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<h4 class="subsection">27.1.3 Thread groups</h4>
<p><small>GDB</small> may be used to debug several processes at the same time.
On some platforms, <small>GDB</small> may support debugging of several
hardware systems, each one having several cores with several different
processes running on each core.  This section describes the MI
mechanism to support such debugging scenarios.
</p>
<p>The key observation is that regardless of the structure of the 
target, MI can have a global list of threads, because most commands that 
accept the &lsquo;<samp>--thread</samp>&rsquo; option do not need to know what process that
thread belongs to.  Therefore, it is not necessary to introduce
neither additional &lsquo;<samp>--process</samp>&rsquo; option, nor an notion of the
current process in the MI interface.  The only strictly new feature
that is required is the ability to find how the threads are grouped
into processes.
</p>
<p>To allow the user to discover such grouping, and to support arbitrary
hierarchy of machines/cores/processes, MI introduces the concept of a
<em>thread group</em>.  Thread group is a collection of threads and other
thread groups.  A thread group always has a string identifier, a type,
and may have additional attributes specific to the type.  A new
command, <code>-list-thread-groups</code>, returns the list of top-level
thread groups, which correspond to processes that <small>GDB</small> is
debugging at the moment.  By passing an identifier of a thread group
to the <code>-list-thread-groups</code> command, it is possible to obtain
the members of specific thread group.
</p>
<p>To allow the user to easily discover processes, and other objects, he
wishes to debug, a concept of <em>available thread group</em> is
introduced.  Available thread group is an thread group that
<small>GDB</small> is not debugging, but that can be attached to, using the
<code>-target-attach</code> command.  The list of available top-level thread
groups can be obtained using &lsquo;<samp>-list-thread-groups --available</samp>&rsquo;.
In general, the content of a thread group may be only retrieved only
after attaching to that thread group.
</p>
<p>Thread groups are related to inferiors (see <a href="Inferiors-Connections-and-Programs.html#Inferiors-Connections-and-Programs">Inferiors Connections and Programs</a>).  Each inferior corresponds to a thread group of a special
type &lsquo;<samp>process</samp>&rsquo;, and some additional operations are permitted on
such thread groups.
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