Accessing struct pointer after passing struct by reference to a function

Question

I have this code, however I am unsure of how to access the struct pointers after passing the struct by reference into a function, the program crashes on this line, accessing the pointers doesnt work.

    scanf("(%lf,%lf)",polygon->xvals[i],polygon->yvals[i]);

FIXED CODE, thanks to all who answered

    struct Polygon{
    double *xvals, *yvals;
    int numverts;
    };
    typedef struct Polygon pol;
    pol getpoly(pol *polygon);


    int main(){
     pol polygon;
     getpoly(&polygon);  
            }


    pol getpoly(pol *polygon){
      polygon->xvals = (double * )malloc(sizeof(double)*polygon->numverts);
      polygon->yvals = (double * )malloc(sizeof(double)*polygon->numverts); 

      check=0;
      int i;

      for(i=0;i<10;i++){

       while(check !=2 ){
        cout<<"enter vertices "<<i<<" (x,y)\n";
        check = scanf("(%lf,%lf)",&polygon->xvals[i],&polygon->yvals[i]);
        _flushall();
       }
      check=0; 
      }
      polygon->xvals[polygon->numverts-1] = polygon->xvals[0];
      polygon->yvals[polygon->numverts-1] = polygon->yvals[0]; 

    return *polygon;    
    }

apologies, i didnt explain what was actually going wrong, the program fails where I try to put stuff into the pointers — Eoin Murray
– Eoin Murray, Commented May 15, 2012 at 9:25
@n.m. There are no references in the code either; it looks like valid C to me (apart from the missing return in main of course). — Mike Seymour
– Mike Seymour, Commented May 15, 2012 at 9:29

hmjd · Accepted Answer · 2012-05-15 09:30:25Z

3

No memory has been allocated for xvals and yvals, they are uninitialsed pointers. numverts is also unintialised. You need to malloc() space for xvals and yvals and initialise numverts:

polygon->numverts = 10;
polygon->xvals = malloc(polygon->numverts * sizeof(double));
polygon->yvals = malloc(polygon->numverts * sizeof(double));

and prevent going beyond the end of these arrays, as this code does:

polygon->xvals[polygon->numverts] = polygon->xvals[0];
polygon->yvals[polygon->numverts] = polygon->yvals[0];

should be:

polygon->xvals[polygon->numverts - 1] = polygon->xvals[0];
polygon->yvals[polygon->numverts - 1] = polygon->yvals[0];

Remember to free() xvals and yvals when no longer required.

edited May 15, 2012 at 9:30

answered May 15, 2012 at 9:25

hmjd

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