"Implementation defined" has a specific meaning in the context of the C standard: it means that an implementation needs to define and document a particular beha

"Implementation defined" 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: https://gcc.gnu.org/onlinedocs/gcc/C-Implementation.html https://gcc.gnu.org/onlinedocs/gcc/C-Implementation.html For undefined behavior, the standard makes no requirements at all on the compiler.