What's wrong with realloc?

Question

Started to learn malloc.h in C. The idea was to create dynamic array. Here is the code:

#include <stdio.h>
#include <conio.h>
#include <malloc.h>

int main() {
    int *array = (int*)malloc(sizeof(int));
    int i, j, val;
    for (i = 0;; i++) {
        array = (int*)realloc(array, sizeof(int) + sizeof(*array));
        printf("Enter array[%d]=", i);
        scanf("%d", (array + i));
        if (*(array + i) == 0) break;
    }
    for (j = 0; j < i; j++) {
        printf("[%d] = %d\n", j, *(array + j));
    }
    getch();
    return 0;
}

The result is

Enter array[0]=1
Enter array[1]=2
Enter array[2]=3
Enter array[3]=4
Enter array[4]=5
Enter array[5]=6
Enter array[6]=7
Enter array[7]=8
Enter array[8]=9
Enter array[9]=10
Enter array[10]=0
[0] = 1
[1] = 2
[2] = 3
[3] = 4
[4] = 5
[5] = 6
[6] = 7
[7] = 8
[8] = 542979931
[9] = 875896893

Each time, >=8 values are random. I have just no idea why it happens so what's wrong?

Answer 1

You undefined behavior in your code. I suppose, doing this:

array=(int*)realloc(array,sizeof(int)+sizeof(*array));

you expect, that sizeof(*array) will return you the size of the whole array, right? But that's not true. sizeof is computed at compile time and doing sizeof(*array) is actually the same as sizeof(int).

So, to make this array extensible, you need to have additional variable, holding the current number of elements and do something like:

array=(int*)realloc(array, sizeof(int) + current_size_of_array * sizeof( int ) );

where current_size_of_array will be incremented on each loop of the for, when you actually add one more element.

Answer 2

sizeof(*array) does not tell you how large the array is. It tells you how many bytes an int occupies. So each call to realloc is only allocating memory for two int values. Change the realloc call to allocate sizeof(int) times the number of ints that you want.