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| author | Chris Cannam |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.3/doc/html/Dynamic-Arrays-in-C_002dThe-Wrong-Way.html Wed Mar 20 15:35:50 2013 +0000 @@ -0,0 +1,93 @@ +<html lang="en"> +<head> +<title>Dynamic Arrays in C-The Wrong Way - FFTW 3.3.3</title> +<meta http-equiv="Content-Type" content="text/html"> +<meta name="description" content="FFTW 3.3.3"> +<meta name="generator" content="makeinfo 4.13"> +<link title="Top" rel="start" href="index.html#Top"> +<link rel="up" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" title="Multi-dimensional Array Format"> +<link rel="prev" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" title="Dynamic Arrays in C"> +<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> +<!-- +This manual is for FFTW +(version 3.3.3, 25 November 2012). + +Copyright (C) 2003 Matteo Frigo. + +Copyright (C) 2003 Massachusetts Institute of Technology. + + Permission is granted to make and distribute verbatim copies of + this manual provided the copyright notice and this permission + notice are preserved on all copies. + + Permission is granted to copy and distribute modified versions of + this manual under the conditions for verbatim copying, provided + that the entire resulting derived work is distributed under the + terms of a permission notice identical to this one. + + Permission is granted to copy and distribute translations of this + manual into another language, under the above conditions for + modified versions, except that this permission notice may be + stated in a translation approved by the Free Software Foundation. + --> +<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> +</head> +<body> +<div class="node"> +<a name="Dynamic-Arrays-in-C-The-Wrong-Way"></a> +<a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a> +<p> +Previous: <a rel="previous" accesskey="p" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C">Dynamic Arrays in C</a>, +Up: <a rel="up" accesskey="u" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a> +<hr> +</div> + +<h4 class="subsection">3.2.5 Dynamic Arrays in C—The Wrong Way</h4> + +<p>A different method for allocating multi-dimensional arrays in C is +often suggested that is incompatible with FFTW: <em>using it will +cause FFTW to die a painful death</em>. We discuss the technique here, +however, because it is so commonly known and used. This method is to +create arrays of pointers of arrays of pointers of <small class="dots">...</small>etcetera. +For example, the analogue in this method to the example above is: + +<pre class="example"> int i,j; + fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */ + + a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **)); + for (i = 0; i < 5; ++i) { + a_bad_array[i] = + (fftw_complex **) malloc(12 * sizeof(fftw_complex *)); + for (j = 0; j < 12; ++j) + a_bad_array[i][j] = + (fftw_complex *) malloc(27 * sizeof(fftw_complex)); + } +</pre> + <p>As you can see, this sort of array is inconvenient to allocate (and +deallocate). On the other hand, it has the advantage that the +(i,j,k)-th element can be referenced simply by +<code>a_bad_array[i][j][k]</code>. + + <p>If you like this technique and want to maximize convenience in accessing +the array, but still want to pass the array to FFTW, you can use a +hybrid method. Allocate the array as one contiguous block, but also +declare an array of arrays of pointers that point to appropriate places +in the block. That sort of trick is beyond the scope of this +documentation; for more information on multi-dimensional arrays in C, +see the <code>comp.lang.c</code> +<a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>. + +<!-- --> + </body></html> +