How to zero out new memory after realloc

Question

What is the best way to zero out new memory after calling realloc while keeping the initially allocated memory intact?

#include 
#include <         


        

Answer 1

There is probably no need to do the memset: you may not be using colors[k] before setting it with something valid later. For example, your code sets colors[i] to a newly allocated color pointer, so you didn't need to set colors[i] to NULL.

But, even if you wanted to "zero it out so everything is nice", or really need the new pointers to be NULL: the C standard doesn't guarantee that all-bits-zero is the null pointer constant (i.e., NULL), so memset() isn't the right solution anyway.

The only portable thing you can do is to set each pointer to NULL in a loop:

size_t k;
for (k=COLORCOUNT; k < i+1; ++k) /* see below for why i+1 */
    colors[k] = NULL;

Your primary problem is that your realloc() call is wrong. realloc() returns a pointer to the resized memory, it doesn't (necessarily) resize it in-place.

So, you should do:

/* i+1 because you later assign to colors[i] */
rgb_t **tmp = realloc(colors, (i+1) * sizeof *tmp);
if (tmp != NULL) {
    /* realloc succeeded, can't use colors anymore */
    colors = tmp;
} else {
    /* realloc failed, colors is still valid */
}

If you really want to know what the memset() call should be, you need to set to zero the memory starting at colors+COLORCOUNT, and set i+1-COLORCOUNT members to zero:

memset(colors+COLORCOUNT, 0, (i+1-COLORCOUNT) * sizeof *colors);

But as I said above, all bytes zero is not guaranteed to be a NULL pointer, so your memset() is useless anyway. You have to use a loop if you want NULL pointers.