C语言越过数组边界访问内存

char firstName[8] = "1234567";
char middleName[8] = "1234567";
char lastName[8] = "1234567";

middleName[-2] = 'X';
middleName[0] = 'X';
middleName[10] = 'X';

printf("%p %s\n",firstName,firstName);
printf("%p %s\n",middleName,middleName);
printf("%p %s\n",lastName,lastName);

116 12X4567
108 X234567
100 123456X