&quot;Implementation defined&quot; has a specific meaning in the context of the C standard: it means that an implementation needs to define and document a particular behavior. E.g. for GCC: <a href="https:&#x2F;&#x2F;gcc.gnu.org&#x2F;onlinedocs&#x2F;gcc&#x2F;C-Implementation.html" rel="nofollow">https:&#x2F;&#x2F;gcc.gnu.org&#x2F;onlinedocs&#x2F;gcc&#x2F;C-Implementation.html</a><p>For undefined behavior, the standard makes no requirements at all on the compiler.