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<title>Debugging with GDB: Range Checking</title>
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<a name="Range-Checking"></a>
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<div class="header">
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<p>
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Previous: <a href="Type-Checking.html#Type-Checking" accesskey="p" rel="previous">Type Checking</a>, Up: <a href="Checks.html#Checks" accesskey="u" rel="up">Checks</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="An-Overview-of-Range-Checking"></a>
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<h4 class="subsection">15.3.2 An Overview of Range Checking</h4>
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<p>In some languages (such as Modula-2), it is an error to exceed the
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bounds of a type; this is enforced with run-time checks. Such range
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checking is meant to ensure program correctness by making sure
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computations do not overflow, or indices on an array element access do
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not exceed the bounds of the array.
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</p>
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<p>For expressions you use in <small>GDB</small> commands, you can tell
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<small>GDB</small> to treat range errors in one of three ways: ignore them,
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always treat them as errors and abandon the expression, or issue
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warnings but evaluate the expression anyway.
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</p>
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<p>A range error can result from numerical overflow, from exceeding an
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array index bound, or when you type a constant that is not a member
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of any type. Some languages, however, do not treat overflows as an
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error. In many implementations of C, mathematical overflow causes the
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result to “wrap around” to lower values—for example, if <var>m</var> is
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the largest integer value, and <var>s</var> is the smallest, then
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</p>
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<div class="smallexample">
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<pre class="smallexample"><var>m</var> + 1 ⇒ <var>s</var>
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</pre></div>
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<p>This, too, is specific to individual languages, and in some cases
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specific to individual compilers or machines. See <a href="Supported-Languages.html#Supported-Languages">Supported Languages</a>, for further details on specific languages.
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</p>
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<p><small>GDB</small> provides some additional commands for controlling the range checker:
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</p>
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<a name="index-set-check-range"></a>
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<a name="index-show-check-range"></a>
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<dl compact="compact">
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<dt><code>set check range auto</code></dt>
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<dd><p>Set range checking on or off based on the current working language.
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See <a href="Supported-Languages.html#Supported-Languages">Supported Languages</a>, for the default settings for
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each language.
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</p>
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</dd>
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<dt><code>set check range on</code></dt>
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<dt><code>set check range off</code></dt>
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<dd><p>Set range checking on or off, overriding the default setting for the
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current working language. A warning is issued if the setting does not
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match the language default. If a range error occurs and range checking is on,
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then a message is printed and evaluation of the expression is aborted.
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</p>
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</dd>
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<dt><code>set check range warn</code></dt>
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<dd><p>Output messages when the <small>GDB</small> range checker detects a range error,
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but attempt to evaluate the expression anyway. Evaluating the
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expression may still be impossible for other reasons, such as accessing
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memory that the process does not own (a typical example from many Unix
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systems).
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</p>
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</dd>
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<dt><code>show range</code></dt>
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<dd><p>Show the current setting of the range checker, and whether or not it is
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being set automatically by <small>GDB</small>.
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</p></dd>
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</dl>
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Previous: <a href="Type-Checking.html#Type-Checking" accesskey="p" rel="previous">Type Checking</a>, Up: <a href="Checks.html#Checks" accesskey="u" rel="up">Checks</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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