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| <title>Debugging with GDB: Arrays</title> |
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| <a name="Artificial-Arrays"></a> |
| <h3 class="section">10.4 Artificial Arrays</h3> |
| |
| <a name="index-artificial-array"></a> |
| <a name="index-arrays"></a> |
| <a name="index-_0040_002c-referencing-memory-as-an-array"></a> |
| <p>It is often useful to print out several successive objects of the |
| same type in memory; a section of an array, or an array of |
| dynamically determined size for which only a pointer exists in the |
| program. |
| </p> |
| <p>You can do this by referring to a contiguous span of memory as an |
| <em>artificial array</em>, using the binary operator ‘<samp>@</samp>’. The left |
| operand of ‘<samp>@</samp>’ should be the first element of the desired array |
| and be an individual object. The right operand should be the desired length |
| of the array. The result is an array value whose elements are all of |
| the type of the left argument. The first element is actually the left |
| argument; the second element comes from bytes of memory immediately |
| following those that hold the first element, and so on. Here is an |
| example. If a program says |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int *array = (int *) malloc (len * sizeof (int)); |
| </pre></div> |
| |
| <p>you can print the contents of <code>array</code> with |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">p *array@len |
| </pre></div> |
| |
| <p>The left operand of ‘<samp>@</samp>’ must reside in memory. Array values made |
| with ‘<samp>@</samp>’ in this way behave just like other arrays in terms of |
| subscripting, and are coerced to pointers when used in expressions. |
| Artificial arrays most often appear in expressions via the value history |
| (see <a href="Value-History.html#Value-History">Value History</a>), after printing one out. |
| </p> |
| <p>Another way to create an artificial array is to use a cast. |
| This re-interprets a value as if it were an array. |
| The value need not be in memory: |
| </p><div class="smallexample"> |
| <pre class="smallexample">(gdb) p/x (short[2])0x12345678 |
| $1 = {0x1234, 0x5678} |
| </pre></div> |
| |
| <p>As a convenience, if you leave the array length out (as in |
| ‘<samp>(<var>type</var>[])<var>value</var></samp>’) <small>GDB</small> calculates the size to fill |
| the value (as ‘<samp>sizeof(<var>value</var>)/sizeof(<var>type</var>)</samp>’: |
| </p><div class="smallexample"> |
| <pre class="smallexample">(gdb) p/x (short[])0x12345678 |
| $2 = {0x1234, 0x5678} |
| </pre></div> |
| |
| <p>Sometimes the artificial array mechanism is not quite enough; in |
| moderately complex data structures, the elements of interest may not |
| actually be adjacent—for example, if you are interested in the values |
| of pointers in an array. One useful work-around in this situation is |
| 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 |
| interesting value, and then repeat that expression via <tt class="key">RET</tt>. For |
| instance, suppose you have an array <code>dtab</code> of pointers to |
| structures, and you are interested in the values of a field <code>fv</code> |
| in each structure. Here is an example of what you might type: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">set $i = 0 |
| p dtab[$i++]->fv |
| <span class="key">RET</span> |
| <span class="key">RET</span> |
| … |
| </pre></div> |
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