Hence standard library function names are "reserved for use as identifiers with external linkage", ie unless your implementation is 'freestanding', you're not a

Hence standard library function names are "reserved for use as identifiers with external linkage", ie unless your implementation is 'freestanding', you're not allowed to re-define them. The code above does not do so. Contrary to what other people have suggested, according to my understanding of the C language standard, the word `memcpy` is not magical in any way, and undefined behaviour only comes in because the code will call `memcpy` with incorrect signature.