Why is the socket function name not being selected in the completion menu?

I am creating ctags for C with the following command:

mkdir .vim/tags
ctags -R -I __THROW -I __attribute_pure__ -I __nonnull -I __attribute__ --file-scope=yes --langmap=c:+.h --languages=c,c++ --links=yes --c-kinds=+p --c++-kinds=+p --fields=+iaS --extra=+q \
-f .vim/tags/systag /usr/include/* --exclude="/usr/include/python2.7" --exclude="/usr/include/python3.4" --exclude="/usr/include/python3.4m"

      

        
        
        
      

    

I have this config in mine .vimrc

:

vim .vimrc
set tags+=$HOME/.vim/tags/systag

      

        
        
        
      

    

Now when I edit the C file, when I press ctrl+ x ctrl+ oafter soc

, a completion menu pops up with many features of the socket function family, but why isn't the feature socket

listed here?