https://fuchsia.googlesource.com/third_party/android.googlesource.com/platform/bionic/+/72f950d28f0d132973acde8a402a1b997aaee5fa/libc/bionic/c16rtomb.cpp
libc/bionic/c16rtomb.cpp - third_party/android.googlesource.com/platform/bionic - Git at Google
https://devdoc.net/c/cplusplus-20170409/en/c/string/multibyte/c16rtomb.html
c16rtomb - cppreference.com