another try

Strange, it seem my mails don't find their way to this list. 
So here we go again: 
I have read the Common Lisp cookbbook and looked at how to call an
external C function. (example (calling gethostname)

Well I extracted that function from the header and let the
foreign-parser do the job

I than have come up with this (IMHO wrong solution, which should
crash)
(defun get-hostname ()
  (let* ((limit 256))
    (fli:with-dynamic-foreign-objects ((buffer (:char)))
      (let ((ret-val (gethostname buffer limit)))
        (if (> ret-val 0)
            (break)
          (fli:convert-from-foreign-string buffer))))))


Well why do I think it should crash? I'm not allocating any space for
the external buffer. 

One correct way seem to be:
(defun get-hostname-2 ()
  (let ((limit 256))
    (fli:with-dynamic-foreign-objects ()
      (let* ((buffer (fli:allocate-dynamic-foreign-object :pointer-type
`(:pointer :char)
                                                          :nelems
limit))
             (ret-val (gethostname buffer limit)))
        (if (= ret-val 0)
            (fli:convert-from-foreign-string buffer)
          (break))))))

Well there may be a better solution available. This seems to be a bit
too ugly. Can anyone give me a hand?

Another thing. It seems the Documenation is wrong. For :ef-mb-string I
find:
:ef-mb-string limit &optional external-format 

but trying it in the listener yields this:
The lambda list of foreign type :EF-MB-STRING is (&KEY LIMIT
EXTERNAL-FORMAT (NULL-TERMINATED-P T))


Well this is probably the correct version: Than this should work
(IIUC)
(defun get-hostname-3 ()
  (let* ((limit 256)
         (buffer (make-string limit)))
    (fli:with-dynamic-foreign-objects ((buffer (:ef-mb-string :limit 256
                                                :external-format
:ascii)))
      (let ((ret-val (gethostname buffer limit)))
        (if (= ret-val 0)
            (fli:convert-from-foreign-string buffer)
          (break))))))
            

according to the sources of cl-http tcp-list where one can find this
(defun getdomainname (&optional (where "/etc/defaultdomain"))
  (or *domainname*
      #+unix
      (fli:with-dynamic-foreign-objects ((buffer (:ef-mb-string
						  :external-format :ascii
						  :limit 256)))
                                        (and (eql (c-getdomainname
buffer 256) 0)
                                             (setq *domainname*
(fli:convert-from-foreign-string buffer))))
      ;; Try using DNS name lookup: on some machines this 
      (let* ((self-name (comm:get-host-entry (machine-instance)
					     :fields '(:name)))
	     (dot (and self-name (position #\. self-name))))
	(and dot (subseq self-name (1+ dot))))
      (if (probe-file where)
	  (with-open-file (stream where :direction :input)
	    (setq *domainname* (read-line stream))))
      (error 'www-utils:domain-resolver-error)))

I extracted the main logic and got this:
(defun getdomainname (&optional (where "/etc/defaultdomain"))
  (or *domainname*
      #+unix
      (fli:with-dynamic-foreign-objects ((buffer (:ef-mb-string
						  :external-format :ascii
						  :limit 256)))
        (and (eql (ipc::c-getdomainname buffer 256) 0)
             (setq *domainname* (fli:convert-from-foreign-string
buffer))))))


Well this seem to work:
CL-USER 73 : 6 > (getdomainname)
"(none)"

but get-hostname-3 fails with:
(get-hostname-3)

Error: #<Pointer to type (:EF-MB-STRING :LIMIT) = #x0805D090> cannot be
converted to foreign type (:POINTER :CHAR).
   1 (abort) Return to level 6.
   2 Return to debug level 6.
   3 Return to level 5.
   4 Return to debug level 5.
   5 Try invoking C-GETDOMAI

well than I tried this 
(fli:define-foreign-function (c-gethostname "gethostname" :source)
    ((name :pointer) (len size-t))
  :result-type :int 
  :language :c)
  
and
(defun get-hostname-4 ()
  (let ((limit 256))
    (fli:with-dynamic-foreign-objects ((buffer (:reference
(:ef-mb-string
                                                           
:external-format :ascii
                                                            :limit
256))))
      (let ((ret-val (c-gethostname buffer limit)))
        (if (= ret-val 0)
            (fli:convert-from-foreign-string buffer)
          (break))))))

this works. 

but not 
(defun get-hostname-4-1 ()
  (let ((limit 256))
    (fli:with-dynamic-foreign-objects ((buffer (:reference
(:ef-mb-string
                                                           
:external-format :ascii
                                                            :limit
limit))))
      (let ((ret-val (c-gethostname buffer limit)))
        (if (= ret-val 0)
            (fli:convert-from-foreign-string buffer)
          (break))))))

WellI got the messag that limit is not bound. But it is!




but again the queston is should it? 

I than checked the knowledgebase and bound 17004, but that confuses me
even more. 

So maybe someone can bring into some light.
What I want to do
I want to allocate a Lisp String (such that it can be handled by the
Lisp GC) than I wont to hand this string over to C which fills it well
on return it should be still a valid Lisp string.

What is the right way doing it?

Thanks
Friedrich