span class="s">$</span>, so communication
</div><div id="l381" class="code_block">between <span class="s">$</span><span class="nb">T_{new}</span><span class="s">$</span> and <span class="s">$</span><span class="nb">T</span><span class="s">$</span> must be through tables, asserted code, or
</div><div id="l382" class="code_block">other side effects.
</div><div id="l383" class="code_block">
</div><div id="l384" class="code_block"><span class="nb">{</span><span class="k">\tt</span> OptionList<span class="nb">}</span> allows optional parameters in the configuration for
</div><div id="l385" class="code_block">the initial size of XSB stacks and to indicate whether <span class="s">$</span><span class="nb">T_{new}</span><span class="s">$</span> is to
</div><div id="l386" class="code_block">be created as detached.  Note that XSB threads allow automatic stack
</div><div id="l387" class="code_block">allocation, so that the size options may be most useful for
</div><div id="l388" class="code_block">applications with very large numbers of threads.  In this case,
</div><div id="l389" class="code_block">setting initial stack sizes to be small may allow more threads to be
</div><div id="l390" class="code_block">created on a given hardware platform.  Also note that only XSB stacks
</div><div id="l391" class="code_block">are affected, the stack size of the underlying pthread remains
</div><div id="l392" class="code_block">unaltered.
</div><div id="l393" class="code_block"><span class="c">%</span>
</div><div id="l394" class="code_block"><span class="k">\bi</span>
</div><div id="l395" class="code_block"><span class="k">\item</span> <span class="nb">{</span><span class="k">\tt</span> glsize(N)<span class="nb">}</span>: create thread with global (heap) plus local
</div><div id="l396" class="code_block">  stack size initially set to <span class="nb">{</span><span class="k">\tt</span> N<span class="nb">}</span> kbytes.  If not specified, the
</div><div id="l397" class="code_block">  default size is used (cf. Section~<span class="k">\ref</span><span class="nb">{</span>sec:EmuOptions<span class="nb">}</span>).
</div><div id="l398" class="code_block"><span class="c">%</span>
</div><div id="l399" class="code_block"><span class="k">\item</span> <span class="nb">{</span><span class="k">\tt</span> tcsize(N)<span class="nb">}</span>: create thread with trail plus choice point 
</div><div id="l400" class="code_block">  stack size initially set to <span class="nb">{</span><span class="k">\tt</span> N<span class="nb">}</span> kbytes.  If not specified, the
</div><div id="l401" class="code_block">  default size is used (cf. Section~<span class="k">\ref</span><span class="nb">{</span>sec:EmuOptions<span class="nb">}</span>).
</div><div id="l402" class="code_block"><span class="c">%</span>
</div><div id="l403" class="code_block"><span class="k">\item</span> <span class="nb">{</span><span class="k">\tt</span> complsize(N)<span class="nb">}</span>: create thread with completion
</div><div id="l404" class="code_block">  stack size initially set to <span class="nb">{</span><span class="k">\tt</span> N<span class="nb">}</span> kbytes. If not specified, the
</div><div id="l405" class="code_block">  default size is used (cf. Section~<span class="k">\ref</span><span class="nb">{</span>sec:EmuOptions<span class="nb">}</span>).
</div><div id="l406" class="code_block"><span class="c">%</span>
</div><div id="l407" class="code_block"><span class="k">\item</span> <span class="nb">{</span><span class="k">\tt</span> pdlsize(N)<span class="nb">}</span>: create thread with <span class="nb">{</span><span class="k">\tt</span> N<span class="nb">}</span> kbytes of
</div><div id="l408" class="code_block">  unification stack.  If not specified, the
</div><div id="l409" class="code_block">  default size is used (cf. Section~<span class="k">\ref</span><span class="nb">{</span>sec:EmuOptions<span class="nb">}</span>).
</div><div id="l410" class="code_block"><span class="c">%</span>
</div><div id="l411" class="code_block"><span class="k">\item</span> <span class="nb">{</span><span class="k">\tt</span> detached<span class="nb">}</span>: creates detached thread.  If not specified, the
</div><div id="l412" class="code_block">  thread created will be joinable.
</div><div id="l413" class="code_block"><span class="k">\ei</span>
</div><div id="l414" class="code_block">
</div><div id="l415" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l416" class="code_block"><span class="k">\bi</span>
</div><div id="l417" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> Goal<span class="nb">}</span> is not callable.
</div><div id="l418" class="code_block"><span class="k">\bi</span>
</div><div id="l419" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> type<span class="k">\_error</span>(callable<span class="k">\_term</span>,Goal)<span class="nb">}</span>
</div><div id="l420" class="code_block"><span class="k">\ei</span>
</div><div id="l421" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> is not a variable
</div><div id="l422" class="code_block"><span class="k">\bi</span>
</div><div id="l423" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> instantiation<span class="k">\_error</span><span class="nb">}</span>
</div><div id="l424" class="code_block"><span class="k">\ei</span>
</div><div id="l425" class="code_block"><span class="k">\item</span>   No more system threads are available (EAGAIN)
</div><div id="l426" class="code_block"><span class="k">\bi</span>
</div><div id="l427" class="code_block"><span class="k">\item</span> <span class="nb">{</span><span class="k">\tt</span> resource<span class="k">\_error</span>(&#39;system threads&#39;)<span class="nb">}</span>
</div><div id="l428" class="code_block"><span class="k">\ei</span>
</div><div id="l429" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> OptionList<span class="nb">}</span> is not instantiated
</div><div id="l430" class="code_block"><span class="k">\bi</span>
</div><div id="l431" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> instantiation<span class="k">\_error</span><span class="nb">}</span>
</div><div id="l432" class="code_block"><span class="k">\ei</span>
</div><div id="l433" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> OptionList<span class="nb">}</span> contains an option, <span class="nb">{</span><span class="k">\tt</span> Option<span class="nb">}</span> not described above
</div><div id="l434" class="code_block"><span class="k">\bi</span>
</div><div id="l435" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> domain<span class="k">\_error</span>(xsb<span class="k">\_thread\_create\_option</span>,Option)<span class="nb">}</span>
</div><div id="l436" class="code_block"><span class="k">\ei</span>
</div><div id="l437" class="code_block"><span class="k">\ei</span>
</div><div id="l438" class="code_block">
</div><div id="l439" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_create</span>(+Goal,-ThreadId)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l440" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_create</span>/2<span class="nb">}}</span>
</div><div id="l441" class="code_block"><span class="c">%</span>
</div><div id="l442" class="code_block">Acts as <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_create</span>(Goal,ThreadId,[])<span class="nb">}</span>.
</div><div id="l443" class="code_block">
</div><div id="l444" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_join</span>(+ThreadIds,-ExitDesignators)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l445" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_join</span>/2<span class="nb">}}</span> 
</div><div id="l446" class="code_block"><span class="c">% </span>
</div><div id="l447" class="code_block">When <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_join</span>/2<span class="nb">}</span> is called by thread <span class="s">$</span><span class="nb">T</span><span class="s">$</span>, <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l448" class="code_block">  ThreadIds<span class="nb">}</span> must be instantiated to either an XSB thread id, or a
</div><div id="l449" class="code_block">list of XSB thread ids; in either case, <span class="nb">{</span><span class="k">\tt</span> ExitDesignators<span class="nb">}</span> must be
</div><div id="l450" class="code_block">uninstantiated.  The action of the predicate is to suspend <span class="s">$</span><span class="nb">T</span><span class="s">$</span> until
</div><div id="l451" class="code_block">all of the threads denoted by <span class="nb">{</span><span class="k">\tt</span> ThreadIds<span class="nb">}</span> have exited.  At this
</div><div id="l452" class="code_block">time, any remaining resources for the threads in <span class="nb">{</span><span class="k">\tt</span> ThreadIds<span class="nb">}</span> is
</div><div id="l453" class="code_block">reclaimed, and <span class="nb">{</span><span class="k">\tt</span> ExitCode<span class="nb">}</span> is either an element of the form:
</div><div id="l454" class="code_block"><span class="k">\begin</span><span class="nb">{</span>center<span class="nb">}</span>
</div><div id="l455" class="code_block"><span class="nb">{</span><span class="k">\tt</span> ExitBall -  ExitCode<span class="nb">}</span> 
</div><div id="l456" class="code_block"><span class="k">\end</span><span class="nb">{</span>center<span class="nb">}</span>
</div><div id="l457" class="code_block"><span class="c">%</span>
</div><div id="l458" class="code_block">or a list of such elements.  For each XSB thread id <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span> in
</div><div id="l459" class="code_block"><span class="nb">{</span>ThreadIds<span class="nb">}</span>, <span class="nb">{</span><span class="k">\tt</span> ExitBall<span class="nb">}</span> is either
</div><div id="l460" class="code_block"><span class="k">\begin</span><span class="nb">{</span>itemize<span class="nb">}</span>
</div><div id="l461" class="code_block"><span class="c">%</span>
</div><div id="l462" class="code_block"><span class="k">\item</span> <span class="nb">{</span><span class="k">\tt</span> no<span class="k">\_error\_ball</span><span class="nb">}</span> if <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span> exited normally (through success or
</div><div id="l463" class="code_block">  failure), without throwing an error or being cancelled.
</div><div id="l464" class="code_block"><span class="c">%</span>
</div><div id="l465" class="code_block"><span class="k">\item</span> An error ball if <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span> threw an error that was not explicitly
</div><div id="l466" class="code_block">  handled by a catch or user error handler.
</div><div id="l467" class="code_block"><span class="c">%</span>
</div><div id="l468" class="code_block"><span class="k">\item</span> A special error ball of the form <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l469" class="code_block">  error(thead<span class="k">\_cancel</span>,<span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span>,Backtrace)<span class="nb">}</span> if <span class="s">$</span><span class="nb">T_</span><span class="m">1</span><span class="s">$</span> was cancelled by some
</div><div id="l470" class="code_block">  other thread.  In this case, <span class="nb">{</span><span class="k">\tt</span> Backtrace<span class="nb">}</span> is an empty backtrace,
</div><div id="l471" class="code_block">  added for conformability with other error forms.
</div><div id="l472" class="code_block"><span class="k">\end</span><span class="nb">{</span>itemize<span class="nb">}</span>
</div><div id="l473" class="code_block"><span class="c">%</span>
</div><div id="l474" class="code_block">ExitCode is either <span class="nb">{</span><span class="k">\tt</span> 0<span class="nb">}</span> for normal termination, or the value
</div><div id="l475" class="code_block">specified by an explicit call to <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_thread\_exit</span>/1<span class="nb">}</span>.
</div><div id="l476" class="code_block">
</div><div id="l477" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l478" class="code_block"><span class="k">\bi</span>
</div><div id="l479" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadIds<span class="nb">}</span> is not instantiated
</div><div id="l480" class="code_block"><span class="k">\bi</span>
</div><div id="l481" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> instantiation<span class="k">\_error</span><span class="nb">}</span>
</div><div id="l482" class="code_block"><span class="k">\ei</span>
</div><div id="l483" class="code_block"><span class="c">%</span>
</div><div id="l484" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadIds<span class="nb">}</span> is not a list of XSB thread ids
</div><div id="l485" class="code_block"><span class="k">\bi</span>
</div><div id="l486" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> domain<span class="k">\_error</span>(listof<span class="k">\_threadids</span>,ThreadIds)<span class="nb">}</span>
</div><div id="l487" class="code_block"><span class="k">\ei</span>
</div><div id="l488" class="code_block"><span class="c">%</span>
</div><div id="l489" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ExitCodes<span class="nb">}</span> is not a variable
</div><div id="l490" class="code_block"><span class="k">\bi</span>
</div><div id="l491" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> instantiation<span class="k">\_error</span><span class="nb">}</span>
</div><div id="l492" class="code_block"><span class="k">\ei</span>
</div><div id="l493" class="code_block"><span class="c">%</span>
</div><div id="l494" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> does not correspond to a valid thread
</div><div id="l495" class="code_block"><span class="k">\bi</span>
</div><div id="l496" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> existence<span class="k">\_error</span>(valid<span class="k">\_thread</span>,ThreadId)<span class="nb">}</span>
</div><div id="l497" class="code_block"><span class="k">\ei</span>
</div><div id="l498" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> does not correspond to a joinable thread
</div><div id="l499" class="code_block">  (i.e. <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> is detached).
</div><div id="l500" class="code_block"><span class="k">\bi</span>
</div><div id="l501" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> permission<span class="k">\_error</span>(join,non<span class="k">\_joinable\_thread</span>,ThreadId)<span class="nb">}</span>
</div><div id="l502" class="code_block"><span class="k">\ei</span>
</div><div id="l503" class="code_block"><span class="k">\ei</span>
</div><div id="l504" class="code_block">
</div><div id="l505" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_exit</span>(+ExitCode)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l506" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_exit</span>/1<span class="nb">}}</span> 
</div><div id="l507" class="code_block"><span class="c">%</span>
</div><div id="l508" class="code_block">Exits a thread <span class="s">$</span><span class="nb">T</span><span class="s">$</span> with <span class="nb">{</span><span class="k">\tt</span> ExitCode<span class="nb">}</span> after releasing any mutexes
</div><div id="l509" class="code_block">held by <span class="s">$</span><span class="nb">T</span><span class="s">$</span>, freeing any thread-specific memory allocated for <span class="s">$</span><span class="nb">T</span><span class="s">$</span> (we
</div><div id="l510" class="code_block">hope).  <span class="nb">{</span><span class="k">\tt</span> ExitCode<span class="nb">}</span> will be used if the caller of <span class="s">$</span><span class="nb">T</span><span class="s">$</span> joins to <span class="s">$</span><span class="nb">T</span><span class="s">$</span>,
</div><div id="l511" class="code_block">but will be ignored in other cases.  There is no need to call this
</div><div id="l512" class="code_block">routine on normal termination of a thread as it is called implicitly
</div><div id="l513" class="code_block">on success or (final) failure of a thread&#39;s goal.
</div><div id="l514" class="code_block">
</div><div id="l515" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l516" class="code_block"><span class="k">\bi</span>
</div><div id="l517" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> is not an integer
</div><div id="l518" class="code_block"><span class="k">\bi</span>
</div><div id="l519" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> type<span class="k">\_error</span>(integer,ExitCode)<span class="nb">}</span>
</div><div id="l520" class="code_block"><span class="k">\ei</span>
</div><div id="l521" class="code_block"><span class="k">\ei</span>
</div><div id="l522" class="code_block">
</div><div id="l523" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_self</span>(-ThreadId)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l524" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_self</span>/1<span class="nb">}}</span> 
</div><div id="l525" class="code_block"><span class="c">%</span>
</div><div id="l526" class="code_block">Returns the XSB thread id of the calling thread.
</div><div id="l527" class="code_block">
</div><div id="l528" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l529" class="code_block"><span class="k">\bi</span>
</div><div id="l530" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> is not a variable
</div><div id="l531" class="code_block"><span class="k">\bi</span>
</div><div id="l532" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> instantiation<span class="k">\_error</span><span class="nb">}</span>
</div><div id="l533" class="code_block"><span class="k">\ei</span>
</div><div id="l534" class="code_block"><span class="k">\ei</span>
</div><div id="l535" class="code_block">
</div><div id="l536" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_detach</span>(+ThreadId)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l537" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_detach</span>/1<span class="nb">}}</span> 
</div><div id="l538" class="code_block"><span class="c">%</span>
</div><div id="l539" class="code_block">Detaches a joinable thread denoted by <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> so that all
</div><div id="l540" class="code_block">resources will be reclaimed upon its exit.  The thread denoted by <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l541" class="code_block">  ThreadId<span class="nb">}</span> will no longer be joinable, once it is detached.
</div><div id="l542" class="code_block">
</div><div id="l543" class="code_block">Note, if it is known at thread creation time that a thread should be
</div><div id="l544" class="code_block">detached, it is better to create the thread as detached via <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l545" class="code_block">  xsb<span class="k">\_thread\_create</span>/3<span class="nb">}</span>.
</div><div id="l546" class="code_block">
</div><div id="l547" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l548" class="code_block"><span class="k">\bi</span>
</div><div id="l549" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> is not an integer
</div><div id="l550" class="code_block"><span class="k">\bi</span>
</div><div id="l551" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> type<span class="k">\_error</span>(integer,ExitCode)<span class="nb">}</span>
</div><div id="l552" class="code_block"><span class="k">\ei</span>
</div><div id="l553" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> does not correspond to a valid thread
</div><div id="l554" class="code_block"><span class="k">\bi</span>
</div><div id="l555" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> existence<span class="k">\_error</span>(valid<span class="k">\_thread</span>,ThreadId)<span class="nb">}</span>
</div><div id="l556" class="code_block"><span class="k">\ei</span>
</div><div id="l557" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> is active but not joinable
</div><div id="l558" class="code_block"><span class="k">\bi</span>
</div><div id="l559" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> permission<span class="k">\_error</span>(thread<span class="k">\_detach</span>,non<span class="k">\_joinable\_thread</span>,ThreadId)<span class="nb">}</span>
</div><div id="l560" class="code_block"><span class="k">\ei</span>
</div><div id="l561" class="code_block"><span class="k">\ei</span>
</div><div id="l562" class="code_block">
</div><div id="l563" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_cancel</span>(+ThreadId)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l564" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_cancel</span>/1<span class="nb">}}</span> 
</div><div id="l565" class="code_block"><span class="c">%</span>
</div><div id="l566" class="code_block">Cancels the XSB thread denoted by <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span>.  The cancellation
</div><div id="l567" class="code_block">does not use pthread cancellation mechanisms, rather it uses XSB&#39;s
</div><div id="l568" class="code_block">interrupt mechanism to set <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span>&#39;s interrupt vector.  When
</div><div id="l569" class="code_block">this interrupt vector is checked, <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> will throw a thread
</div><div id="l570" class="code_block">cancellation error, which can be caught within <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> like any
</div><div id="l571" class="code_block">other error.  Usually, this means that <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> will abort and
</div><div id="l572" class="code_block">exit.  Note that the high-level upon which thread cancellation is
</div><div id="l573" class="code_block">implemented allows for simplicity and portability of thread
</div><div id="l574" class="code_block">cancellation.  <span class="k">\footnote</span><span class="nb">{</span>While interrupt vector is not checked upon
</div><div id="l575" class="code_block">  every SLG-WAM instruction, it is checked on every <span class="nb">{</span><span class="k">\sf</span> call<span class="nb">}</span> and
</div><div id="l576" class="code_block">  <span class="nb">{</span><span class="k">\sf</span> execute<span class="nb">}</span> instruction, and should should be suitable for almost
</div><div id="l577" class="code_block">  any need for cancellation.<span class="nb">}</span>.
</div><div id="l578" class="code_block">
</div><div id="l579" class="code_block">The main XSB thread cannot be cancelled; apart from that any thread
</div><div id="l580" class="code_block">can cancel any other thread.
</div><div id="l581" class="code_block">
</div><div id="l582" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l583" class="code_block"><span class="k">\bi</span>
</div><div id="l584" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> is not instantiated
</div><div id="l585" class="code_block"><span class="k">\bi</span>
</div><div id="l586" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> instantiation<span class="k">\_error</span><span class="nb">}</span>
</div><div id="l587" class="code_block"><span class="k">\ei</span>
</div><div id="l588" class="code_block"><span class="c">%</span>
</div><div id="l589" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> does not correspond to valid thread
</div><div id="l590" class="code_block"><span class="k">\bi</span>
</div><div id="l591" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> existence<span class="k">\_error</span>(valid<span class="k">\_thread</span>,ThreadId)<span class="nb">}</span>
</div><div id="l592" class="code_block"><span class="k">\ei</span>
</div><div id="l593" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> ThreadId<span class="nb">}</span> denotes the main thread.
</div><div id="l594" class="code_block"><span class="k">\bi</span>
</div><div id="l595" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> permission<span class="k">\_error</span>(cancel,main<span class="k">\_thread</span>,ThreadId)<span class="nb">}</span>
</div><div id="l596" class="code_block"><span class="k">\ei</span>
</div><div id="l597" class="code_block"><span class="k">\ei</span>
</div><div id="l598" class="code_block">
</div><div id="l599" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_thread\_yield</span><span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l600" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_thread\_yield</span>/0<span class="nb">}}</span> 
</div><div id="l601" class="code_block"><span class="c">% </span>
</div><div id="l602" class="code_block">Make the calling thread ready to be run <span class="nb">{</span><span class="k">\em</span> after<span class="nb">}</span> other threads of
</div><div id="l603" class="code_block">the same priority.  This predicate relies on the real-time extensions
</div><div id="l604" class="code_block">to pthreads specified in Posix 1b, and may not be available on all
</div><div id="l605" class="code_block">platforms.
</div><div id="l606" class="code_block">
</div><div id="l607" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l608" class="code_block"><span class="k">\bi</span>
</div><div id="l609" class="code_block"><span class="k">\item</span> 	The current platform does not support Posix real-time extensions
</div><div id="l610" class="code_block"><span class="k">\bi</span>
</div><div id="l611" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> misc<span class="k">\_error</span><span class="nb">}</span>
</div><div id="l612" class="code_block"><span class="k">\ei</span>
</div><div id="l613" class="code_block"><span class="k">\ei</span>
</div><div id="l614" class="code_block">
</div><div id="l615" class="code_block"><span class="k">\comment</span><span class="nb">{</span>
</div><div id="l616" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_user\_mutex\_lock</span>(MutexId)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l617" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_user\_mutex\_lock</span>/1<span class="nb">}}</span> 
</div><div id="l618" class="code_block"><span class="c">%</span>
</div><div id="l619" class="code_block">Locks a user-mutex, currently numbered 0 through 9, with 0
</div><div id="l620" class="code_block">corresponding to the mutex that controls I/O to the console.  These
</div><div id="l621" class="code_block">mutexes are initialized automatically at process startup, and in
</div><div id="l622" class="code_block">addition each of the mutexes have a <span class="nb">{</span><span class="k">\tt</span> pthread<span class="k">\_mutexattr\_t</span><span class="nb">}</span> type
</div><div id="l623" class="code_block">of <span class="nb">{</span><span class="k">\tt</span> NORMAL<span class="nb">}</span>.
</div><div id="l624" class="code_block">
</div><div id="l625" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l626" class="code_block"><span class="k">\bi</span>
</div><div id="l627" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> is not an integer
</div><div id="l628" class="code_block"><span class="k">\bi</span>
</div><div id="l629" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> type<span class="k">\_error</span>(integer,MutexId)<span class="nb">}</span>
</div><div id="l630" class="code_block"><span class="k">\ei</span>
</div><div id="l631" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> does is not the index of a user mutex
</div><div id="l632" class="code_block"><span class="k">\bi</span>
</div><div id="l633" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> domain<span class="k">\_error</span>(mutex<span class="k">\_id</span>,MutexId)<span class="nb">}</span>
</div><div id="l634" class="code_block"><span class="k">\ei</span>
</div><div id="l635" class="code_block"><span class="k">\item</span> 	Locking <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> would lead to a deadlock
</div><div id="l636" class="code_block"><span class="k">\bi</span>
</div><div id="l637" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> permission<span class="k">\_error</span>(&#39;mutex lock&#39;,&#39;deadlocking mutex&#39;,MutexId)<span class="nb">}</span>
</div><div id="l638" class="code_block"><span class="k">\ei</span>
</div><div id="l639" class="code_block"><span class="k">\ei</span>
</div><div id="l640" class="code_block">
</div><div id="l641" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_user\_mutex\_trylock</span>(MutexId)<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l642" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_user_mutex_trylock</span>/1<span class="nb">}}</span> 
</div><div id="l643" class="code_block"><span class="c">%</span>
</div><div id="l644" class="code_block">If the user-mutex denoted by <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> is not busy, this predicate
</div><div id="l645" class="code_block">locks <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> and succeeds; if <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> is busy the
</div><div id="l646" class="code_block">predicate fails, rather than waiting for the lock as with <span class="nb">{</span><span class="k">\tt</span>
</div><div id="l647" class="code_block">  xsb<span class="k">\_user\_mutex\_trylock</span>/1<span class="nb">}</span>
</div><div id="l648" class="code_block">
</div><div id="l649" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l650" class="code_block"><span class="k">\bi</span>
</div><div id="l651" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> is not an integer
</div><div id="l652" class="code_block"><span class="k">\bi</span>
</div><div id="l653" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> type<span class="k">\_error</span>(integer,MutexId)<span class="nb">}</span>
</div><div id="l654" class="code_block"><span class="k">\ei</span>
</div><div id="l655" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> does is not the index of a user mutex
</div><div id="l656" class="code_block"><span class="k">\bi</span>
</div><div id="l657" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> domain<span class="k">\_error</span>(mutex<span class="k">\_id</span>,MutexId)<span class="nb">}</span>
</div><div id="l658" class="code_block"><span class="k">\ei</span>
</div><div id="l659" class="code_block"><span class="k">\ei</span>
</div><div id="l660" class="code_block">
</div><div id="l661" class="code_block"><span class="k">\ournewitem</span><span class="nb">{</span>xsb<span class="k">\_user\_mutex\_unlock</span>/0<span class="nb">}{</span>thread<span class="nb">}</span>
</div><div id="l662" class="code_block"><span class="k">\index</span><span class="nb">{</span><span class="k">\texttt</span><span class="nb">{</span>xsb<span class="k">\_user\_mutex\_unlock</span>/1<span class="nb">}}</span> 
</div><div id="l663" class="code_block"><span class="c">%</span>
</div><div id="l664" class="code_block">Unlocks a user mutex (user mutexes are described above).
</div><div id="l665" class="code_block">
</div><div id="l666" class="code_block"><span class="nb">{</span><span class="k">\bf</span> Error Cases<span class="nb">}</span>
</div><div id="l667" class="code_block"><span class="k">\bi</span>
</div><div id="l668" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> is not an integer
</div><div id="l669" class="code_block"><span class="k">\bi</span>
</div><div id="l670" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> type<span class="k">\_error</span>(integer,MutexId)<span class="nb">}</span>
</div><div id="l671" class="code_block"><span class="k">\ei</span>
</div><div id="l672" class="code_block"><span class="k">\item</span>   <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> does is not the index of a user mutex
</div><div id="l673" class="code_block"><span class="k">\bi</span>
</div><div id="l674" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> domain<span class="k">\_error</span>(mutex<span class="k">\_id</span>,MutexId)<span class="nb">}</span>
</div><div id="l675" class="code_block"><span class="k">\ei</span>
</div><div id="l676" class="code_block"><span class="k">\item</span> 	The thread calling <span class="nb">{</span><span class="k">\tt</span> xsb<span class="k">\_user\_mutex\_unlock</span>/1<span class="nb">}</span> does not
</div><div id="l677" class="code_block">  own the lock for <span class="nb">{</span><span class="k">\tt</span> MutexId<span class="nb">}</span> 
</div><div id="l678" class="code_block"><span class="k">\bi</span>
</div><div id="l679" class="code_block"><span class="k">\item</span> 	<span class="nb">{</span><span class="k">\tt</span> permission<span class="k">\_error</span>(&#39;mutex unlock&#39;,mutex,MutexId)<span class="nb">}</span>
</div><div id="l680" class="code_block"><span class="k">\ei</span>
</div><div id="l681" class="code_block"><span class="k">\ei</span>
</div><div id="l682" class="code_block"><span class="nb">}</span>
</div><div id="l683" class="code_block"><span class="k">\end</span><span class="nb">{</span>description<span class="nb">}</span>
</div></pre></div></td></tr></table></div>

      
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