Valgrind… 4 bytes inside a block of size 8 free

Question

I'm getting this error in Valgrind after attempting to free a list. print_list dumps the list to the syslog. I'm pretty confident that output is correct.

Valgrind:

==7028== 1 errors in context 1 of 10:
==7028== Invalid read of size 4
==7028==    at 0x8049603: free_list (list.c:239)
==7028==    by 0x80488B5: m61_close_for_valgrind (m61.c:36)
==7028==    by 0x8048825: main (mytest.c:19)
==7028==  Address 0x420006c is 4 bytes inside a block of size 8 free'd
==7028==    at 0x4028F0F: free (vg_replace_malloc.c:446)
==7028==    by 0x804960C: free_list (list.c:239)
==7028==    by 0x80488B5: m61_close_for_valgrind (m61.c:36)
==7028==    by 0x8048825: main (mytest.c:19)
==7028== 

mytest.c:

15  char *temp = malloc(10);
16  char *temp2 = malloc(10);
17  free(temp);
18  free(temp2);
19  m61_close_for_valgrind();

list.h

typedef struct lnode {
    ACTIVE_ALLOCATION *value;
    struct lnode *next;
} lnode;

list.c (Called by m61_close_for_valgrind()

void free_list(LIST *s) {

    lnode **nptr = &s->head;

    print_list(s);
    while (*nptr) {
        lnode **tmp = nptr;
        tmp = nptr;

        if ((*tmp)->value) {
            syslog(LOG_NOTICE,"Freeing (*tmp)->value=%p\n", (*tmp)->value);
            //printf("%p\n",(*nptr)->value);
            free((*tmp)->value);    //Free active allocation metadata
        }

        nptr = &(*nptr)->next;
        syslog(LOG_NOTICE,"New *nptr value=%p\n", (*nptr));

        syslog(LOG_NOTICE,"Freeing (*tmp)=%p\n", (*tmp));
        free(*tmp);             //Free node

    }

}

syslog

Sep 19 00:37:02 appliance mytest[7759]:   -- Start List Dump --
Sep 19 00:37:02 appliance mytest[7759]:   (*nptr)=0x903f220 (*nptr)->value=0x903f208   (*nptr)->next=0x903f260  (*nptr)->value->ptr=0x903f1f0
Sep 19 00:37:02 appliance mytest[7759]: (*nptr)->value->ptr=0x903f1f0
Sep 19 00:37:02 appliance mytest[7759]:   (*nptr)=0x903f260 (*nptr)->value=0x903f248   (*nptr)->next=(nil)  (*nptr)->value->ptr=0x903f230
Sep 19 00:37:02 appliance mytest[7759]: (*nptr)->value->ptr=0x903f230
Sep 19 00:37:02 appliance mytest[7759]:   -- End List Dump --
Sep 19 00:37:02 appliance mytest[7759]: Freeing (*tmp)->value=0x903f208
Sep 19 00:37:02 appliance mytest[7759]: New *nptr value=0x903f260
Sep 19 00:37:02 appliance mytest[7759]: Freeing (*tmp)=0x903f220
Sep 19 00:37:02 appliance mytest[7759]: Freeing (*tmp)->value=0x903f248
Sep 19 00:37:02 appliance mytest[7759]: New *nptr value=(nil)
Sep 19 00:37:02 appliance mytest[7759]: Freeing (*tmp)=0x903f260

Answer 1

In each iteration other than the first, tmp points at the next pointer from the previous node - but you've already freed that node (in the previous iteration), so tmp points into a freed block and you can't dereference it.

Answer 2

As caf already wrote, you're accessing memory that has just been freed.

To fix that, just don't use double pointers, single pointers will do very well here.

So replace

lnode **nptr = &s->head;

by

lnode *nptr = s->head;

Same for

lnode **tmp = nptr;

in the loop. Make it

lnode *tmp = nptr;

and drop the double assignment just when you at it.

Then access value and next by

tmp->value

and

tmp->next

directly