141 lines
8.1 KiB
HTML
141 lines
8.1 KiB
HTML
<html lang="en">
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<head>
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<title>Auto Display - Debugging with GDB</title>
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<link rel="prev" href="Memory.html#Memory" title="Memory">
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<body>
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<div class="node">
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<a name="Auto-Display"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Print-Settings.html#Print-Settings">Print Settings</a>,
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Previous: <a rel="previous" accesskey="p" href="Memory.html#Memory">Memory</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">10.7 Automatic Display</h3>
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<p><a name="index-automatic-display-665"></a><a name="index-display-of-expressions-666"></a>
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If you find that you want to print the value of an expression frequently
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(to see how it changes), you might want to add it to the <dfn>automatic
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display list</dfn> so that <span class="sc">gdb</span> prints its value each time your program stops.
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Each expression added to the list is given a number to identify it;
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to remove an expression from the list, you specify that number.
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The automatic display looks like this:
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<pre class="smallexample"> 2: foo = 38
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3: bar[5] = (struct hack *) 0x3804
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</pre>
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<p class="noindent">This display shows item numbers, expressions and their current values. As with
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displays you request manually using <code>x</code> or <code>print</code>, you can
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specify the output format you prefer; in fact, <code>display</code> decides
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whether to use <code>print</code> or <code>x</code> depending your format
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specification—it uses <code>x</code> if you specify either the ‘<samp><span class="samp">i</span></samp>’
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or ‘<samp><span class="samp">s</span></samp>’ format, or a unit size; otherwise it uses <code>print</code>.
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<a name="index-display-667"></a>
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<dl><dt><code>display </code><var>expr</var><dd>Add the expression <var>expr</var> to the list of expressions to display
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each time your program stops. See <a href="Expressions.html#Expressions">Expressions</a>.
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<p><code>display</code> does not repeat if you press <RET> again after using it.
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<br><dt><code>display/</code><var>fmt</var> <var>expr</var><dd>For <var>fmt</var> specifying only a display format and not a size or
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count, add the expression <var>expr</var> to the auto-display list but
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arrange to display it each time in the specified format <var>fmt</var>.
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See <a href="Output-Formats.html#Output-Formats">Output Formats</a>.
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<br><dt><code>display/</code><var>fmt</var> <var>addr</var><dd>For <var>fmt</var> ‘<samp><span class="samp">i</span></samp>’ or ‘<samp><span class="samp">s</span></samp>’, or including a unit-size or a
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number of units, add the expression <var>addr</var> as a memory address to
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be examined each time your program stops. Examining means in effect
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doing ‘<samp><span class="samp">x/</span><var>fmt</var> <var>addr</var></samp>’. See <a href="Memory.html#Memory">Examining Memory</a>.
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</dl>
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<p>For example, ‘<samp><span class="samp">display/i $pc</span></samp>’ can be helpful, to see the machine
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instruction about to be executed each time execution stops (‘<samp><span class="samp">$pc</span></samp>’
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is a common name for the program counter; see <a href="Registers.html#Registers">Registers</a>).
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<a name="index-delete-display-668"></a>
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<a name="index-undisplay-669"></a>
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<dl><dt><code>undisplay </code><var>dnums</var><code>...</code><dt><code>delete display </code><var>dnums</var><code>...</code><dd>Remove items from the list of expressions to display. Specify the
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numbers of the displays that you want affected with the command
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argument <var>dnums</var>. It can be a single display number, one of the
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numbers shown in the first field of the ‘<samp><span class="samp">info display</span></samp>’ display;
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or it could be a range of display numbers, as in <code>2-4</code>.
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<p><code>undisplay</code> does not repeat if you press <RET> after using it.
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(Otherwise you would just get the error ‘<samp><span class="samp">No display number ...</span></samp>’.)
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<p><a name="index-disable-display-670"></a><br><dt><code>disable display </code><var>dnums</var><code>...</code><dd>Disable the display of item numbers <var>dnums</var>. A disabled display
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item is not printed automatically, but is not forgotten. It may be
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enabled again later. Specify the numbers of the displays that you
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want affected with the command argument <var>dnums</var>. It can be a
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single display number, one of the numbers shown in the first field of
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the ‘<samp><span class="samp">info display</span></samp>’ display; or it could be a range of display
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numbers, as in <code>2-4</code>.
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<p><a name="index-enable-display-671"></a><br><dt><code>enable display </code><var>dnums</var><code>...</code><dd>Enable display of item numbers <var>dnums</var>. It becomes effective once
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again in auto display of its expression, until you specify otherwise.
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Specify the numbers of the displays that you want affected with the
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command argument <var>dnums</var>. It can be a single display number, one
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of the numbers shown in the first field of the ‘<samp><span class="samp">info display</span></samp>’
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display; or it could be a range of display numbers, as in <code>2-4</code>.
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<br><dt><code>display</code><dd>Display the current values of the expressions on the list, just as is
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done when your program stops.
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<p><a name="index-info-display-672"></a><br><dt><code>info display</code><dd>Print the list of expressions previously set up to display
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automatically, each one with its item number, but without showing the
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values. This includes disabled expressions, which are marked as such.
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It also includes expressions which would not be displayed right now
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because they refer to automatic variables not currently available.
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</dl>
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<p><a name="index-display-disabled-out-of-scope-673"></a>If a display expression refers to local variables, then it does not make
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sense outside the lexical context for which it was set up. Such an
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expression is disabled when execution enters a context where one of its
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variables is not defined. For example, if you give the command
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<code>display last_char</code> while inside a function with an argument
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<code>last_char</code>, <span class="sc">gdb</span> displays this argument while your program
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continues to stop inside that function. When it stops elsewhere—where
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there is no variable <code>last_char</code>—the display is disabled
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automatically. The next time your program stops where <code>last_char</code>
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is meaningful, you can enable the display expression once again.
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</body></html>
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