How to implement this create function correctly? - c

Question

I'm trying to make a function to make a priority queue. My structs are this:
struct node {
char *item;
struct node *next;
};
struct queue {
struct node *start;
struct node *end;
};
struct priority_queue {
struct queue **aoq;
int x;
};
My function that I want to implement is:
struct priority_queue *priority_queue_create(int x);
Here, as seen in the structure priority_queue, struct queue **aoq is essentially an array of queues, which is want I want. int x, as in the function header, is the number of queues in the array.
My take on it is this:
struct priority_queue *priority_queue_create(int x) {
struct priority_queue *pq = malloc(sizeof(struct priority_queue));
pq->x = x;
pq->aoq = malloc(x* sizeof(struct queue));
return pq;
}
Where I put the 3 comment lines in the code above is where I suspect my error to be. I want to be able to do something like: if x = 3, there should be an array with 3 queues in it, and I suppose I'll be able to access it by doing something like
pq->aoq[0] /// to access the first queue in the array, or
pq->aoq[2] /// to access the third queue in the array
Can anyone help my fix my implementation? Thanks in advance.
Edit:
Other implementation I've tried:
struct priority_queue *priority_queue_create(int x) {
struct priority_queue *pq = malloc(sizeof(struct priority_queue *));
pq->x = x;
pq->aoq = malloc(x * sizeof(struct queue *));
for (int i = 0; i < x; ++i) {
pq->aoq[i] = malloc(sizeof(struct queue));
}
return pq;
}

Answer 1

From the code that you posted, I think this is the functionality that you want. You were missing the allocation of memory for the queues inside struct priority_queue, that is:
pq-> aoq = malloc( x * sizeof(struct queue *));
Adding this and keeping what you already had (with a sample main function to test the function priority_queue_create()), we have the following code:
#include <stdio.h>
#include <stdlib.h>
struct node {
char *item;
struct node *next;
};
struct queue {
struct node *start;
struct node *end;
};
struct priority_queue {
struct queue **aoq;
int x;
};
struct priority_queue *priority_queue_create(int x) {
struct priority_queue *pq = malloc(sizeof(struct priority_queue *));
pq->x = x;
pq-> aoq = malloc( x * sizeof(struct queue *));
for (int i = 0; i < x; ++i) {
pq->aoq[i] = malloc(sizeof(struct queue)); ///
}
return pq;
}
int main(){
struct priority_queue * pq = priority_queue_create(3);
char * str1 = "abcd";
char * str2 = "abcde";
pq->aoq[0]->start = malloc(sizeof(struct node));
pq->aoq[2]->start = malloc(sizeof(struct node));
pq->aoq[0]->start->item = str1;
pq->aoq[2]->start->item = str2;
printf("%s - %s\n", pq->aoq[0]->start->item, pq->aoq[2]->start->item);
free(pq->aoq[0]->start);
free(pq->aoq[2]->start);
free(pq->aoq[0]);
free(pq->aoq[1]);
free(pq->aoq[2]);
free(pq->aoq);
free(pq);
}