[cffi-devel] CFFI and wait (2)

[Lorenzo Villani]

Hi,

I'm trying to use CFFI in order to wrap execve (2). The wrapper also
uses fork (2) and wait (2). While I have absolutely no problems with
execve or fork I'm stumbling on a problem with wait.

Wait takes a pointer to an int to store child process' status. The
problem is: after the call to wait the value stored in the pointer is
the same as before the call.

I am using CFFI  0.10.5 and GNU CLISP 2.47 on Fedora 13.

The code follows:

(defcfun ("fork" c-fork) :int)
(defcfun ("execve" c-execve) :int (filename :string) (args :pointer) (env :pointer))
(defcfun ("wait" c-wait) :int (status :pointer))

(defun execute-program (path argv env)
  (let ((child-pid (c-fork)))
    (if (= child-pid -1)
	(error "Unable to create child process")
	(if (= child-pid 0)
	    ;; Child
	    (progn
	      ;; Remember: (nth 0 argv) must be the full path to the executable.
	      (let ((c-argv (foreign-alloc :string :initial-contents (append (list path) argv)
					   :null-terminated-p t))
		    (c-env (foreign-alloc :string :initial-contents env :null-terminated-p t)))
		(c-execve path c-argv c-env)
		(foreign-free c-argv)
		(foreign-free c-env)))
	    ;; Parent code
	    (progn
	      (let ((wait-status (foreign-alloc :int)))
		(c-wait wait-status)
		;; This test condition is a reimplementation of __WIFEXITED macro defined at
		;; /usr/include/bits/waitstatus.h
		(if (= 0 (logand (mem-ref wait-status :int) #x7f))
		    ;; This test condition is a reimplementation of __WEXITSTATUS macro defined at
		    ;; /usr/include/bits/waitstatus.h
		    (when (/= 0 (ash (logand (mem-ref wait-status :int) #xff00) -8))
			(error "Child returned an error"))
		    (error "Failed to execute child process"))
		(foreign-free wait-status)))))))


-- 
Lorenzo Villani <lvillani at binaryhelix.net>
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