Thread: Linker error

Hello.I am working on a simple math project using C.

I have these three files:

queue.h

Code:

#include <stdlib.h>

/* Simply linked lists */
typedef struct ListNode_tag {
   int data;
   struct ListNode_tag * next;
} ListNode;


/* Type queue */
typedef struct {
   ListNode * first;
   ListNode * last;
} queue;

/* Empty queue */
const queue queueEmpty = { NULL, NULL };

/* Insert */
void queueInsert (queue * qp, int t);

/* Remove */
int queueRemove (queue * qp);

/* Head */
int queueHead (queue q);

queue.c

Code:

#include <stdlib.h>

/* Simply linked lists */
typedef struct ListNode_tag {
   int data;
   struct ListNode_tag * next;
} ListNode;


/* Type queue */
typedef struct {
   ListNode * first;
   ListNode * last;
} queue;

/* Empty queue */
const queue queueEmpty = { NULL, NULL };

/* Insert */
void queueInsert (queue * qp, int t);

/* Remove */
int queueRemove (queue * qp);

/* Head */
int queueHead (queue q);

main.c

Code:

#include "queue.h"

int main()
{
	//queue q = queueEmpty;
	return 1;
}

When I compile the project I get
fatal error LNK1169: one or more multiply defined symbols found

Can you help?Thanks

Right, the way the muiltiple files and the linking works is, you need to have too the function prototypes in the .h file and the function definiations in the .c file. In your case, you have the same copy of the .h file in the .c file which is why yoiu get the muiltiply defined symbols. Error.

ssharish

Wrong copy paste sorry...

queue.c

Code:

#include <stdio.h>
#include "queue.h"
/* Insert */
void queueInsert (queue * qp, int t)
{
   ListNode * n = (ListNode *) malloc(sizeof(ListNode));

   /* Check if malloc succeeded */
   if (n == NULL) {
      fprintf(stderr, "Out of memory\n");
      exit(1);
   }

   /* Copy the data */
   n->data = t;
   n->next = NULL;

   /* If the queue was empty, just add this one element */
   if (qp->last == NULL)
      qp->first = qp->last = n;

   /* Otherwise, add the new element at the end */
   else {
      qp->last->next = n;
      qp->last = n;
   }
}

/* Remove */
int queueRemove (queue * qp)
{
   ListNode * n;
   int result;

   /* Error, if the queue was empty */
   if (qp->first == NULL) {
      fprintf(stderr, "Nothing to remove from an empty queue\n");
      exit(1);
   }

   /* Remove and free the first element */
      n = qp->first;
   result = qp->first->data;
   qp->first = qp->first->next;
   free(n);

   /* If the queue is now empty, set the 'last' pointer too */
   if (qp->first == NULL)
      qp->last = NULL;
   
   return result;
}


/* Head */
int queueHead (queue q)
{
   /* Error, if the queue was empty */
   
   if (q.first == NULL) {
      fprintf(stderr, "Nothing to see in an empty queue\n");
      exit(1);
   }
   
   return q.first->data;
}

Code:

const queue queueEmpty = { NULL, NULL };

Should be just

Code:

const queue queueEmpty;

If you want to initialise them, then dereference it and then initialise.

ssharish