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POPassing C++ array to Fortran subroutine causes nan values to appear in result
 

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  1. POPassing C++ array to Fortran subroutine causes nan values to appear in result
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    <p>I'm trying to use Fortran code from a C++ application. Specifically, I am trying to interface with SLATEC's <a href="http://www.netlib.org/slatec/src/drc3jj.f" rel="nofollow" title="drc3jj.f source code">drc3jj.f</a>. However, the Fortran subroutine returns an array which size depends on the parameters passed to the function. </p> <p>If the size of the array is 1, then the C++ array that I print contains the appropriate value. However, if this size is greater than one, the C++ array contains NaNs where there should be output values. </p> <p>Below is the code I use. This merely links the Fortran subroutine to the C++ application.</p> <pre><code>#ifndef FORTRANLINKAGE_H #define FORTRANLINKAGE_H extern "C" { extern void drc3jj_(double*,double*,double*,double*,double*, double*,double [],int*,int*); } #endif // FORTRANLINKAGE_H </code></pre> <p>The meat is down here. We actually call the Fortran subroutine from C++ and print the output:</p> <pre><code>#include "fortranLinkage.h" #include &lt;iostream&gt; #include &lt;stdlib.h&gt; using namespace std; void wigner3j(double l2, double l3, double m2, double m3, double coeff []) { double l1min,l1max; int ierr,size(3); drc3jj_(&amp;l2,&amp;l3,&amp;m2,&amp;m3,&amp;l1min,&amp;l1max,coeff,&amp;size,&amp;ierr); cout &lt;&lt; "Min: " &lt;&lt; l1min &lt;&lt; "\t Max: " &lt;&lt; l1max &lt;&lt; "\t Err: " &lt;&lt; ierr &lt;&lt; endl; } int main(int argc, char const *argv[]) { int l1(atoi(argv[1])),l2(atoi(argv[2])),m2(atoi(argv[3])),m3(atoi(argv[4])); double coeff [3]; wigner3j(l1,l2,m2,m3,coeff); for (int i=0;i&lt;3;i++) { cout &lt;&lt; coeff[i] &lt;&lt; endl; } return 0; } </code></pre> <p>If we call the program with <code>./myProgram 2 8 2 8</code>, it properly outputs 1/sqrt(21). However, if we try <code>./myProgram 2 8 2 7</code>, where the size of the array is actually 2, we get this result:</p> <pre><code>Min: 9 Max: 10 Err: 0 -nan -nan 2.08175e-317 </code></pre> <p>The NaNs actually have the proper sign. </p> <p>Anyway, is there another (proper) way to pass C++ arrays to Fortran? Is this even the issue? </p>
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    1. USJoey Dumont
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      1. POPassing C++ array to Fortran subroutine causes nan values to appear in result
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    1. COlooks like you need to pass as pointer &coeff. dont the docs for the fortranlinkage module address this?
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    2. CO@george I tried, compiler says: `test.cpp:13:58: error: cannot convert ‘double**’ to ‘double*’ for argument ‘7’`. When I try to pass it as `&coeff[0]`, I get the same error as before. There is no module, the code I posted is the whole code I use.
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      1. USJoey Dumont
    3. COfix double [] to double * in the template..
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