These provide standard APIs, but are interposed by the threading library
to support cancelation points or are implemented using another
underlying system call (e.g., pipe implemented in terms of pipe2).
After discussion with kib (see D44111), I've concluded that it is
better to keep the public interfaces in libc with at little
as possible in libsys.
These provide standard APIs, but are not implemented directly on a
syscall of the same name. They are often implemnted on top of newer
syscalls (e.g., pipe implemented in terms of pipe2).