108 lines
5.4 KiB
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108 lines
5.4 KiB
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<title>Arrays - Debugging with GDB</title>
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Data.html#Data" title="Data">
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<link rel="prev" href="Variables.html#Variables" title="Variables">
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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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<body>
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<div class="node">
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<a name="Arrays"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Output-Formats.html#Output-Formats">Output Formats</a>,
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Previous: <a rel="previous" accesskey="p" href="Variables.html#Variables">Variables</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">10.4 Artificial Arrays</h3>
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<p><a name="index-artificial-array-646"></a><a name="index-arrays-647"></a><a name="index-g_t_0040_0040_0040r_007b_002c-referencing-memory-as-an-array_007d-648"></a>It is often useful to print out several successive objects of the
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same type in memory; a section of an array, or an array of
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dynamically determined size for which only a pointer exists in the
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program.
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<p>You can do this by referring to a contiguous span of memory as an
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<dfn>artificial array</dfn>, using the binary operator ‘<samp><span class="samp">@</span></samp>’. The left
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operand of ‘<samp><span class="samp">@</span></samp>’ should be the first element of the desired array
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and be an individual object. The right operand should be the desired length
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of the array. The result is an array value whose elements are all of
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the type of the left argument. The first element is actually the left
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argument; the second element comes from bytes of memory immediately
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following those that hold the first element, and so on. Here is an
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example. If a program says
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<pre class="smallexample"> int *array = (int *) malloc (len * sizeof (int));
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</pre>
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<p class="noindent">you can print the contents of <code>array</code> with
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<pre class="smallexample"> p *array@len
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</pre>
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<p>The left operand of ‘<samp><span class="samp">@</span></samp>’ must reside in memory. Array values made
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with ‘<samp><span class="samp">@</span></samp>’ in this way behave just like other arrays in terms of
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subscripting, and are coerced to pointers when used in expressions.
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Artificial arrays most often appear in expressions via the value history
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(see <a href="Value-History.html#Value-History">Value History</a>), after printing one out.
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<p>Another way to create an artificial array is to use a cast.
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This re-interprets a value as if it were an array.
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The value need not be in memory:
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<pre class="smallexample"> (gdb) p/x (short[2])0x12345678
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$1 = {0x1234, 0x5678}
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</pre>
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<p>As a convenience, if you leave the array length out (as in
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‘<samp><span class="samp">(</span><var>type</var><span class="samp">[])</span><var>value</var></samp>’) <span class="sc">gdb</span> calculates the size to fill
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the value (as ‘<samp><span class="samp">sizeof(</span><var>value</var><span class="samp">)/sizeof(</span><var>type</var><span class="samp">)</span></samp>’:
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<pre class="smallexample"> (gdb) p/x (short[])0x12345678
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$2 = {0x1234, 0x5678}
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</pre>
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<p>Sometimes the artificial array mechanism is not quite enough; in
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moderately complex data structures, the elements of interest may not
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actually be adjacent—for example, if you are interested in the values
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of pointers in an array. One useful work-around in this situation is
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to use a convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Variables</a>) as a counter in an expression that prints the first
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interesting value, and then repeat that expression via <RET>. For
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instance, suppose you have an array <code>dtab</code> of pointers to
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structures, and you are interested in the values of a field <code>fv</code>
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in each structure. Here is an example of what you might type:
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<pre class="smallexample"> set $i = 0
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p dtab[$i++]->fv
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<RET>
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<RET>
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...
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</pre>
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</body></html>
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