Question

I have filled a dynamic allocated float multi array in a function.

A second function has to get the values of the array exploiting the pointer to the first element of the array defined in the former function.

The second function do not access to the correct memory location so it doesn’t work but it does if the multy array is defined in a static way.

Does somebody know why?

eval_cell should get values defined in div_int

float f_imp(float x, float y){
     return pow(x,2)+pow(y,2)-1;
}


int eval_cell(float* p){
    int s[4];
    s[0] = f_imp(*p, *(p+1)) <= 0;
    printf("%f %f\n",*p, *(p+1));

    s[1] = f_imp(*(p+3), *(p+4)) <= 0;
    printf("%f %f\n",*(p+3), *(p+4));

    s[2] = f_imp(*(p+9), *(p+10)) <= 0;
    printf("%f %f\n",*(p+9), *(p+10));

    s[3] = f_imp(*(p+6), *(p+7))  <= 0;
    printf("%f %f\n",*(p+6), *(p+7));

    printf("%d%d%d%d\n",s[0],s[1],s[2],s[3]);
    return s[0];
}

void div_int(float* x1, float* y1,float* x2,float* y2,
             float* f0, float* f2,float* f6,float* f8){
    int i,j,m;
    float* p;
    float** a_cell; // array 9x3 contente coordinate vertici e valore funzione
    *a_cell = (float**) malloc(9*sizeof(float*));
    for (i=0;i<9;i++){
       a_cell[i] = (float*) malloc(3*sizeof(float));
    }
    a_cell[0][0] = *x1;
    a_cell[0][1] = *y1;
    a_cell[0][2] = *f0;
    a_cell[2][0] = *x2;
    a_cell[2][1] = *y1;
    a_cell[2][2] = *f2;
    a_cell[6][0] = *x1;
    a_cell[6][1] = *y2;
    a_cell[6][2] = *f6;
    a_cell[8][0] = *x2;
    a_cell[8][1] = *y2;
    a_cell[8][2] = *f8;
/***   calcolo dei valori incogniti di a_cell            ***/
    a_cell[1][0] = (*x1+*x2)/2;
    a_cell[1][1] = *y1;
    a_cell[1][2] = f_imp(a_cell[1][0], a_cell[1][1]);
    a_cell[3][0] = *x1;
    a_cell[3][1] = (*y1+*y2)/2;
    a_cell[3][2] = f_imp(a_cell[3][0], a_cell[3][1]);;
    a_cell[4][0] = (*x2+*x1)/2;
    a_cell[4][1] = (*y2+*y1)/2;
    a_cell[4][2] = f_imp(a_cell[4][0], a_cell[4][1]);
    a_cell[5][0] = *x2;
    a_cell[5][1] = (*y2+*y1)/2;
    a_cell[5][2] = f_imp(a_cell[5][0], a_cell[5][1]);
    a_cell[7][0] = (*x1+*x2)/2;
    a_cell[7][1] = *y2;
    a_cell[7][2] = f_imp(a_cell[7][0], a_cell[7][1]);

    for (j=0;j<2;j++){
       m = j*3;
       for(i=0;i<2;i++){
       m += i;
       eval_cell(&a_cell[m][0]);
       }
    }
    p = *a_cell;
    for (i=0;i<9;i++){
        for (j=0;j<3;j++){
          printf("%f \n",*(p+3*i+j));
          printf("%f \n",a_cell[i][j]);
      printf("\n");
       }
    }
   free(a_cell);
   return;
}

Answer 1

It’s because you using pointer in incorrect way:
See a_cell is pointer to dynamic array of 9 pointers to dynamic array of 3 floats.
So when you do eval_cell(&a_cell[m][0]) (or just eval_cell(a_cell[m]) this is actually the same) you actually get pointer to array of 3 floats. And after that you do:

int eval_cell(float* p){

...
s[2] = f_imp(*(p+9), *(p+10)) <= 0;

*(p+9) will get 9th element in array of 3 floats, so this is incorrect.

It works in static way, because static multi dimension array in memory is just one dimension array for which you was given multi indexing (by compiler). That’s why in static you will probably address valid memory area.

See picture for more explanation: