chromium/native_client_sdk/src/libraries/nacl_io/syscalls/README

This folder contains the implementations of libc/POSIX
entry points and redirects them to nacl_io implementations.

Not all C library entry points are implemented in this way.
In many cases nacl_io instead intercepts the calls at a
lower level within the C library.  The way this is done
differs for each C library.  See kernel_wrap_*.cc for
details.