Core dumped trying to create newVector(array 2D) in a struct

Question

So I'm trying to create a simple array 2D inside a struct, and to read the values of some coordinates that are in a file.

At this moment I've got all inside functions and i get a segmentation fault when i try to get the coordinates on the 'get_coords' function. If I write all the exactly same code on a 'int main' instead of using functions it works.

code:

            #include <stdio.h>
            #include <stdlib.h>

            // structure
            typedef struct coordinates{
                double **a;
            } coord;

            // counts the nmbr of points 
            int count(){
                int i;
                int a1, a2, a3;
                FILE *fp;
                fp = fopen("abc.txt", "r+");
                while (fscanf(fp, "%d %d %d", &a1, &a2, &a3) != EOF){
                    i++;
                    if (feof(fp)){
                        break;          
                    }   
                }
                fclose(fp);
                return(i);
            }

            // creates new structure with the right size of memory allocated
            coord *newVector(size_t s){
                coord *v;
                int j;
                v = malloc(sizeof(coord));

                v->a = malloc(sizeof(double*)*3);

                for (j=0; j<3; j++){
                    v->a[j] = malloc(sizeof(double)*s);
                }

            }

            void get_coords(coord *points){
                int i=0;
                FILE *fp;
                fp = fopen("abc.txt", "r+");
                while (fscanf(fp, "%le %le %le", &points->a[i][0], &points->a[i][1], &points->a[i][2]) != EOF){
                    i++;
                }
                fclose(fp);
            }


            int main(){
                int i = 0, j=0;

                coord *points;

                i = count();

                points = newVector(i);

                get_coords(points);

                for (i=0; i<3; i++){
                        printf("%lf %lf %lf\n", points->a[i][0], points->a[i][1], points->a[i][2]);
                }
            }

abc.txt:

1 2 3
4 5 6
7 8 9

Thanks all for your help.

Cumps, Dylan.

PS: This is just a prototype for what i want.

Answer 1

In count() , you are incrementing an uninitialized variable, invoking Undefined Behavior (UB).

Change this:

int count() {
  int i;

to this:

int count() {
  int i = 0;

---

newVector() is not returning the dynamic allocated memory.

Change this:

coord *newVector(size_t s) {
  ...
}

to this:

coord *newVector(size_t s) {
  ...
  return v;
}

---

After fixing these issues, you should see this output:

1.000000 2.000000 3.000000
4.000000 5.000000 6.000000
7.000000 8.000000 9.000000

---

Not the problem, but I would use %lf as the format specifiers in fscanf() , instead of %le .

Moreover, in count() , this return(i); is the same as this return i; . The parentheses are redundant.

TODO: Free the dynamically allocated memory (I assume that you skipped that part for providing the MCVE).

---

Pro-tip: Compile with warning flags enabled next time. They would have already found the issue for you in this case. In GCC for example, I would get this:

gsamaras@myPc:~$ gcc -Wall  main.c
main.c: In function ‘main’:
main.c:51:28: warning: unused variable ‘j’ [-Wunused-variable]
                 int i = 0, j=0;
                            ^
main.c: In function ‘newVector’:
main.c:37:13: warning: control reaches end of non-void function [-Wreturn-type]
             }
             ^

where the last warning is the second point in my answer.

Answer 2

You forgot to return v in newVector function:

        // creates new structure with the right size of memory allocated
        coord *newVector(size_t s){
            coord *v;
            int j;
            v = malloc(sizeof(coord));

            v->a = malloc(sizeof(double*)*3);

            for (j=0; j<3; j++){
                v->a[j] = malloc(sizeof(double)*s);
            }

            return v;

        }