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date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>Allocating aligned memory in Fortran - FFTW 3.3.3</title>
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49 <a name="Allocating-aligned-memory-in-Fortran"></a>
50 <p>
51 Next:&nbsp;<a rel="next" accesskey="n" href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran">Accessing the wisdom API from Fortran</a>,
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56
57 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
58
59 <p><a name="index-alignment-560"></a><a name="index-fftw_005falloc_005freal-561"></a><a name="index-fftw_005falloc_005fcomplex-562"></a>In order to obtain maximum performance in FFTW, you should store your
60 data in arrays that have been specially aligned in memory (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Enforcing alignment also permits you to
61 safely use the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>) to apply a given plan to more than one pair of in/out
62 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
63 any alignment guarantees. The <em>only</em> way to allocate aligned
64 memory in standard Fortran is to allocate it with an external C
65 function, like the <code>fftw_alloc_real</code> and
66 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
67 a simple way to associate such allocated memory with a standard Fortran
68 array pointer that you can then use normally.
69
70 <p>We therefore recommend allocating all your input/output arrays using
71 the following technique:
72
73 <ol type=1 start=1>
74
75 <li>Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
76 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
77 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
78 a(:,:,:)</code> for a 3d complex array.
79
80 <li>The number of elements to allocate must be an
81 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
82 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
83 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
84 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
85 <code>int(L * M * N, C_SIZE_T)</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N array.
86
87 <li>Declare a <code>type(C_PTR) :: p</code> to hold the return value from
88 FFTW's allocation routine. Set <code>p = fftw_alloc_real(sz)</code> for a real array, or <code>p = fftw_alloc_complex(sz)</code> for a complex array.
89
90 <li><a name="index-c_005ff_005fpointer-563"></a>Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
91 using the standard <code>c_f_pointer</code> subroutine: <code>call
92 c_f_pointer(p, arr, [...dimensions...])</code>, where
93 <code>[...dimensions...])</code> are an array of the dimensions of the array
94 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
95 [L,M,N])</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N array. (Alternatively, you can
96 omit the dimensions argument if you specified the shape explicitly
97 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
98 multidimensional array.
99
100 <li>When you are done using the array, deallocate the memory by <code>call
101 fftw_free(p)</code> on <code>p</code>.
102
103 </ol>
104
105 <p>For example, here is how we would allocate an L&nbsp;&times;&nbsp;M 2d real array:
106
107 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
108 type(C_PTR) :: p
109 p = fftw_alloc_real(int(L * M, C_SIZE_T))
110 call c_f_pointer(p, arr, [L,M])
111 <em>...use arr and arr(i,j) as usual...</em>
112 call fftw_free(p)
113 </pre>
114 <p>and here is an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N 3d complex array:
115
116 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
117 type(C_PTR) :: p
118 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
119 call c_f_pointer(p, arr, [L,M,N])
120 <em>...use arr and arr(i,j,k) as usual...</em>
121 call fftw_free(p)
122 </pre>
123 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
124 single array and associating both real and complex array pointers with
125 it, for in-place real-to-complex transforms.
126
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