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| <title>Debugging with GDB: Value History</title> |
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| <a name="Value-History-1"></a> |
| <h3 class="section">10.10 Value History</h3> |
| |
| <a name="index-value-history"></a> |
| <a name="index-history-of-values-printed-by-GDB"></a> |
| <p>Values printed by the <code>print</code> command are saved in the <small>GDB</small> |
| <em>value history</em>. This allows you to refer to them in other expressions. |
| Values are kept until the symbol table is re-read or discarded |
| (for example with the <code>file</code> or <code>symbol-file</code> commands). |
| When the symbol table changes, the value history is discarded, |
| since the values may contain pointers back to the types defined in the |
| symbol table. |
| </p> |
| <a name="index-_0024"></a> |
| <a name="index-_0024_0024"></a> |
| <a name="index-history-number"></a> |
| <p>The values printed are given <em>history numbers</em> by which you can |
| refer to them. These are successive integers starting with one. |
| <code>print</code> shows you the history number assigned to a value by |
| printing ‘<samp>$<var>num</var> = </samp>’ before the value; here <var>num</var> is the |
| history number. |
| </p> |
| <p>To refer to any previous value, use ‘<samp>$</samp>’ followed by the value’s |
| history number. The way <code>print</code> labels its output is designed to |
| remind you of this. Just <code>$</code> refers to the most recent value in |
| the history, and <code>$$</code> refers to the value before that. |
| <code>$$<var>n</var></code> refers to the <var>n</var>th value from the end; <code>$$2</code> |
| is the value just prior to <code>$$</code>, <code>$$1</code> is equivalent to |
| <code>$$</code>, and <code>$$0</code> is equivalent to <code>$</code>. |
| </p> |
| <p>For example, suppose you have just printed a pointer to a structure and |
| want to see the contents of the structure. It suffices to type |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">p *$ |
| </pre></div> |
| |
| <p>If you have a chain of structures where the component <code>next</code> points |
| to the next one, you can print the contents of the next one with this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">p *$.next |
| </pre></div> |
| |
| <p>You can print successive links in the chain by repeating this |
| command—which you can do by just typing <tt class="key">RET</tt>. |
| </p> |
| <p>Note that the history records values, not expressions. If the value of |
| <code>x</code> is 4 and you type these commands: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">print x |
| set x=5 |
| </pre></div> |
| |
| <p>then the value recorded in the value history by the <code>print</code> command |
| remains 4 even though the value of <code>x</code> has changed. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-show-values"></a> |
| </dd> |
| <dt><code>show values</code></dt> |
| <dd><p>Print the last ten values in the value history, with their item numbers. |
| This is like ‘<samp>p $$9</samp>’ repeated ten times, except that <code>show |
| values</code> does not change the history. |
| </p> |
| </dd> |
| <dt><code>show values <var>n</var></code></dt> |
| <dd><p>Print ten history values centered on history item number <var>n</var>. |
| </p> |
| </dd> |
| <dt><code>show values +</code></dt> |
| <dd><p>Print ten history values just after the values last printed. If no more |
| values are available, <code>show values +</code> produces no display. |
| </p></dd> |
| </dl> |
| |
| <p>Pressing <tt class="key">RET</tt> to repeat <code>show values <var>n</var></code> has exactly the |
| same effect as ‘<samp>show values +</samp>’. |
| </p> |
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