implementing a queue of structure

Posted on 1998-08-15
Last Modified: 2010-04-15
i would like to get help on how to implement a FIFO queue of data structures using c. i would like to use it as an entry point in my program i am currently coding for tcp/ip routing program. It should have a menu.I would appreciate if i got help soon .I am a student at the university.
Question by:zigla
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Author Comment

ID: 1252128
Edited text of question

Expert Comment

ID: 1252129
what does the menu have to do with the fifo queue ?

Accepted Solution

Answers2000 earned 100 total points
ID: 1252130
Create a struct something like this

typedef tagMYSTRUCT
struct tagMYSTRUCT * pNext ; /* pointer to next element */
/* Your data goes here */
char exampledata ;

You also need a variable which points to the top element of the queue, e.g.
MYSTRUCT * pTopElement = NULL ;

To add element to queue do this

int AddItem( char data ) /* return 0 for success, -1 for error */
 /* Allocate new item */
 MYSTRUCT * newitem = (MYSTRUCT *)malloc( sizeof(MYSTRUCT) ) ;

 /* malloc could fail */
 if ( newitem == NULL ) return -1 ;

 /* if queue is previously empty then this is top element */
 if ( pTopElement == NULL )
    pTopElement = newitem ;
    newitem->exampledata = data ;
    newitem->pNext = NULL ; /* Last item in queue */
    return 0;

 newitem->pNext = pTopElement ;
 newitem->exampledata = data ;
 pTopElement = newitem ;
 return 0 ;

To remove an item

int PopItem( char * pdata ) /* return 0 for success, -1 for error */
   MYSTRUCT * pTemp ;

   if ( pTopElement == NULL )
      /* Nothing in queue, return error */
      return -1 ;

   /* Get data from top item */
   *pdata = pTopElement->exampledata ;

   /* Free top item, and make 2nd from top the top */
   pTemp = pTopElement->pNext ;
   free(pTopElement) ;
   pTopElement = pTemp ;

   return 0 ;


To clean up when you're finished completely with the queue
void FreeQueue( void )
  char cDummy ;

  while ( pTopElement != NULL )
       PopItem( &cDummy ) ;

Replace the example data (in my case a character) with the data you really need.  If you want each item to have a block of data I suggest you memcpy into the structure.

That should be enough to get you started (sorry for any typos above, but the algorithm should be there for you).

And don't forget to check the return values from AddItem and PopItem, because they can fail.  AddItem will fail if you run out of memory.  PopItem will fail if you calling code attempts to pull more stuff out the queue than you've added to it.

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Expert Comment

ID: 1252131
Answers2000.  You've implemented a LIFO.  You're adding to the front of the list and removing from the front.  Try something like:

#include <assert.h>
#include <malloc.h>

struct queue_item {
  void *data;
  struct queue_item *next;

typedef struct queue *Queue;
struct queue {
  struct queue_item *head;
  struct queue_item *last;
} ;

Queue Queue_create(void)
Queue queue = calloc((size_t) 1, sizeof(*queue));
  return queue;

void *Queue_add_item(Queue queue, void *data)
struct queue_item *item;

  assert(queue != 0);

  if ((item = malloc(sizeof(*item)))) {
   item-> data = data;
   item-> next = 0;
   if (! queue-> head) {
     queue-> head = queue-> last = item;
   } else {
     queue-> last = queue-> last-> next = item;
  return item;

void *Queue_get_head(Queue queue)
struct queue_item *next = 0;
void *data = 0;
  assert(queue != 0);
  if (queue-> head) {
    data = queue-> head-> data;
    next = queue-> head-> next;
    free(queue-> head);
    if (! (queue-> head = next)) {
      queue-> last = 0;
  return data;


Expert Comment

ID: 1252132
Zigla: I don't see how a menu relates to a FIFO queue though.  Can you elaborate on the problem a little bit more, please?

Expert Comment

ID: 1252133
OOps ecw is right, misread the question.  Not tried his code

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