This is documented[0]: > Note that it is assumed that a freestanding environment will additionally provide memcpy, memmove, memset, and memcmp implementations,

This is documented[0]: > Note that it is assumed that a freestanding environment will additionally provide memcpy, memmove, memset, and memcmp implementations, as these are needed for efficient codegen for many programs. GCC has effectively the same stipulation[1]: > GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp. The main thing I'm confused about is why you didn't get a linker error. 0. https://clang.llvm.org/docs/CommandGuide/clang.html#cmdoption-ffreestanding https://clang.llvm.org/docs/CommandGuide/clang.html#cmdoptio... 1. https://gcc.gnu.org/onlinedocs/gcc/Standards.html https://gcc.gnu.org/onlinedocs/gcc/Standards.html