| <html lang="en"> |
| <head> |
| <title>Arrays - Debugging with GDB</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Debugging with GDB"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Data.html#Data" title="Data"> |
| <link rel="prev" href="Variables.html#Variables" title="Variables"> |
| <link rel="next" href="Output-Formats.html#Output-Formats" title="Output Formats"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, |
| 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 |
| Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software'' and ``Free Software Needs |
| Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,'' |
| and with the Back-Cover Texts as in (a) below. |
| |
| (a) The FSF's Back-Cover Text is: ``You are free to copy and modify |
| this GNU Manual. Buying copies from GNU Press supports the FSF in |
| developing GNU and promoting software freedom.''--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Arrays"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Output-Formats.html#Output-Formats">Output Formats</a>, |
| Previous: <a rel="previous" accesskey="p" href="Variables.html#Variables">Variables</a>, |
| Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">10.4 Artificial Arrays</h3> |
| |
| <p><a name="index-artificial-array-514"></a><a name="index-arrays-515"></a><a name="index-g_t_0040_0040_0040r_007b_002c-referencing-memory-as-an-array_007d-516"></a>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>You can do this by referring to a contiguous span of memory as an |
| <dfn>artificial array</dfn>, using the binary operator ‘<samp><span class="samp">@</span></samp>’. The left |
| operand of ‘<samp><span class="samp">@</span></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 |
| |
| <pre class="smallexample"> int *array = (int *) malloc (len * sizeof (int)); |
| </pre> |
| <p class="noindent">you can print the contents of <code>array</code> with |
| |
| <pre class="smallexample"> p *array@len |
| </pre> |
| <p>The left operand of ‘<samp><span class="samp">@</span></samp>’ must reside in memory. Array values made |
| with ‘<samp><span class="samp">@</span></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>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: |
| <pre class="smallexample"> (gdb) p/x (short[2])0x12345678 |
| $1 = {0x1234, 0x5678} |
| </pre> |
| <p>As a convenience, if you leave the array length out (as in |
| ‘<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 |
| 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>’: |
| <pre class="smallexample"> (gdb) p/x (short[])0x12345678 |
| $2 = {0x1234, 0x5678} |
| </pre> |
| <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 <RET>. 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: |
| |
| <pre class="smallexample"> set $i = 0 |
| p dtab[$i++]->fv |
| <RET> |
| <RET> |
| ... |
| </pre> |
| </body></html> |
| |