Queue implementing by linked list in c

Question

I am implementing queue with a linked list but I am facing problem in the insertion() function. I am able to insert only one data, whenever I insert another data then again previous data insert whatever did I insert at first time.

#include <stdio.h>
#include <stdlib.h>
struct Queue
{
    int data;
    struct Queue *next;
};

struct Queue *rear = NULL;
struct Queue *front = NULL;

void insertion(int data)
{
    struct Queue *n;
    n = (struct Queue *)malloc(sizeof(struct Queue));
    n->data = data;
    n->next = NULL;
    if (rear == NULL)
    {
        front = n;
        rear = n;
    }
    else
    {
        rear->next = n;
        rear = n;
    }
}

void deletion()
{
    if (front == NULL)
        printf("\n Underflow");
    else if (front == rear)
    {
        front = NULL;
        rear = NULL;
    }
    else
        front = front->next;
}
void viewList()
{
    struct Queue *t = front;
    if (t == NULL)
        printf("\n there is no item for view...............");
    else
    {
        while (t != NULL)
        {
            printf(" %d", front->data);
            t = t->next;
        }
    }
}
int main()
{
    struct Queue *q = NULL;
    insertion(5);
    insertion(10);
    // deletion();
    viewList();
    printf("\n");
    viewList();
    return 0;
}

Answer 1

The function deletion produces a memory leak because it does not free the memory of the deleted node. The function can look at least like

void deletion() {
      if(front == NULL)
                printf("\n Underflow");
      else
      {
          stuct Queue *n = front;

          front = front->next;
          if ( front == NULL ) rear = NULL;

          free( n );
      }
}

Within the function viewList you are always outputting the value stored in the node pointed to by the pointer front

printf(" %d", front->data);

You have to write

printf(" %d", t->data);

This declaration in main

struct Queue *q = NULL;

is redundant and does not make a sense.

Pay attention to that it is a bad idea when functions depend on global variables. You could define the queue the following way

struct Node 
{
    int data;
    struct Node *next;
};

struct Queue 
{
    struct Node *front;
    struct Node *rear;
};

And in main you could define an object of the queue the following way

struct Queue queue = { .front = NULL, .rear = NULL };

And the functions should be rewritten such a way that they would accept a pointer to the queue. For example

int insertion( struct Queue *queue, int data );

The function can be defined like

int insertion( struct Queue *queue, int data )
{
    struct Node *new_node = malloc( sizeof( struct Node ) );
    int success = new_node != NULL;

    if ( success )
    {
        new_node->data = data;
        new_node->next = NULL;

        if ( queue->front == NULL )
        {
            queue->front = new_node;
        }
        else
        {
            queue->rear->next = new_node;
        }

        queue->rear = new_node;
    }

    return success;
}   

And the function will be called like

int main( void )
{
    struct Queue queue = { .front = NULL, .rear = NULL };

    insertion( &queue, 5 );
    insertion( &queue, 10 );
    //...

you can check the returned value of the function to determine whether the insertion was successful as for example.

    if ( !insertion( &queue, 5 ) ) puts( "Error. Not enough memory." );

Here is a demonstrative program.

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

struct Node 
{
    int data;
    struct Node *next;
};

struct Queue 
{
    struct Node *front;
    struct Node *rear;
};

int insertion( struct Queue *queue, int data )
{
    struct Node *new_node = malloc( sizeof( struct Node ) );
    int success = new_node != NULL;

    if ( success )
    {
        new_node->data = data;
        new_node->next = NULL;

        if ( queue->front == NULL )
        {
            queue->front = new_node;
        }
        else
        {
            queue->rear->next = new_node;
        }

        queue->rear = new_node;
    }

    return success;
}

int empty( const struct Queue *queue )
{
    return queue->front == NULL;
}

void viewList( const struct Queue *queue )
{
    if ( empty( queue ) )
    {
        printf("\n there is no item for view...............");
    }       
    else
    {
        for ( struct Node *current = queue->front; current != NULL;  current = current->next )
        {
            printf( " %d",  current->data );
        }
    }
}

int main(void) 
{
    struct Queue queue = { .front = NULL, .rear = NULL };
    
    insertion( &queue, 5 );
    insertion( &queue, 10 );
    
    viewList( &queue );
    
    return 0;
}

The program output is

 5 10