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POPassing 4D C Array to FORTRAN Function
 

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  1. POPassing 4D C Array to FORTRAN Function
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    <p>I'm attempting to pass a 4D array, initialized in C, to a FORTRAN function. Following the advise from a previous <a href="https://stackoverflow.com/questions/18881139/passing-multidimensional-c-array-to-fortran-function-for-modification">question</a>, I first allocate the data array as contiguous data with a single malloc, and then allocate pointers to pointers, making them ultimately point inside the data array. This method works for a 3D array. However, when I go to a 4D array I don't get the values expected in FORTRAN. Does something need to change when I malloc the memory for C arrays > 3 dimensions or is there a bug in the alloc_4D_double function?</p> <p>Code Excerpt:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; double f_function(double ****); double ****alloc_4D_double(int wlen, int xlen, int ylen, int zlen, double *ary) { int i,j,k; double ****multiDary = (double****)malloc(wlen*sizeof(double***)); for (i = 0; i &lt; wlen; i++) { multiDary[i] = (double***)malloc(xlen*sizeof(double**)); for (j = 0; j &lt; xlen; j++) { multiDary[i][j] = (double**)malloc(ylen*sizeof(double*)); for (k = 0; k &lt; ylen; k++) { multiDary[i][j][k] = (double*)malloc(zlen*sizeof(double)); } } } for (i = 0; i &lt; wlen; i++) { for (j = 0; j &lt; xlen; j++) { for (k = 0; k &lt; ylen; k++) { multiDary[i][j][k] = &amp;ary[zlen*(k + ylen*(j + xlen*i))]; } } } return multiDary; } int main ( void ) { // create contiguous memory double *data1D = (double*)malloc(2 * 4 * 10 * 10 * sizeof(double)); // map memory locations to allow use of array syntax double ****exposure4D = alloc_4D_double(2, 4, 10, 10, exposure1D); // assign a value data[0][1][4][4] = 0.777; // pass the pointer to FORTRAN f_funtion(data1D) return 0; } </code></pre> <p>and</p> <pre><code>integer(4) function f_function(data) BIND(C) use, intrinsic :: iso_c_binding implicit none ! real(C_DOUBLE) , intent(in), dimension(2,4,10,10) :: data // this value does not evaluate to 0.7777 write(*,*), data(1,2,5,5) // this value does evaluate to 0.7777 write(*,*), data(1,1,9,2) end function f_function </code></pre>
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    1. USCommunity
    1. USjavacavaj
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    1. COIs the difference due to C using row-major and Fortran using column-major storage? You might just have gotten lucky with the 3D array, perhaps because all the dimensions were the same.
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      1. POPassing 4D C Array to FORTRAN Function
      1. USJonathan Leffler
    2. COThat's very clever. It looks ok to me. Maybe @JonathanLeffler is right?
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      1. POPassing 4D C Array to FORTRAN Function
      1. USCharlie Burns
    3. COA little index arithmetic suggests that @JonathanLeffler is right, that element `[0,1,4,4]` in an indexed-from-0 row-major array corresponds to element `[1,1,9,2]` in an indexed-from-1 column-major array.
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      1. POPassing 4D C Array to FORTRAN Function
      1. USHigh Performance Mark
 

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