Adding new function to libc

How can I add a new libc function and also call it from C programs? The new function is a not a wrappe开发者_如何学Gor to any kernel level system calls. Its function will be done in user space.

Put it in its own library file and link it with -llibrary_name_here. The only things that belong in libc are already there (along with plenty of things that don't belong there).