放在 /usr/share/vim/vim80/syntax/c.vim 最后:

syn match cFunctions "\<[a-zA-Z_][a-zA-Z_0-9]*\>[^()]*)("me=e-2
syn match cFunctions "\<[a-zA-Z_][a-zA-Z_0-9]*\>\s*("me=e-1
hi cFunctions ctermfg=3 gui=bold
syn keyword cXhc         std io
syn keyword cType       LL Iter uint
hi cXhc gui=bold