Question

for(k=i; k<MAXRECORDS; k++) {
    if(slist->servers_ptr[k+1] != NULL) {
        slist->servers_ptr[k] = slist->servers_ptr[k+1];
    } else slist->servers_ptr[k] = NULL;
}

When I run valgrind, I get an error of invalid size 8.

I assume this has something to do with a border case in my for loop, but I don’t understand logically how it’s happening.

EDIT: It was pointed out that on the last round of the for loop, accessing servers_ptr[k+1] is outside of the array, causing valgrind errors. I have since updated my code to:

for(k=i; k<MAXRECORDS-1; k++) {
        if(slist->servers_ptr[k+1] != NULL) {
                slist->servers_ptr[k] = slist->servers_ptr[k+1];
                if(k==MAXRECORDS-2)slist->servers_ptr[k+1] = NULL;
        } else slist->servers_ptr[k] = NULL;
}

I still get the errors in valgrind. Did I update it incorrectly?

Answer 1

It’s almost certainly because you’re going beyond the end of the array. The maximum value of k is MAXRECORDS-1 and you’re using k+1 in your expressions.

That means you’ll be accessing array[MAXRECORDS] where the index should be limited to between 0 and MAXRECORDS - 1 inclusive.

It’s difficult to see what you’re trying to do without more context but the fix may be as simple as using k < MAXRECORDS - 1 as the for loop continuing condition (the bit in the middle):

for (k = i; k < MAXRECORDS - 1; k++) {

The other possibility is an invalid value of i, like -1 for example, which would cause the problem at the other end of the array. This is probably less likely since I’m assuming you’re deleting element i by shifting all the other elements down (as in: i will be set to a valid index).

---

That’s not a memory leak by the way, simply a memory corruption. Memory leaks are when you allocate memory and then lose the pointers to them so that they can never be freed, something like:

char *x;
for (i = 0; i < 10; i++)
    x = malloc (64);

where only the last allocation is accessible.

---

By the way, if a shuffling deletion is what you’re after, it would be better done (in my opinion) as:

// For every element where there's a non-NULL next element,
// shift that element down. Then force the last element to
// be NULL (it will have been shifted down already).  

for (k = i; (k < MAXRECORDS - 1) && (slist->servers_ptr[k+1] != NULL); k++)
    slist->servers_ptr[k] = slist->servers_ptr[k+1];
slist->servers_ptr[k] = NULL;

The extra condition stops where the next element is NULL and places NULL into that position. That should work fine and has the advantage of being less complex.