C Memory Management -> Queue - c

Question

Okay so i have this Queue implementation that works, but i'm having some memory leak that doesn't let a if else operations runs after that.
File queue_arr.h
#define MaxQ 100
typedef struct {
int array[MaxQ];
int front;
int back;
int size;
}*Queue;
Queue init();
int enqueue(Queue q, int v);
int dequeue(Queue q, int *v);
int front(Queue q, int *v);
int isEmpty(Queue q);
File queue_arr.c
#include "queue_arr.h"
#include <stdlib.h>
#include <stdio.h>
Queue init() {
Queue q = (Queue)malloc(sizeof(Queue));
q->front = 0;
q->back = 0;
q->size = 0;
return q;
}
int enqueue(Queue q, int v) {
if(q->size==MaxQ)
return 0;
q->array[q->back] = v;
q->back++;
if(q->back == MaxQ)
q->back = 0;
q->size++;
return 1;
}
int dequeue(Queue q, int *v) {
if(q->size == 0)
return 0;
*v = q->array[q->front];
q->front++;
if(q->front == MaxQ)
q->front =0;
q->size--;
return 1;
}
int front(Queue q, int *v) {
if(q->size==0)
return 0;
*v = q->array[q->front];
return 1;
}
int isEmpty(Queue q) {
if(q->size == 0)
return 1;
return 0;
}
int main(){
Queue teste = init();
int *aux;
*aux = -1;
printf("Value : %d\n",*aux );
enqueue(teste,5);
enqueue(teste,10);
enqueue(teste,15);
front(teste,aux);
printf("Next costumer: %d\n",*aux );
dequeue(teste,aux);
printf("Costumer %d left queue\n",*aux );
dequeue(teste,aux);
printf("Costumer %d left queue\n",*aux );
dequeue(teste,aux);
printf("Costume %d left queue\n",*aux );
int random = 10;
if(random == 10)
printf("asdasdasd\n");
}
The last if and else statement doest not work, i noticed cause i was trying to do a isEmpty if clause, and keep leading me to segmentation fault.
Without the last three lines, the code compiles and runs, with no errors. But with that i just keep getting a segmentation fault.
Does anybody knows the problem ?

Answer 1

Your code can't decide whether a Queue is a pointer to something or the something.
typedef struct {
int array[MaxQ];
int front;
int back;
int size;
}*Queue;
This says a Queue is a pointer to a bunch of things.
Queue q = (Queue)malloc(sizeof(Queue));
q->front = 0;
This allocates enough bytes to hold a Queue, which is just a pointer, and then attempts to use the thing it points to. But you never allocated space for anything but a pointer.

Answer 2

Queue q = (Queue)malloc(sizeof(Queue));
Congratulations, sizeof(Queue) is 4 (or 8) bytes since it's a pointer.

Answer 3

Your segmentaion fault because of this
int *aux;
*aux = -1;
you should
aux = malloc(sizeof(int));
before
*aux = -1;
but I don't think that is what you want, instead you should just do it
int aux = -1;
and when calling dequeue
dequeue(teste, &aux);
Also I would recomend you fix these other issues
You don't need to cast malloc
You should check that malloc didn't return NULL otherwise you will dereference a
NULL pointer which is undefined behavior.
Also, your malloc call is passing the wrong size. A possible fix is to use
Queue q = malloc(sizeof(*q));
But I would actually recommend to avoid hiding the fact that variables are pointers completely.
You do have a memory leak but that is not a cause for a segmentaion fault, you should just call free(teste) at the end of main().
Your main() doesn't return you should, also add return 0; at the end of main() after free()
I fixed your code, hope this helps
#include <stdlib.h>
#include <stdio.h>
#define MaxQ 100
typedef {
int array[MaxQ];
int front;
int back;
int size;
} *Queue;
Queue init() {
Queue q = malloc(sizeof(*q));
if (q == NULL)
return NULL;
q->front = 0;
q->back = 0;
q->size = 0;
return q;
}
int enqueue(Queue q, int v) {
if (q == NULL)
return 0;
if(q->size==MaxQ)
return 0;
q->array[q->back] = v;
q->back++;
if(q->back == MaxQ)
q->back = 0;
q->size++;
return 1;
}
int dequeue(Queue q, int *v) {
if (q == NULL)
return 0;
if(q->size == 0)
return 0;
*v = q->array[q->front];
q->front++;
if(q->front == MaxQ)
q->front =0;
q->size--;
return 1;
}
int front(Queue q, int *v) {
if (q == NULL)
return 0;
if(q->size==0)
return 0;
*v = q->array[q->front];
return 1;
}
int isEmpty(Queue q) {
if (q == NULL)
return 0;
if(q->size == 0)
return 1;
return 0;
}
int main(){
Queue teste = init();
if (teste == NULL)
return -1;
int aux;
aux = -1;
printf("Value : %d\n", aux);
enqueue(teste,5);
enqueue(teste,10);
enqueue(teste,15);
front(teste, &aux);
printf("Next costumer: %d\n", aux );
dequeue(teste, &aux);
printf("Costumer %d left queue\n", aux );
dequeue(teste, &aux);
printf("Costumer %d left queue\n", aux );
dequeue(teste, &aux);
printf("Costume %d left queue\n", aux );
int random = 10;
if(random == 10)
printf("asdasdasd\n");
free(teste);
return 0;
}