How does a pointer points to [-1]th index of the array produce legal output everytime. What is actually happening in the pointer assignment?

#include<stdio.h>
int main()
{
        int realarray[10];
        int *array = &realarray[-1];

        printf("%p\n", (void *)array);
        return 0;
}

Code output:

manav@workstation:~/knr$ gcc -Wall -pedantic ptr.c
manav@workstation:~/knr$ ./a.out
0xbf841140

EDIT: If this scenario is valid, then can i use this to define an array whose index start from 1 instead of 0, namely: array[1], array[2],...

c++ c arrays pointers
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