c - 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 <stdlib.h>
#include <assert.h>
#include <string.h>
#include <stdio.h>

size_t COLORCOUNT = 4;

typedef struct rgb_t {
    int r;
    int g;
    int b;
} rgb_t;

rgb_t** colors;

void addColor(size_t i, int r, int g, int b) {
    rgb_t* color;
    if (i >= COLORCOUNT) {
        // new memory wont be NULL
        colors = realloc(colors, sizeof(rgb_t*) * i);
       //something messy like this...
        //memset(colors[COLORCOUNT-1],0 ,sizeof(rgb_t*) * (i - COLORCOUNT - 1));

         // ...or just do this (EDIT)
        for (j=COLORCOUNT; j<i; j++) {
            colors[j] = NULL;
        }

        COLORCOUNT = i;
    }

    color = malloc(sizeof(rgb_t));
    color->r = r;
    color->g = g;
    color->b = b;

    colors[i] = color;
}

void freeColors() {
    size_t i;
    for (i=0; i<COLORCOUNT; i++) {
        printf("%x
", colors[i]);
        // can't do this if memory isn't NULL
       // if (colors[i])
         //   free(colors[i]);

    }
}


int main() {
    colors = malloc(sizeof(rgb_t*) * COLORCOUNT);
    memset(colors,0,sizeof(rgb_t*) * COLORCOUNT);
    addColor(0, 255, 0, 0);
    addColor(3, 255, 255, 0);
    addColor(7, 0, 255, 0);


    freeColors();
    getchar();
}

Answer 1

There is no way to solve this as a general pattern. The reason why is that in order to know what part of the buffer is new you need to know how long the old buffer was. It's not possible to determine this in C and hence prevents a general solution.

However you could write a wrapper like so

void* realloc_zero(void* pBuffer, size_t oldSize, size_t newSize) {
  void* pNew = realloc(pBuffer, newSize);
  if ( newSize > oldSize && pNew ) {
    size_t diff = newSize - oldSize;
    void* pStart = ((char*)pNew) + oldSize;
    memset(pStart, 0, diff);
  }
  return pNew;
}