Misalignment of assigning the first element in void* arrays and malloc assigning more memory than necessary

I’m confused as to why the following code still runs even though i have not allocated enough memory for the void* arr[]; i’m still able to write to those locations and not run into a segfault.

void** arr;

arr = realloc(NULL, 1); // 1 Byte allocated
arr[0] = (void *)1;     // able to store a 8B pointer

arr = realloc(arr, 2); // 2 Bytes allocated
arr[1] = (void *)2;    // able to store yet another 8B pointer

arr = realloc(arr, 4); // 4 Bytes allocated
arr[2] = (void *)3;    // able to store yet another 8B pointer
arr[3] = (void *)4;    // able to store yet another 8B pointer

// Error
arr = realloc(arr, 8); // realloc():invalid next size thrown presumaby 
                       // because we wrote into the space for malloc metadata

Also, looking at this it seems that the first assignment a[0] = 1 takes up only the first 4B chunk even though void*s are 8Bytes. The rest of the assignments do take up 8Bs so i’m also curious why the first a[0] only takes up 4Bytes.

Thank you!

>Solution :

It is undefined behavour. In C nothing prevents you from writing to the invalid or not allocated memory.

it seems that the first assignment a[0] = 1 takes up only the first 4B
chunk even though void*s are 8Bytes.

No, it is called little endian and less significant bytes are stored first.

example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    unsigned long long x[] = {1,2,3};
    unsigned y[sizeof(x) / sizeof(unsigned)];

    memcpy(y,x,sizeof(x));
    for(int i = 0; i < sizeof(y) / sizeof(y[0]); i ++)
        printf("0x%08x ", y[i]);
    printf("\n");
}

result:

0x00000001 0x00000000 0x00000002 0x00000000 0x00000003 0x00000000 

https://godbolt.org/z/a7vrrPWGx

You can also see it on the bete level:
https://godbolt.org/z/b7xMMfxhc