Enqueue function of queue using linked list in c - 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.