setting elements of a structure to random double values using an array of pointers to the structure

Question

I am trying to make an array of pointers to a structure the program runs, however it just says 'Segmentation fault: 11' any help would be be very much appreciated

typedef struct
{
  int id;
  double x;
  double y;
  char name[30];
  int named;
} Star;


Star *stars[10];


int bang (int n_stars)
{
  if ((n_stars >= 1) && (n_stars <= 10))
{
    for (int i = 0; i < n_stars; i++)
    {
      double x = rand() % 10;
      double y = rand() % 10;
      stars[i] = (Star*) malloc(sizeof(x) + sizeof(y));
      stars[i]->x = x;
      stars[i]->y = y;
      free(stars[i]);
      return 1;
    }
}
else
{
    return 0;
}
return 0;
}

Answer 1

For the sake of the argument, let us disregard struct alignment and assume a target (such as x86) where:

sizeof(double) == 8 , and sizeof(int) == 4 .

Now, you're allocating a block of memory 16 bytes in size:

stars[i] = (Star*) malloc(sizeof(x) + sizeof(y));

The definition of the struct is:

typedef struct
{
  int id;
  double x;
  double y;
  char name[30];
  int named;
} Star;

Note the offset of y is 12 bytes. When you perform:

stars[i]->y = y;

The value of y is written to the address &stars[i] + 12 bytes. Double taking up 8 bytes, this will overflow your allocation (12+8 > 16), mentioned above.

You most likely want to change the allication to stars[i] = malloc(sizeof(Star)); , to allocate the proper amount of space for your struct.

EDIT: If you are planning to use the allocated data elsewhere, you should remove the call to free , and instead free the data after you are done with it in your program. The return statement on the following line may possibly be a mistake too, as it exits the loop on the first iteration, which is most likely not what you want.