Using qsort() inside of struct with array of structs does not access the right elements but 0

Question

I have the following 2 structs:

typedef struct {
  char fullName[40];
  int yearOfBirth;
} Ancestor;

typedef struct {
  Ancestor **ancestor;
  int capacity;
  int size;
} AncestorList;

and I would like to sort the Ancestor elements os the array by yearOfBirth. Here is how I call qsort():

qsort(listOfGreatUncles->ancestor, listOfGreatUncles->size, sizeof(Ancestor), compare); //listOfGreatUncles is a AncestorList struct

and here is my compare procedure:

int compare(const void *s1, const void *s2) {
  Ancestor *a1 = (Ancestor *)s1;
  Ancestor *a2 = (Ancestor *)s2;

  printf("a1 yearOfBirth %d\n", a1->yearOfBirth);
  printf("a2 yearOfBirth %d\n", a2->yearOfBirth);

  return (a1->yearOfBirth - a2->yearOfBirth);
  }
}

Now my output for a1 and a2 are 0. What am I doing wrong?

Antti Haapala · Accepted Answer · 2018-12-24 05:58:23Z

3

The elements in the array are of type Ancestor * and that's what you should use as the operand to sizeof. The pointers given to the comparison function are pointers to the element type cast to void *, hence you cast them back to Ancestor ** and dereference.

qsort(listOfGreatUncles->ancestor, listOfGreatUncles->size, sizeof (Ancestor *), compare);

or the form that always gives the right size if the array itself is properly typed:

qsort(listOfGreatUncles->ancestor, 
      listOfGreatUncles->size, 
      sizeof listOfGreatUncles->ancestor[0],
      compare);

i.e.

qsort(array, length, sizeof array[0], compfunc);

And finally

int compare(const void *s1, const void *s2) {
    Ancestor *a1 = *(Ancestor **)s1;
    Ancestor *a2 = *(Ancestor **)s2;

    printf("a1 yearOfBirth %d\n", a1->yearOfBirth);
    printf("a2 yearOfBirth %d\n", a2->yearOfBirth);

    return (a1->yearOfBirth - a2->yearOfBirth);
}

And the return value should actually be

return (a1->yearOfBirth > a2->yearOfBirth) - (a1->yearOfBirth < a2->yearOfBirth);

to avoid undefined behaviour on extreme values of int.

edited Dec 24, 2018 at 5:58

answered Dec 23, 2018 at 21:44

Antti Haapala

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1 Comment

chux Over a year ago

a1->yearOfBirth - a2->yearOfBirth may overflow and provide the wrong answer and is UB. return (a1->yearOfBirth > a2->yearOfBirth) - (a1->yearOfBirth < a2->yearOfBirth); nicely handles that. Of course extreme years are not expected, such values may occur as sentinels.

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