3.15 Emacs

<C-h C-i> (info-lookup-symbol) is a good way to find documentation on C functions while editing (see (emacs)Info Lookup section ‘Info Documentation Lookup’ in The Emacs Editor).

The GMP manual can be included in such lookups by putting the following in your ‘.emacs’,

(eval-after-load "info-look"
  '(let ((mode-value (assoc 'c-mode (assoc 'symbol info-lookup-alist))))
     (setcar (nthcdr 3 mode-value)
             (cons '("(gmp)Function Index" nil "^ -.* " "\\>")
                   (nth 3 mode-value)))))