Why does this LibraryLink function crash?

Question

Recently, I have rewritten pure Wolfram Language implementation CAGDBSplineSurface[] with C. Below is main LibraryLink code:

LibraryLink Code

#include "WolframLibrary.h"

/************************************************************************/
/*            BSplineSurface(P,U,V,p,q,u,v)                             */
/************************************************************************/ 
DLLEXPORT int BSplineSurface(WolframLibraryData libData, mint Argc, MArgument *Args, MArgument Res) {
    /*define the argument-variable*/
    MTensor tensor_P, tensor_U, tensor_V;
    mreal *P, *U, *V;
    mint p, q;
    mreal u, v;
    /*define the program-variable*/
    MTensor tensor_S;
    mreal *S;
    mint k;
    mint dims[1];
    int err;
    mint const *P_dims;
    mint c;
    /*assign the argument-value to argument-variable*/
    tensor_P = MArgument_getMTensor(Args[0]);
    P = libData->MTensor_getRealData(tensor_P);
    tensor_U = MArgument_getMTensor(Args[1]);
    U = libData->MTensor_getRealData(tensor_U);
    tensor_V = MArgument_getMTensor(Args[2]);
    V = libData->MTensor_getRealData(tensor_V);
    p = MArgument_getInteger(Args[3]);
    q = MArgument_getInteger(Args[4]);
    u = MArgument_getReal(Args[5]);
    v = MArgument_getReal(Args[6]);
    /*achieve the dimensions of the tensor*/
    P_dims = libData->MTensor_getDimensions(tensor_P);
    c = P_dims[2];
    /*build the tensor*/
    dims[0] = c;
    err = libData->MTensor_new(MType_Real, 1, dims, &tensor_S);
    S = libData->MTensor_getRealData(tensor_S);
    /*initialize the S vector*/
    for (k = 0; k <= c - 1; k++){
        S[k] = 0.0;
    }
    /*main implementation*/
    calc_bspline_surf(P, U, V, p, q, u, v, P_dims, S);
    /*return the result*/
    MArgument_setMTensor(Res, tensor_S);
    return LIBRARY_NO_ERROR; 
}

However, when I compiled it to dll file, it always caused Mathematica crash.

---

Maybe someone would say this is just a C probelm, however, this time I have ran it in C and calc_bspline_surf() could return right result. I think it is a LibraryLink wrapper related question.

pts =
  {{{1, 1, 0.724003}, {1, 2, 0.583991}, {1, 3, 0.76682}, {1, 4, -0.739215}, {1, 5, 0.665493}}, 
  {{2, 1, -0.769059}, {2, 2, 0.549204}, {2, 3, 0.815537}, {2, 4, 0.512306}, {2, 5, 0.200062}}, 
  {{3, 1, -0.0984508}, {3, 2, 0.123631}, {3, 3, -0.938963}, {3, 4, 0.579558}, {3, 5, 0.612359}}, 
  {{4, 1, 0.377609}, {4, 2, 0.454769}, {4, 3, 0.87932}, {4, 4, -0.502242}, {4, 5, -0.603295}}, 
  {{5, 1, -0.645871}, {5, 2, -0.398243}, {5, 3, -0.510713}, {5, 4, -0.144209}, {5, 5, 0.145951}},
  {{6, 1, -0.334002}, {6, 2, -0.435887}, {6, 3, 0.182128}, {6, 4, 0.0293587}, {6, 5, 0.0583313}}};

f = BSplineFunction[pts];

Attachment

You could download the notebook via this link(thanks for happy fish help), and that Mathematica notebook contains the complete librarylink code.

Answer 1

This is a incompatible pointer problem. Just change int const *dims to mint const *dims (a total of 2 places), and the function will give correct results.

My attempt to explain the difference:

(Since I don't use C, I may make mistakes. Feel free to edit, or if there are any serious mistakes, I will remove this paragraph)

mint refers to long in my system (maybe different on different system according to the typedef in WolframLibrary.h). A long integer occupies 32 bits while int occupies 16 bits (usually). A pointer originally a mint * would regard its address and its following 32 bits as the number. If it is forced to be converted to int *, the same address and its following 16 bits will be the number. This will cause a difference in results.