> libpthread needs to supply a function called "read" that provides
  > a blocking interface to the programmer, but actually does non-blocking
  > calls to the kernel.

If the file descriptor has been set to non-blocking mode, _sys_read
may not work as expected, no?  Another example would be write(), which
is used internally inside libc.  Even if, using weak symbols, the libc
routine correctly calls _sys_write(), it may not be expecting an EAGAIN
error return.