(defun c-switch-to-header ()
  "Switch to or from the header of a C source file."
  (interactive)
  (when (string-match "\\([a-zA-Z0-9\-\_\/]+\\)\\.\\([ch]\\)" (buffer-name))
    (if (string= "c" (match-string 2 (buffer-name)))
	(setq ext ".h")
      (setq ext ".c"))
    (find-file (concat (match-string 1 (buffer-name)) ext))))

(global-set-key [f8] 'c-switch-to-header)