That design might be for portability, since pread() and pwrite() ought to be thread safe, so one global FD per file for your multi threaded app should be enough

That design might be for portability, since pread() and pwrite() ought to be thread safe, so one global FD per file for your multi threaded app should be enough, at least on Linux and freebsd. It does however not help if some other part of your application "accidentally" opens that file too (some lib for example).