mingw/info/gdb/Value-History.html

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  19 <h3 class="section">Value history</h3>
  20
  21    <p>Values printed by the <code>print</code> command are saved in the GDB
  22 <dfn>value history</dfn>.  This allows you to refer to them in other expressions.
  23 Values are kept until the symbol table is re-read or discarded
  24 (for example with the <code>file</code> or <code>symbol-file</code> commands).
  25 When the symbol table changes, the value history is discarded,
  26 since the values may contain pointers back to the types defined in the
  27 symbol table.
  28
  29    <p>The values printed are given <dfn>history numbers</dfn> by which you can
  30 refer to them.  These are successive integers starting with one.
  31 <code>print</code> shows you the history number assigned to a value by
  32 printing <code>$</code><var>num</var><code> = </code> before the value; here <var>num</var> is the
  33 history number.
  34
  35    <p>To refer to any previous value, use <code>$</code> followed by the value's
  36 history number.  The way <code>print</code> labels its output is designed to
  37 remind you of this.  Just <code>$</code> refers to the most recent value in
  38 the history, and <code>$$</code> refers to the value before that.
  39 <code>$$</code><var>n</var><code></code> refers to the <var>n</var>th value from the end; <code>$$2</code>
  40 is the value just prior to <code>$$</code>, <code>$$1</code> is equivalent to
  41 <code>$$</code>, and <code>$$0</code> is equivalent to <code>$</code>.
  42
  43    <p>For example, suppose you have just printed a pointer to a structure and
  44 want to see the contents of the structure.  It suffices to type
  45
  46 <pre class="example">     p *$
  47      </pre>
  48
  49    <p>If you have a chain of structures where the component <code>next</code> points
  50 to the next one, you can print the contents of the next one with this:
  51
  52 <pre class="example">     p *$.next
  53      </pre>
  54
  55 <p>You can print successive links in the chain by repeating this
  56 command--which you can do by just typing &lt;RET&gt;.
  57
  58    <p>Note that the history records values, not expressions.  If the value of
  59 <code>x</code> is 4 and you type these commands:
  60
  61 <pre class="example">     print x
  62      set x=5
  63      </pre>
  64
  65 <p>then the value recorded in the value history by the <code>print</code> command
  66 remains 4 even though the value of <code>x</code> has changed.
  67
  68      <dl>
  69 <dt><code>show values</code>
  70      <dd>Print the last ten values in the value history, with their item numbers.
  71 This is like <code>p $$9</code> repeated ten times, except that <code>show
  72 values</code> does not change the history.
  73
  74      <br><dt><code>show values </code><var>n</var><code></code>
  75      <dd>Print ten history values centered on history item number <var>n</var>.
  76
  77      <br><dt><code>show values +</code>
  78      <dd>Print ten history values just after the values last printed.  If no more
  79 values are available, <code>show values +</code> produces no display.
  80 </dl>
  81
  82    <p>Pressing &lt;RET&gt; to repeat <code>show values </code><var>n</var><code></code> has exactly the
  83 same effect as <code>show values +</code>.
  84
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  86