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<a name="What-the-Stub-Can-Do-for-You"></a>
<h4 class="subsection">20.5.1 What the Stub Can Do for You</h4>

<a name="index-remote-serial-stub"></a>
<p>The debugging stub for your architecture supplies these three
subroutines:
</p>
<dl compact="compact">
<dt><code>set_debug_traps</code></dt>
<dd><a name="index-set_005fdebug_005ftraps"></a>
<a name="index-remote-serial-stub_002c-initialization"></a>
<p>This routine arranges for <code>handle_exception</code> to run when your
program stops.  You must call this subroutine explicitly in your
program&rsquo;s startup code.
</p>
</dd>
<dt><code>handle_exception</code></dt>
<dd><a name="index-handle_005fexception"></a>
<a name="index-remote-serial-stub_002c-main-routine"></a>
<p>This is the central workhorse, but your program never calls it
explicitly&mdash;the setup code arranges for <code>handle_exception</code> to
run when a trap is triggered.
</p>
<p><code>handle_exception</code> takes control when your program stops during
execution (for example, on a breakpoint), and mediates communications
with <small>GDB</small> on the host machine.  This is where the communications
protocol is implemented; <code>handle_exception</code> acts as the <small>GDB</small>
representative on the target machine.  It begins by sending summary
information on the state of your program, then continues to execute,
retrieving and transmitting any information <small>GDB</small> needs, until you
execute a <small>GDB</small> command that makes your program resume; at that point,
<code>handle_exception</code> returns control to your own code on the target
machine.
</p>
</dd>
<dt><code>breakpoint</code></dt>
<dd><a name="index-breakpoint-subroutine_002c-remote"></a>
<p>Use this auxiliary subroutine to make your program contain a
breakpoint.  Depending on the particular situation, this may be the only
way for <small>GDB</small> to get control.  For instance, if your target
machine has some sort of interrupt button, you won&rsquo;t need to call this;
pressing the interrupt button transfers control to
<code>handle_exception</code>&mdash;in effect, to <small>GDB</small>.  On some machines,
simply receiving characters on the serial port may also trigger a trap;
again, in that situation, you don&rsquo;t need to call <code>breakpoint</code> from
your own program&mdash;simply running &lsquo;<samp>target remote</samp>&rsquo; from the host
<small>GDB</small> session gets control.
</p>
<p>Call <code>breakpoint</code> if none of these is true, or if you simply want
to make certain your program stops at a predetermined point for the
start of your debugging session.
</p></dd>
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