Enqueue function of queue using linked list in c

Question

I'm having a problem when using linked list to build a queue program. Here's the full code.

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#define ERROR_VALUE -300000

typedef struct LinkedNode {
    int data;
    struct LinkdedNode* link;
}Node;
Node* front;
Node* rear;

void init_queue() { front = rear = NULL; }
int is_empty() { return (front = NULL && rear == NULL); }


int size() {
    Node* p; 
    int count = 0;
    if (is_empty())
        return 0; 
    for (p = front; p != rear; p = p->link) {
        count++;
        return count + 1; 
    }
}
void enqueue(int e) { 
    Node* p = (Node*)malloc(sizeof(Node));
    p->data = e;
    p->link = NULL; 
    if (is_empty())
        front = rear = p;
    else {
        rear->link = p;
        rear = p;
    }
}
int dequeue() { 
    Node* p = front; 
    int e;
    if (is_empty()) {
        printf("Queue Empty Error!\n");
        return ERROR_VALUE;
    }
    else if (size() == 1) {
        front = rear = NULL;
    }
    else
        front = p->link;
    e = p->data;
    free(p);
    return e;
}
int peek() { 
    if (is_empty()) {
        printf("Queue Empty Error!\n");
        return ERROR_VALUE;
    }
    return front->data;
}

void print_queue() {
    Node* p;
    printf("QUEUE STATUS: size=%d\n", size());
    if (is_empty())
        return;
    for (p = front; p != NULL; p = p->link)
        printf("[%2d] ", p->data);
    printf("\n");
}
int main(void) {
    int val, sel;

    init_queue();
    while (1) {
        do {
            printf("1.ENQUEUE 2.DEQUEUE 3.PEEK 4.STATUS 0.EXIT :");
            scanf("%d", &sel);
        } while (sel < 0 || sel > 4);
        if (sel == 1) {
            printf("1.ENQUEUE VALUE ? ");
            scanf("%d", &val);
            enqueue(val);
        }
        else if (sel == 2) {
            val = dequeue();
            if (val != ERROR_VALUE)
                printf("2.DEQUEUE VALUE = %d\n", val);
        }
        else if (sel == 3) {
            val = peek();
            if (val != ERROR_VALUE)
                printf("3.PEEK VALUE = %d\n", val);
        }
        else if (sel == 4)
            print_queue();
        else if (sel == 0) break;
    }
    return 0;
}

I didn't made is_full() function because linked list is "dynamic". When debugging, the program stops when I try enqueuing value. My guess is that there is something wrong in enqueue function, but cannot find what.

Answer 1

This is wrong:

int is_empty() { return (front = NULL && rear == NULL); }

Note the front = NULL . That means every time you call is_empty() , front gets set to NULL , which then causes is_empty() to return 0 because front = NULL evaluates to NULL .

You need to change is_empty() to

int is_empty() { return (front == NULL && rear == NULL); }

And this is exactly why many programmers use "Yoda conditions" like NULL == front - they prevent this type of bug because if you write = instead of == the code will fail to compile.

And, as you've noticed, such bugs are very hard to spot in your own code.