So you want to add an interface to libc...

CASE I: internal symbols

  A) used in a single file
     Duh: use whatever name you want and make it static

  B) used in multiple files
     Give it a name in the implementation namespace (leading underbar)
     and declare it in a __{BEGIN,END}_HIDDEN_DECLS block in a .h
     file inside libc.  If it's used in just a single subdir of
     libc *and* that subdir has an appropriate .h file in it, then
     declare it there.
     Example: stdio/local.h.
     Otherwise, declare it in one of the hidden/* files.
     Example: _mktemp() in hidden/stdio.h

CASE II: external symbols

  First of all, add your symbol to Symbols.list.  MD symbols go in
  arch/*/Symbols.list (shock, I know)

  Declare your function in the appropriate header.  It almost certainly
  should be in a public header installed under /usr/include/.  Exceptions
  are symbols that are just shared between libc and libpthread/csu/ld.so
  which are only declared in libc/include/* or just in each .c file.

  A) objects (variables)
     That's it, you're done.

  B) plain C functions (not syscalls)
     1) functions that are *not* called from inside libc

	If this function is specified in the ISO C standard or its
	name begins with an underbar, then in the hidden/* version
	of the header where you declared the function, add this line:
		PROTO_STD_DEPRECATED(your_function_name);

	Otherwise, this is *not* a function specified in the ISO C
	standard and its name begins with a letter.  In the hidden/*
	version of the header where you declared the function, add
	this line:
		PROTO_DEPRECATED(your_function_name);

     2) functions that are called from inside libc

	In the hidden/* version of the header where you declared
	the function, add this line:
		PROTO_NORMAL(your_function_name);

	Then, in the .c file(s) where the function is defined:
	 - if the function is specified in the ISO C standard or
	   its name begins with an underbar, add
		DEF_STRONG(your_function_name);

	 - otherwise, add:
		DEF_WEAK(your_function_name);

  C) syscalls that require cancellation or other libpthread wrappers
     You're done in libc, but go to libpthread and add the wrapper there.

  D) syscalls that require libc wrappers for other reasons
     First of all, make sure this really is the Right Thing.  Talk
     with kettenis, deraadt, miod, and guenther.

     1) If the actual syscall doesn't have the same calling convention
	as the C interface, then maybe it shouldn't be exported at
	all and you should just have an ASM stub, like SYS___tfork
	--> __tfork_thread() or SYS_break --> brk() and sbrk().  If
	it _could_ be called from C, then give the syscall a name
	different from the C API.  Syscalls that fail this for
	historical reasons (e.g., getlogin) are generated with
	PSEUDO in libc/sys/Makefile.inc, so the ASM stub has a
	leading underbar.  Go read gen/getlogin.c for an example
	of how to do this.

     2) Otherwise, you're sane and have a syscall like sigaction()
	which requires a wrapper but whose syscall API is the same
	as the C API.  Then:
         - generate the ASM stub with HIDDEN in libc/sys/Makefile.inc
         - in the proper hidden/*.h file add
		PROTO_WRAP(your_function_name)
	 - the wrapper function should be defined in
		libc/sys/w_your_function_name.c
	   which should define WRAP(your_function_name) and
	   follow it with DEF_WRAP(your_function_name).  Look at
	   libc/sys/w_sigaction.c for an example.
	 - by default, libc code that calls your_function_name()
	   will get the real syscall and *not* the wrapper.  libc
	   calls that needd to go through the wrapper should invoke
	   WRAP(your_function_name)

  E) syscalls that don't require any wrapping
     In the hidden/* version of the header where you declared the
     function, add this line:
	PROTO_NORMAL(your_function_name);