It's probably more likely to be implementation-defined behaviour (as with the signedness of 'char') rather than undefined behaviour. The third alternative bein

It's probably more likely to be implementation-defined behaviour (as with the signedness of 'char') rather than undefined behaviour. The third alternative being unspecified behaviour - see comp.lang.c FAQ 11.33 for more info: https://c-faq.com/ansi/undef.html https://c-faq.com/ansi/undef.html