use these links:
Complete C library quick Reference and API Documentation:
this is "Official POSIX 2008 library documentation on opengroup(dot)org"
it cover most of GNU C headers as well as standard C include networking ,IO, threads,...
, , & so on.
it writed very similar to "JAVA API DOC" (e.g. very quick lookup ,Html format , link every function to its page , details for every function, list all functions by alphabet or by header , even have section "see also" at the end of every page )
i have searched a lot to find such a nice thing.
you can download it and use it offline from "downloads" link at the bottom of main index page.