112 lines
5.5 KiB
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112 lines
5.5 KiB
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<title>Value History - Debugging with GDB</title>
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<link rel="up" href="Data.html#Data" title="Data">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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<div class="node">
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<a name="Value-History"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>,
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Previous: <a rel="previous" accesskey="p" href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>,
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Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
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<hr>
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</div>
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<h3 class="section">10.10 Value History</h3>
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<p><a name="index-value-history-717"></a><a name="index-history-of-values-printed-by-_0040value_007bGDBN_007d-718"></a>Values printed by the <code>print</code> command are saved in the <span class="sc">gdb</span>
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<dfn>value history</dfn>. This allows you to refer to them in other expressions.
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Values are kept until the symbol table is re-read or discarded
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(for example with the <code>file</code> or <code>symbol-file</code> commands).
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When the symbol table changes, the value history is discarded,
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since the values may contain pointers back to the types defined in the
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symbol table.
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<p><a name="index-g_t_0040code_007b_0024_007d-719"></a><a name="index-g_t_0040code_007b_0024_0024_007d-720"></a><a name="index-history-number-721"></a>The values printed are given <dfn>history numbers</dfn> by which you can
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refer to them. These are successive integers starting with one.
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<code>print</code> shows you the history number assigned to a value by
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printing ‘<samp><span class="samp">$</span><var>num</var><span class="samp"> = </span></samp>’ before the value; here <var>num</var> is the
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history number.
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<p>To refer to any previous value, use ‘<samp><span class="samp">$</span></samp>’ followed by the value's
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history number. The way <code>print</code> labels its output is designed to
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remind you of this. Just <code>$</code> refers to the most recent value in
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the history, and <code>$$</code> refers to the value before that.
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<code>$$</code><var>n</var> refers to the <var>n</var>th value from the end; <code>$$2</code>
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is the value just prior to <code>$$</code>, <code>$$1</code> is equivalent to
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<code>$$</code>, and <code>$$0</code> is equivalent to <code>$</code>.
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<p>For example, suppose you have just printed a pointer to a structure and
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want to see the contents of the structure. It suffices to type
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<pre class="smallexample"> p *$
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</pre>
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<p>If you have a chain of structures where the component <code>next</code> points
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to the next one, you can print the contents of the next one with this:
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<pre class="smallexample"> p *$.next
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</pre>
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<p class="noindent">You can print successive links in the chain by repeating this
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command—which you can do by just typing <RET>.
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<p>Note that the history records values, not expressions. If the value of
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<code>x</code> is 4 and you type these commands:
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<pre class="smallexample"> print x
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set x=5
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</pre>
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<p class="noindent">then the value recorded in the value history by the <code>print</code> command
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remains 4 even though the value of <code>x</code> has changed.
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<a name="index-show-values-722"></a>
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<dl><dt><code>show values</code><dd>Print the last ten values in the value history, with their item numbers.
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This is like ‘<samp><span class="samp">p $$9</span></samp>’ repeated ten times, except that <code>show
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values</code> does not change the history.
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<br><dt><code>show values </code><var>n</var><dd>Print ten history values centered on history item number <var>n</var>.
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<br><dt><code>show values +</code><dd>Print ten history values just after the values last printed. If no more
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values are available, <code>show values +</code> produces no display.
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</dl>
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<p>Pressing <RET> to repeat <code>show values </code><var>n</var> has exactly the
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same effect as ‘<samp><span class="samp">show values +</span></samp>’.
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</body></html>
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