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<title>SWI-Prolog 7.3.6 Reference Manual: Section 9.2</title><link rel="home" href="index.html">
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<h2 id="sec:thmonitor"><a id="sec:9.2"><span class="sec-nr">9.2</span> <span class="sec-title">Monitoring
threads</span></a></h2>
<a id="sec:thmonitor"></a>
<p>Normal multithreaded applications should not need the predicates from
this section because almost any usage of these predicates is unsafe. For
example checking the existence of a thread before signalling it is of no
use as it may vanish between the two calls. Catching exceptions using <a id="idx:catch3:1732"></a><a class="pred" href="exception.html#catch/3">catch/3</a>
is the only safe way to deal with thread-existence errors.
<p>These predicates are provided for diagnosis and monitoring tasks. See
also <a class="sec" href="thutil.html">section 9.5</a>, describing more
high-level primitives.
<dl class="latex">
<dt class="pubdef"><a id="thread_property/2"><strong>thread_property</strong>(<var>?Id,
?Property</var>)</a></dt>
<dd class="defbody">
True if thread <var>Id</var> has <var>Property</var>. Either or both
arguments may be unbound, enumerating all relations on backtracking.
Calling <a id="idx:threadproperty2:1733"></a><a class="pred" href="thmonitor.html#thread_property/2">thread_property/2</a>
does not influence any thread. See also
<a id="idx:threadjoin2:1734"></a><a class="pred" href="threadcreate.html#thread_join/2">thread_join/2</a>.
For threads that have an alias name, this name is returned in <var>Id</var>
instead of the opaque thread identifier. Defined properties are:
<dl class="latex">
<dt><strong>alias</strong>(<var>Alias</var>)</dt>
<dd class="defbody">
<var>Alias</var> is the alias name of thread <var>Id</var>.
</dd>
<dt><strong>detached</strong>(<var>Boolean</var>)</dt>
<dd class="defbody">
Current detached status of the thread.
</dd>
<dt><strong>status</strong>(<var>Status</var>)</dt>
<dd class="defbody">
Current status of the thread. <var>Status</var> is one of:
<dl class="latex">
<dt><strong>running</strong></dt>
<dd class="defbody">
The thread is running. This is the initial status of a thread. Please
note that threads waiting for something are considered running too.</dd>
<dt><strong>false</strong></dt>
<dd class="defbody">
The <var>Goal</var> of the thread has been completed and failed.</dd>
<dt><strong>true</strong></dt>
<dd class="defbody">
The <var>Goal</var> of the thread has been completed and succeeded.</dd>
<dt><strong>exited</strong>(<var>Term</var>)</dt>
<dd class="defbody">
The <var>Goal</var> of the thread has been terminated using <a id="idx:threadexit1:1735"></a><a class="pred" href="threadcreate.html#thread_exit/1">thread_exit/1</a>
with <var>Term</var> as argument. If the underlying native thread has
exited (using pthread_exit()) <var>Term</var> is unbound.</dd>
<dt><strong>exception</strong>(<var>Term</var>)</dt>
<dd class="defbody">
The <var>Goal</var> of the thread has been terminated due to an uncaught
exception (see <a id="idx:throw1:1736"></a><a class="pred" href="exception.html#throw/1">throw/1</a>
and <a id="idx:catch3:1737"></a><a class="pred" href="exception.html#catch/3">catch/3</a>).
</dd>
</dl>
</dd>
</dl>
<p>See also <a id="idx:threadstatistics3:1738"></a><a class="pred" href="thmonitor.html#thread_statistics/3">thread_statistics/3</a>
to obtain resource usage information and
<a id="idx:messagequeueproperty2:1739"></a><a class="pred" href="threadcom.html#message_queue_property/2">message_queue_property/2</a>
to get the number of queued messages for a thread.</dd>
<dt class="pubdef"><a id="thread_statistics/3"><strong>thread_statistics</strong>(<var>+Id,
+Key, -Value</var>)</a></dt>
<dd class="defbody">
Obtains statistical information on thread <var>Id</var> as <a id="idx:statistics2:1740"></a><a class="pred" href="statistics.html#statistics/2">statistics/2</a>
does in single-threaded applications. This call supports all keys of <a id="idx:statistics2:1741"></a><a class="pred" href="statistics.html#statistics/2">statistics/2</a>,
although only stack sizes, <code>cputime</code>,
<code>inferences</code> and <code>epoch</code> yield different values
for each thread.<sup class="fn">144<span class="fn-text">There is no
portable interface to obtain thread-specific CPU time and some operating
systems provide no access to this information at all. On such systems
the total process CPU is returned. Thread CPU time is supported on
MS-Windows, Linux and MacOSX.</span></sup></dd>
<dt class="pubdef"><a id="mutex_statistics/0"><strong>mutex_statistics</strong></a></dt>
<dd class="defbody">
Print usage statistics on internal mutexes and mutexes associated with
dynamic predicates. For each mutex two numbers are printed: the number
of times the mutex was acquired and the number of <em>collisions</em>:
the number of times the calling thread has to wait for the mutex.
Generally collision count is close to zero on single-CPU hardware.
</dd>
</dl>
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