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Mar 8th, 2005

How to free up memory used for queues?

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I have a first-in-first-out queue implementation such that when it is first initialized, a malloc is called to set aside memory for items that will be placed on the queue. When I am done with the queue, how do I free up the memory?
My code for the queue is as follows:

C Syntax (Toggle Plain Text)
typedef Edge Item;
void QUEUEinit(int);
int QUEUEempty(void);
void QUEUEput(Item);
Item QUEUEget(void);
 
static Item *q;
static int N,head,tail;
 
void QUEUEinit(int maxN)
{q = malloc((maxN+1)*sizeof(Item));
  N = maxN+1; head=N; tail=0;}
 
int QUEUEempty()
{ return head % N == tail;}
 
void QUEUEput(Item item)
{ q[tail++] = item;
  tail = tail % N;}
 
Item QUEUEget()
{ head = head % N;
  return q[head++];}
typedef Edge Item;
void QUEUEinit(int);
int QUEUEempty(void);
void QUEUEput(Item);
Item QUEUEget(void);

static Item *q;
static int N,head,tail;

void QUEUEinit(int maxN)
{q = malloc((maxN+1)*sizeof(Item));
  N = maxN+1; head=N; tail=0;}

int QUEUEempty()
{ return head % N == tail;}

void QUEUEput(Item item)
{ q[tail++] = item;
  tail = tail % N;}

Item QUEUEget()
{ head = head % N;
  return q[head++];}

kloony

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Mar 8th, 2005

Re: How to free up memory used for queues?

As defined in stdlib.h and malloc.h

C Syntax (Toggle Plain Text)

free (q);

will do the trick. Assure only pointers that were created with malloc, calloc or realloc are passed to free (), otherwise other calls to get memory may have unpredictable results.

Tight_Coder_Ex

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Mar 8th, 2005

Re: How to free up memory used for queues?

I did try the free(q), but was surprised when the following thing happened:

when there were still items on my queue, i did a free(q) and then tried to access items on my queue, it turns out that the first remaining item was not accessible but the 2nd, 3rd item so on were accessible. any explanation for this?

kloony

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Mar 8th, 2005

Re: How to free up memory used for queues?

Quote originally posted by kloony ...

I did try the free(q), but was surprised when the following thing happened:

when there were still items on my queue, i did a free(q) and then tried to access items on my queue, it turns out that the first remaining item was not accessible but the 2nd, 3rd item so on were accessible. any explanation for this?

Once you've freed it, consider all of it inaccessible. If it appears that items are still accessible, that's just a temporary illusion to make you think you're doing something you're allowed to.

Dave Sinkula

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long time no c

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Mar 8th, 2005

Re: How to free up memory used for queues?

thanks! help appreciated.

kloony

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Mar 9th, 2005

Re: How to free up memory used for queues?

The free() function is still puzzling me in some sense. I have a simple code below which creates a structure called graph. The code compiles without any problem and when it is executed, the output prints out 2 "10"'s, one before the free(), another one after. Did the memory set aside for G really got freed?

C Syntax (Toggle Plain Text)
#include<stdio.h>
#include<stdlib.h>
 
main()
{       typedef struct graph *Graph;
        struct graph{int V; int E;};
 
        int V=10;
        int E=10;
 
        Graph G=malloc(sizeof *G);
        G->V=V;
        G->E=E;
 
        printf("%d\n", G->E);
        free(*G);
        printf("%d\n", G->E);
        return 0;}
#include<stdio.h>
#include<stdlib.h>

main()
{       typedef struct graph *Graph;
        struct graph{int V; int E;};

        int V=10;
        int E=10;

        Graph G=malloc(sizeof *G);
        G->V=V;
        G->E=E;

        printf("%d\n", G->E);
        free(*G);
        printf("%d\n", G->E);
        return 0;}

kloony

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Mar 9th, 2005

Re: How to free up memory used for queues?

Apologies for my previous code....the free function should be

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free(G);

I mistyped it. Any help would be greatly appreciated.

kloony

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Mar 9th, 2005

Re: How to free up memory used for queues?

Hello,

When free() is called, a block of memory previously allocated by a call to malloc, calloc or realloc is freed:

(Toggle Plain Text)

Graph G=malloc(sizeof *G); /* Memory is dynamically allocated */
if (G != NULL) { /* G successfully points to an empty block of memory */
	/* Do task here */
}else if (G == NULL) {
	return 0; /* Memory allocation failed */
}

free(G); /* Dynamically allocated memory is freed */

Further documentation:

C Reference: malloc()
C Reference: free()

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Mar 9th, 2005

Re: How to free up memory used for queues?

Thanks, that's why I thought that after I "free(G)", the code

C Syntax (Toggle Plain Text)
printf("%d\n", G->E);
printf("%d\n", G->E);

should not be able to return me a value of 10. when it does, it surprises me, which makes me wonder whether the memory has really been freed.

kloony

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Mar 9th, 2005

Re: How to free up memory used for queues?

Hello,

Also, be sure to set your variable to NULL after you free the memory. Example:

(Toggle Plain Text)

free(G); /* Dynamically allocated memory is freed */
G = NULL; /* This ensures that we point our pointer to NULL */

This can ensure a few things. Firstly, it ensures our pointer doesn't point to our freed address. Secondly, it secures your code from crashing if you call free() twice on the same pointer. For instance:

(Toggle Plain Text)

free(G); /* Dynamically allocated memory is freed */
free(G); /* Will cause a crash. */

Where in fact:

(Toggle Plain Text)

free(G); /* Dynamically allocated memory is freed */
G = NULL; /* This ensures that we point our pointer to NULL */
free(G); /* Won't cause a crash because G points to NULL. */

Hope this helps.

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