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<title>Debugging with GDB: Data</title>
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<p>
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Next: <a href="Optimized-Code.html#Optimized-Code" accesskey="n" rel="next">Optimized Code</a>, Previous: <a href="Source.html#Source" accesskey="p" rel="previous">Source</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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</div>
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<hr>
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<a name="Examining-Data"></a>
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<h2 class="chapter">10 Examining Data</h2>
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<a name="index-printing-data"></a>
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<a name="index-examining-data"></a>
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<a name="index-print"></a>
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<a name="index-inspect"></a>
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<p>The usual way to examine data in your program is with the <code>print</code>
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command (abbreviated <code>p</code>), or its synonym <code>inspect</code>. It
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evaluates and prints the value of an expression of the language your
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program is written in (see <a href="Languages.html#Languages">Using <small>GDB</small> with
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Different Languages</a>). It may also print the expression using a
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Python-based pretty-printer (see <a href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>).
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</p>
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<dl compact="compact">
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<dt><code>print [[<var>options</var>] --] <var>expr</var></code></dt>
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<dt><code>print [[<var>options</var>] --] /<var>f</var> <var>expr</var></code></dt>
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<dd><p><var>expr</var> is an expression (in the source language). By default the
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value of <var>expr</var> is printed in a format appropriate to its data type;
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you can choose a different format by specifying ‘<samp>/<var>f</var></samp>’, where
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<var>f</var> is a letter specifying the format; see <a href="Output-Formats.html#Output-Formats">Output
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Formats</a>.
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</p>
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<a name="print-options"></a><p>The <code>print</code> command supports a number of options that allow
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overriding relevant global print settings as set by <code>set print</code>
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subcommands:
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</p>
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<dl compact="compact">
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<dt><code>-address [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing of addresses.
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Related setting: <a href="Print-Settings.html#set-print-address">set print address</a>.
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</p>
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</dd>
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<dt><code>-array [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Pretty formatting of arrays.
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Related setting: <a href="Print-Settings.html#set-print-array">set print array</a>.
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</p>
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</dd>
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<dt><code>-array-indexes [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing of array indexes.
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Related setting: <a href="Print-Settings.html#set-print-array_002dindexes">set print array-indexes</a>.
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</p>
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</dd>
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<dt><code>-elements <var>number-of-elements</var>|<code>unlimited</code></code></dt>
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<dd><p>Set limit on string chars or array elements to print. The value
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<code>unlimited</code> causes there to be no limit. Related setting:
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<a href="Print-Settings.html#set-print-elements">set print elements</a>.
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</p>
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</dd>
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<dt><code>-max-depth <var>depth</var>|<code>unlimited</code></code></dt>
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<dd><p>Set the threshold after which nested structures are replaced with
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ellipsis. Related setting: <a href="Print-Settings.html#set-print-max_002ddepth">set print max-depth</a>.
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</p>
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</dd>
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<dt><code>-null-stop [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing of char arrays to stop at first null char. Related
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setting: <a href="Print-Settings.html#set-print-null_002dstop">set print null-stop</a>.
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</p>
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</dd>
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<dt><code>-object [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing C<tt>++</tt> virtual function tables. Related setting:
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<a href="Print-Settings.html#set-print-object">set print object</a>.
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</p>
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</dd>
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<dt><code>-pretty [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set pretty formatting of structures. Related setting: <a href="Print-Settings.html#set-print-pretty">set print pretty</a>.
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</p>
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</dd>
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<dt><code>-raw-values [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set whether to print values in raw form, bypassing any
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pretty-printers for that value. Related setting: <a href="Print-Settings.html#set-print-raw_002dvalues">set print raw-values</a>.
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</p>
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</dd>
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<dt><code>-repeats <var>number-of-repeats</var>|<code>unlimited</code></code></dt>
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<dd><p>Set threshold for repeated print elements. <code>unlimited</code> causes
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all elements to be individually printed. Related setting: <a href="Print-Settings.html#set-print-repeats">set print repeats</a>.
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</p>
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</dd>
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<dt><code>-static-members [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing C<tt>++</tt> static members. Related setting: <a href="Print-Settings.html#set-print-static_002dmembers">set print static-members</a>.
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</p>
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</dd>
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<dt><code>-symbol [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing of symbol names when printing pointers. Related setting:
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<a href="Print-Settings.html#set-print-symbol">set print symbol</a>.
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</p>
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</dd>
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<dt><code>-union [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing of unions interior to structures. Related setting:
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<a href="Print-Settings.html#set-print-union">set print union</a>.
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</p>
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</dd>
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<dt><code>-vtbl [<code>on</code>|<code>off</code>]</code></dt>
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<dd><p>Set printing of C++ virtual function tables. Related setting:
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<a href="Print-Settings.html#set-print-vtbl">set print vtbl</a>.
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</p></dd>
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</dl>
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<p>Because the <code>print</code> command accepts arbitrary expressions which
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may look like options (including abbreviations), if you specify any
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command option, then you must use a double dash (<code>--</code>) to mark
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the end of option processing.
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</p>
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<p>For example, this prints the value of the <code>-p</code> expression:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) print -p
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</pre></div>
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<p>While this repeats the last value in the value history (see below)
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with the <code>-pretty</code> option in effect:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) print -p --
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</pre></div>
|
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<p>Here is an example including both on option and an expression:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) print -pretty -- *myptr
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$1 = {
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next = 0x0,
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flags = {
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sweet = 1,
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sour = 1
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},
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meat = 0x54 "Pork"
|
}
|
</pre></div>
|
|
</dd>
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<dt><code>print [<var>options</var>]</code></dt>
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<dt><code>print [<var>options</var>] /<var>f</var></code></dt>
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<dd><a name="index-reprint-the-last-value"></a>
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<p>If you omit <var>expr</var>, <small>GDB</small> displays the last value again (from the
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<em>value history</em>; see <a href="Value-History.html#Value-History">Value History</a>). This allows you to
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conveniently inspect the same value in an alternative format.
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</p></dd>
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</dl>
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<p>A more low-level way of examining data is with the <code>x</code> command.
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It examines data in memory at a specified address and prints it in a
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specified format. See <a href="Memory.html#Memory">Examining Memory</a>.
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</p>
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<p>If you are interested in information about types, or about how the
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fields of a struct or a class are declared, use the <code>ptype <var>exp</var></code>
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command rather than <code>print</code>. See <a href="Symbols.html#Symbols">Examining the Symbol
|
Table</a>.
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</p>
|
<a name="index-exploring-hierarchical-data-structures"></a>
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<a name="index-explore"></a>
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<p>Another way of examining values of expressions and type information is
|
through the Python extension command <code>explore</code> (available only if
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the <small>GDB</small> build is configured with <code>--with-python</code>). It
|
offers an interactive way to start at the highest level (or, the most
|
abstract level) of the data type of an expression (or, the data type
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itself) and explore all the way down to leaf scalar values/fields
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embedded in the higher level data types.
|
</p>
|
<dl compact="compact">
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<dt><code>explore <var>arg</var></code></dt>
|
<dd><p><var>arg</var> is either an expression (in the source language), or a type
|
visible in the current context of the program being debugged.
|
</p></dd>
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</dl>
|
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<p>The working of the <code>explore</code> command can be illustrated with an
|
example. If a data type <code>struct ComplexStruct</code> is defined in your
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C program as
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</p>
|
<div class="smallexample">
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<pre class="smallexample">struct SimpleStruct
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{
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int i;
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double d;
|
};
|
|
struct ComplexStruct
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{
|
struct SimpleStruct *ss_p;
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int arr[10];
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};
|
</pre></div>
|
|
<p>followed by variable declarations as
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">struct SimpleStruct ss = { 10, 1.11 };
|
struct ComplexStruct cs = { &ss, { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } };
|
</pre></div>
|
|
<p>then, the value of the variable <code>cs</code> can be explored using the
|
<code>explore</code> command as follows.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">(gdb) explore cs
|
The value of `cs' is a struct/class of type `struct ComplexStruct' with
|
the following fields:
|
|
ss_p = <Enter 0 to explore this field of type `struct SimpleStruct *'>
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arr = <Enter 1 to explore this field of type `int [10]'>
|
|
Enter the field number of choice:
|
</pre></div>
|
|
<p>Since the fields of <code>cs</code> are not scalar values, you are being
|
prompted to chose the field you want to explore. Let’s say you choose
|
the field <code>ss_p</code> by entering <code>0</code>. Then, since this field is a
|
pointer, you will be asked if it is pointing to a single value. From
|
the declaration of <code>cs</code> above, it is indeed pointing to a single
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value, hence you enter <code>y</code>. If you enter <code>n</code>, then you will
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be asked if it were pointing to an array of values, in which case this
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field will be explored as if it were an array.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">`cs.ss_p' is a pointer to a value of type `struct SimpleStruct'
|
Continue exploring it as a pointer to a single value [y/n]: y
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The value of `*(cs.ss_p)' is a struct/class of type `struct
|
SimpleStruct' with the following fields:
|
|
i = 10 .. (Value of type `int')
|
d = 1.1100000000000001 .. (Value of type `double')
|
|
Press enter to return to parent value:
|
</pre></div>
|
|
<p>If the field <code>arr</code> of <code>cs</code> was chosen for exploration by
|
entering <code>1</code> earlier, then since it is as array, you will be
|
prompted to enter the index of the element in the array that you want
|
to explore.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">`cs.arr' is an array of `int'.
|
Enter the index of the element you want to explore in `cs.arr': 5
|
|
`(cs.arr)[5]' is a scalar value of type `int'.
|
|
(cs.arr)[5] = 4
|
|
Press enter to return to parent value:
|
</pre></div>
|
|
<p>In general, at any stage of exploration, you can go deeper towards the
|
leaf values by responding to the prompts appropriately, or hit the
|
return key to return to the enclosing data structure (the <i>higher</i>
|
level data structure).
|
</p>
|
<p>Similar to exploring values, you can use the <code>explore</code> command to
|
explore types. Instead of specifying a value (which is typically a
|
variable name or an expression valid in the current context of the
|
program being debugged), you specify a type name. If you consider the
|
same example as above, your can explore the type
|
<code>struct ComplexStruct</code> by passing the argument
|
<code>struct ComplexStruct</code> to the <code>explore</code> command.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">(gdb) explore struct ComplexStruct
|
</pre></div>
|
|
<p>By responding to the prompts appropriately in the subsequent interactive
|
session, you can explore the type <code>struct ComplexStruct</code> in a
|
manner similar to how the value <code>cs</code> was explored in the above
|
example.
|
</p>
|
<p>The <code>explore</code> command also has two sub-commands,
|
<code>explore value</code> and <code>explore type</code>. The former sub-command is
|
a way to explicitly specify that value exploration of the argument is
|
being invoked, while the latter is a way to explicitly specify that type
|
exploration of the argument is being invoked.
|
</p>
|
<dl compact="compact">
|
<dt><code>explore value <var>expr</var></code></dt>
|
<dd><a name="index-explore-value"></a>
|
<p>This sub-command of <code>explore</code> explores the value of the
|
expression <var>expr</var> (if <var>expr</var> is an expression valid in the
|
current context of the program being debugged). The behavior of this
|
command is identical to that of the behavior of the <code>explore</code>
|
command being passed the argument <var>expr</var>.
|
</p>
|
</dd>
|
<dt><code>explore type <var>arg</var></code></dt>
|
<dd><a name="index-explore-type"></a>
|
<p>This sub-command of <code>explore</code> explores the type of <var>arg</var> (if
|
<var>arg</var> is a type visible in the current context of program being
|
debugged), or the type of the value/expression <var>arg</var> (if <var>arg</var>
|
is an expression valid in the current context of the program being
|
debugged). If <var>arg</var> is a type, then the behavior of this command is
|
identical to that of the <code>explore</code> command being passed the
|
argument <var>arg</var>. If <var>arg</var> is an expression, then the behavior of
|
this command will be identical to that of the <code>explore</code> command
|
being passed the type of <var>arg</var> as the argument.
|
</p></dd>
|
</dl>
|
|
<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="Expressions.html#Expressions" accesskey="1">Expressions</a>:</td><td> </td><td align="left" valign="top">Expressions
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Ambiguous-Expressions.html#Ambiguous-Expressions" accesskey="2">Ambiguous Expressions</a>:</td><td> </td><td align="left" valign="top">Ambiguous Expressions
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Variables.html#Variables" accesskey="3">Variables</a>:</td><td> </td><td align="left" valign="top">Program variables
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Arrays.html#Arrays" accesskey="4">Arrays</a>:</td><td> </td><td align="left" valign="top">Artificial arrays
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Output-Formats.html#Output-Formats" accesskey="5">Output Formats</a>:</td><td> </td><td align="left" valign="top">Output formats
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Memory.html#Memory" accesskey="6">Memory</a>:</td><td> </td><td align="left" valign="top">Examining memory
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Auto-Display.html#Auto-Display" accesskey="7">Auto Display</a>:</td><td> </td><td align="left" valign="top">Automatic display
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Print-Settings.html#Print-Settings" accesskey="8">Print Settings</a>:</td><td> </td><td align="left" valign="top">Print settings
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Pretty-Printing.html#Pretty-Printing" accesskey="9">Pretty Printing</a>:</td><td> </td><td align="left" valign="top">Python pretty printing
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Value-History.html#Value-History">Value History</a>:</td><td> </td><td align="left" valign="top">Value history
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>:</td><td> </td><td align="left" valign="top">Convenience variables
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Convenience-Funs.html#Convenience-Funs">Convenience Funs</a>:</td><td> </td><td align="left" valign="top">Convenience functions
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Registers.html#Registers">Registers</a>:</td><td> </td><td align="left" valign="top">Registers
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Floating-Point-Hardware.html#Floating-Point-Hardware">Floating Point Hardware</a>:</td><td> </td><td align="left" valign="top">Floating point hardware
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Vector-Unit.html#Vector-Unit">Vector Unit</a>:</td><td> </td><td align="left" valign="top">Vector Unit
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="OS-Information.html#OS-Information">OS Information</a>:</td><td> </td><td align="left" valign="top">Auxiliary data provided by operating system
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Memory-Region-Attributes.html#Memory-Region-Attributes">Memory Region Attributes</a>:</td><td> </td><td align="left" valign="top">Memory region attributes
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Dump_002fRestore-Files.html#Dump_002fRestore-Files">Dump/Restore Files</a>:</td><td> </td><td align="left" valign="top">Copy between memory and a file
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Core-File-Generation.html#Core-File-Generation">Core File Generation</a>:</td><td> </td><td align="left" valign="top">Cause a program dump its core
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Character-Sets.html#Character-Sets">Character Sets</a>:</td><td> </td><td align="left" valign="top">Debugging programs that use a different
|
character set than GDB does
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Caching-Target-Data.html#Caching-Target-Data">Caching Target Data</a>:</td><td> </td><td align="left" valign="top">Data caching for targets
|
</td></tr>
|
<tr><td align="left" valign="top">• <a href="Searching-Memory.html#Searching-Memory">Searching Memory</a>:</td><td> </td><td align="left" valign="top">Searching memory for a sequence of bytes
|
</td></tr>
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<tr><td align="left" valign="top">• <a href="Value-Sizes.html#Value-Sizes">Value Sizes</a>:</td><td> </td><td align="left" valign="top">Managing memory allocated for values
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