Sorting a struct with array pointer is not working

Question

I'm creating two struct's: Deck and Card. I want sort the cards of the deck using qsort but this is not working. When I run an exception occurs: Segmentation fault. Follow my code:

typedef struct Card {
   int value;
}Card;

typedef struct Deck{
   Card *c[100];
   int top;
}Deck;

Card * newCard(int value) {
   Card * aux = (Card *)malloc(sizeof(Card));
   aux->value = value;

   return aux;
}

Deck * newDeck() {
   Deck * deck = (Deck *)malloc(sizeof(Deck));
   deck->top = 0;

   return deck;
}

void addCard(Deck *b, Card *c) {
   b->top++;
   b->c[b->top] = c;
}

int compare(const void *x, const void *y) {
   Card * xa = *(Card **) x;
   Card * ya = *(Card **) y;

   if(xa->value == ya->value)
      return 0;

   if(xa->value > ya->value)
      return 1;

   return -1;
}

void sort(Deck *b) {
   qsort(b->c, b->top, sizeof(struct Card*), compare);
}

int main() {
   Deck * b = newDeck();
   addCard(b,newCard(11));
   addCard(b,newCard(12));
   addCard(b,newCard(11));
   addCard(b,newCard(1));
   addCard(b,newCard(1));

   sort(b);

   return 0;
}

Someone can help me? I just want sort the cards of the deck but something is wrong with the logic. Probably something like pointer(malloc or calloc?).

Please provide a minimal compiled example that reproduces the problem. — Vlad from Moscow
– Vlad from Moscow, Commented Aug 13, 2019 at 17:06
your code is incomplete. please add full code. Off top of my head, you are accessing Card in Deck without allocating it.... — Afshin
– Afshin, Commented Aug 13, 2019 at 17:06
b->top++; b->c[b->top] = c; Increase the variable after adding, not before. — Lundin
– Lundin, Commented Aug 14, 2019 at 9:01

Afshin · Accepted Answer · 2019-08-13 17:58:58Z

Hm....Your code seems like "fill in the blanks" since you have not posted your complete code and the code that you have posted is correct. So I completed your code in a way it works:

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

// Your code
typedef struct Card {
   int value;
}Card;

typedef struct Deck{
   Card *c[100];
   int top;
}Deck;

int compare(const void *x, const void *y) {
   Card*  xa = *(Card **) x;
   Card*  ya = *(Card **) y;

   if(xa->value == ya->value)
      return 0;

   if(xa->value > ya->value)
      return 1;

   return -1;
}

void sort(Deck *b) {
   qsort(b->c, b->top, sizeof(struct Card*), compare);
}

// My main
int main() {
    Deck d;
    d.c[0] = calloc(1, sizeof(Card));
    d.c[0]->value = 5;
    d.c[1] = calloc(1, sizeof(Card));
    d.c[1]->value = 1;
    d.c[2] = calloc(1, sizeof(Card));
    d.c[2]->value = 3;
    d.top = 3;
    sort(&d);
    printf("%d %d %d", d.c[0]->value, d.c[1]->value, d.c[2]->value);
    return 0;
}

My guess in your segmentation fault is that you are accessing Card without allocating it.

Update:

your addCard() is incorrect. Here is correct one:

void addCard(Deck *b, Card *c) {
   b->c[b->top] = c;
   b->top++;
}

When you are adding cards, your top points place that you need to add card. So you need to increase it after adding. Just remember that you need to add checks for it too to prevent overflowing Deck->c.

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