在将元素推送到队列之前增加队列的大小如何让我出错而相反的工作正常？

Question

Like I said if I put q->size++ (in push_queue function) before the if statement which contains the code of pushing an element to the queue the running of the program crashes however if I put the line after the if statement it works perfectly even though there is nothing related to the size of the queue, here is the code:

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

struct queue_node
{
    int value;
    struct queue_node *next;
};

typedef struct queue_node queue_node;
struct queue
{
    queue_node *front ;
    queue_node *back ;
    int size;
};
typedef struct queue queue;


queue * create_queue()
{
    queue * q = malloc(sizeof(queue));
    q->back = q->front = NULL;
    q->size = 0;
    return q;
}
bool empty(queue *q)
{
    return (q->size == 0) ;
}

void push_queue(queue *q , int value)
{
    queue_node *qn = malloc(sizeof(queue_node));
    qn->next = NULL;
    qn->value = value;
    q->size++;

        if (empty(q))
    {
        q->back = q->front = qn;

    }
    else
    {
        q->back->next = qn;
        q->back = qn;

    }

}

int pop_queue(queue *q)
{
    if (empty(q))
    {
        printf("vide");
        return 0;

    }
        int value = q->front->value;
        queue_node *qn = q->front;
        q->front = q->front->next;
        free(qn);
        q->size--;
        return value;
}


int main ()
{
    queue *q = create_queue();
    push_queue(q,5);
    push_queue(q,9);
    push_queue(q,7);

    while (!empty(q))
        printf("%d\t",pop_queue(q));
    free(q);
    return 0;
}

Answer 1

push_queue is first checking if the queue is empty. empty uses size . If you increment size before checking empty , empty will always be false. On the first push q->back will be null and q->back->next will segfault.

Note: since this is a FIFO (First In First Out) queue , "pop" is the wrong term. "push" and "pop" are associated with stacks and LIFO queues . Consider "enqueue" and "dequeue".