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<title>Debugging with GDB: Range Checking</title>

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<a name="Range-Checking"></a>
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<a name="An-Overview-of-Range-Checking"></a>
<h4 class="subsection">15.3.2 An Overview of Range Checking</h4>

<p>In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks.  Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.
</p>
<p>For expressions you use in <small>GDB</small> commands, you can tell
<small>GDB</small> to treat range errors in one of three ways: ignore them,
always treat them as errors and abandon the expression, or issue
warnings but evaluate the expression anyway.
</p>
<p>A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member
of any type.  Some languages, however, do not treat overflows as an
error.  In many implementations of C, mathematical overflow causes the
result to &ldquo;wrap around&rdquo; to lower values&mdash;for example, if <var>m</var> is
the largest integer value, and <var>s</var> is the smallest, then
</p>
<div class="smallexample">
<pre class="smallexample"><var>m</var> + 1 &rArr; <var>s</var>
</pre></div>

<p>This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines.  See <a href="Supported-Languages.html#Supported-Languages">Supported Languages</a>, for further details on specific languages.
</p>
<p><small>GDB</small> provides some additional commands for controlling the range checker:
</p>
<a name="index-set-check-range"></a>
<a name="index-show-check-range"></a>
<dl compact="compact">
<dt><code>set check range auto</code></dt>
<dd><p>Set range checking on or off based on the current working language.
See <a href="Supported-Languages.html#Supported-Languages">Supported Languages</a>, for the default settings for
each language.
</p>
</dd>
<dt><code>set check range on</code></dt>
<dt><code>set check range off</code></dt>
<dd><p>Set range checking on or off, overriding the default setting for the
current working language.  A warning is issued if the setting does not
match the language default.  If a range error occurs and range checking is on,
then a message is printed and evaluation of the expression is aborted.
</p>
</dd>
<dt><code>set check range warn</code></dt>
<dd><p>Output messages when the <small>GDB</small> range checker detects a range error,
but attempt to evaluate the expression anyway.  Evaluating the
expression may still be impossible for other reasons, such as accessing
memory that the process does not own (a typical example from many Unix
systems).
</p>
</dd>
<dt><code>show range</code></dt>
<dd><p>Show the current setting of the range checker, and whether or not it is
being set automatically by <small>GDB</small>.
</p></dd>
</dl>

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