It seems that 0 wasn&#x27;t a usable address on PDP-11 so K&amp;R decided it&#x27;s fine to use it as null pointer constant.<p>And of course dereferencing 0 isn&#x27;t impossible <i>in practice</i> on arches which support this. Compilers for such machines usually can be coerced to generate code which accesses 0, you just have to live with the fact that your code isn&#x27;t considered &quot;valid, portable C&quot; anymore.