title: A POSIX compliant OS usually extends an existing implementation of the C Standard Library?
body: <p>One of the <em>Units of Functionality</em> that POSIX states an OS needs to provide to be POSIX compliant is POSIX_C_LANG_SUPPORT. Basically this is the whole C Standard Library with some more things.</p>
  
  <p>My question is simple: developers of POSIX compliant OSes usually just download an open source version of C Standard Library (e.g. glib or uClibc) and adapt it to fit POSIX or they implement everything from scratch? Is there any advantage in rewriting the C Library instead of just picking one of the very known implementations and adjust it to my needs?</p>