Implementation of simple queue data structure in C

Question

I was trying to implement a simple queue in C programming. But I encountered with the following error. Can anyone find me what is wrong in my program

#include<stdio.h>
#include<stdlib.h>
#define QUENE_SIZE 10
struct queue_structure{
  unsigned int queue_front, queue_rear;
  int queue_array[10];
};
void enqueue(struct queue_structure *,int value);
int dequeue(struct queue_structure *);
int queue_full(struct queue_structure *);
int queue_empty(struct queue_structure *);
int main(){
  struct queue_structure *queue_pointer = malloc(sizeof(struct queue_structure));
  int option,number,i,dequeue_number;
  queue_pointer->queue_front = queue_pointer->queue_rear = -1;
  while(1){
    printf("Enter the option:\n");
    scanf("%d",&option);
    switch(option){
      case 1:
        printf("Enter the number to be enqueued:\n");
        scanf("%d",&number);
        if(queue_full(queue_pointer) != 1){
          enqueue(queue_pointer,number);
        }
        else{
          printf("The queue is full");
        }
        break;
      case 2:
        if(queue_empty(queue_pointer) != 1){
          dequeue_number = dequeue(queue_pointer);
          printf("%d has been dequeued",dequeue_number);
        }
        else{
          printf("Your queue is empty\n");
        }
        break;
      case 3:
        for(i = 0; i < 10 ; i++ ){
          printf("%d",queue_pointer->queue_array[i]);
        }
        break;
      case 4:
        exit(0);
        break;
    }
  }  

}
void enqueue(struct queue_structure *qs, int number){   
  if(qs -> queue_front == qs -> queue_rear){
    qs->queue_front = qs->queue_rear = -1;
  }
  else{
    (qs -> queue_rear)++;
    qs->queue_array[qs->queue_rear] = number;    
  }  
}
int dequeue(struct queue_structure *qs){
  int i;
  if(qs->queue_front == qs->queue_rear){
    qs->queue_front = qs->queue_rear = -1; 
  }
  else{
    for(i = qs->queue_front; i < qs->queue_rear ; i++){
      qs->queue_array[i] = qs->queue_array[i + 1];
    }
  }
}
int queue_full(struct queue_structure *qs){
  if((qs->queue_rear == 10) && (qs->queue_front == 0)){
    return 1;
  }
  else{
    return 0;
  }
int queue_empty(struct queue_structure *qs){
  if((qs->queue_rear && qs->queue_front) == -1){
    return 1;
  }
  else{
    return 0;
  }
}

}

I am getting the following error

/tmp/ccLJHnMG.o: In function main': queue1.c:(.text+0xda): undefined reference toqueue_empty' collect2: ld returned 1 exit status**

This is not the code you're running! It has at least one syntax error! — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 26, 2015 at 13:52

Armen Tsirunyan · Accepted Answer · 2011-07-30 11:57:49Z

2

int queue_full(struct queue_structure *qs){
  if((qs->queue_rear == 10) && (qs->queue_front == 0)){
    return 1;
  }
  else{
    return 0;
  }
} <<<<<<<<<<<<<<<<<<<<<<
int queue_empty(struct queue_structure *qs){
...

answered Jul 30, 2011 at 11:57

Armen Tsirunyan

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Comments

NPE · Accepted Answer · 2011-07-30 11:58:55Z

The final curly brace is misplaced, it should be like this:

int queue_full(struct queue_structure *qs){
  if((qs->queue_rear == 10) && (qs->queue_front == 0)){
    return 1;
  }
  else{
    return 0;
  }
} /* added the }*/

int queue_empty(struct queue_structure *qs){
  if((qs->queue_rear && qs->queue_front) == -1){
    return 1;
  }
  else{
    return 0;
  }
}
/* there was a } here that I've removed */

Otherwise queue_empty is defined within queue_full. This is not standard C, but it appears to be supported by gcc as an extension, hence no errors during compilation.

When your code is compiled with -pedantic, gcc does flag it up:

aix@aix:~$ gcc -pedantic qq.c
qq.c: In function ‘queue_full’:
qq.c:78: warning: ISO C forbids nested functions
qq.c:78: warning: ISO C90 forbids mixed declarations and code

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