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| author | Chris Cannam <cannam@all-day-breakfast.com> |
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| date | Tue, 18 Oct 2016 13:40:26 +0100 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.5/doc/html/Dynamic-Arrays-in-C_002dThe-Wrong-Way.html Tue Oct 18 13:40:26 2016 +0100 @@ -0,0 +1,120 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This manual is for FFTW +(version 3.3.5, 30 July 2016). + +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. --> +<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> +<head> +<title>FFTW 3.3.5: Dynamic Arrays in C-The Wrong Way</title> + +<meta name="description" content="FFTW 3.3.5: Dynamic Arrays in C-The Wrong Way"> +<meta name="keywords" content="FFTW 3.3.5: Dynamic Arrays in C-The Wrong Way"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format"> +<link href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" rel="next" title="Words of Wisdom-Saving Plans"> +<link href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" rel="prev" title="Dynamic Arrays in C"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a> +<div class="header"> +<p> +Previous: <a href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" accesskey="p" rel="prev">Dynamic Arrays in C</a>, Up: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" accesskey="u" rel="up">Multi-dimensional Array Format</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> +</div> +<hr> +<a name="Dynamic-Arrays-in-C_002d_002d_002dThe-Wrong-Way"></a> +<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 …etcetera. +For example, the analogue in this method to the example above is: +</p> +<div class="example"> +<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></div> + +<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 +<em>(i,j,k)</em>-th element can be referenced simply by +<code>a_bad_array[i][j][k]</code>. +</p> +<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>. +</p> + + + +</body> +</html>