shows an overwrite in c (not enough space in the array greeting)

makefile

# needed to include the no-stack-protector to make gcc
# be fast and loose with the stack
junk : test.c
  gcc -o junk -g -fno-stack-protector test.c

clean :
  rm -f ./junk
  

test.c

#include <stdio.h>
int main() {

    // Example 1: An array with not enough space?
    
  // step 1
  // run this and enter ab for the greeting and everything is good
  
  // step 2
  // run this and enter abc and one of the variables will be over written (on my computer)
  
  // step 3
  // run this and enter abcd and BOTH! of the variables will be over written
  
  // step 4
  // run this and enter a string of 12 or 13 characters and you will get a segmentation fault
    char a;
    char greeting[3];
    char b;

    a = 0xff;
    b = 0xff;

    printf("value of a b before %d %d\n",a,b);

    printf("Enter a greeting: ");
    scanf("%s", greeting);

    printf("value of greeting is %s\n", greeting);

    printf("value of a b after %d %d\n",a,b);

    return 0;
}