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| <title>Debugging with GDB: Range Checking</title> |
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| <a name="Range-Checking"></a> |
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| Previous: <a href="Type-Checking.html#Type-Checking" accesskey="p" rel="prev">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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| <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 “wrap around” to lower values—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 ⇒ <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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