cl-stream/unistd-stream

diff --git a/unistd-stream.lisp b/unistd-stream.lisp
index 9b1fa08..5fa8423 100644
--- a/unistd-stream.lisp
+++ b/unistd-stream.lisp
@@ -160,32 +160,35 @@ UNISTD:READ and UNISTD:WRITE."))
   "Creates a buffered input/output stream for file descriptor FD."
   (make-instance 'unistd-io-stream :fd fd))
 
+(defun compute-flags (read write append non-blocking create)
+  (logior (if append              fcntl:+o-append+   0)
+          (if non-blocking        fcntl:+o-nonblock+ 0)
+          (if create              fcntl:+o-creat+    0)
+          (cond ((and read write) fcntl:+o-rdwr+)
+                (read             fcntl:+o-rdonly+)
+                (write            fcntl:+o-wronly+)
+                (t 0))))
+
+(defun compute-class (read write)
+  (cond ((and read write) 'unistd-io-stream)
+        (read             'unistd-input-stream)
+        (write            'unistd-output-stream)))
+
 (defun unistd-stream-open (pathname &key
-                                      (append nil)
-                                      (blocking t)
-                                      (create t)
-                                      (direction :io)
-                                      (input-buffer-size *stream-default-buffer-size*)
-                                      (mode #o777)
-                                      (output-buffer-size *stream-default-buffer-size*))
-  (let* ((flags (logior (if append
-                            fcntl:+o-append+
-                            0)
-                        (if blocking
-                            0
-                            fcntl:+o-nonblock+)
-                        (if create
-                            fcntl:+o-creat+
-                            0)
-                        (ecase direction
-                          ((:input) fcntl:+o-rdonly+)
-                          ((:output) fcntl:+o-wronly+)
-                          ((:io) fcntl:+o-rdwr+))))
-         (fd (fcntl:open pathname flags mode)))
-    (make-instance (case direction
-                     ((:input) 'unistd-input-stream)
-                     ((:output) 'unistd-output-stream)
-                     ((:io) 'unistd-io-stream))
+                                      read write append
+                                      non-blocking
+                                      (create #o777)
+                                      (input-buffer-size
+                                       *stream-default-buffer-size*)
+                                      (output-buffer-size
+                                       *stream-default-buffer-size*))
+  (assert (or read write)
+          (read write)
+          "Open not for reading nor writing.")
+  (let* ((flags (compute-flags read write append non-blocking create))
+         (fd (fcntl:open pathname flags (or create 0)))
+         (class (compute-class read write)))
+    (make-instance class
                    :fd fd
                    :input-buffer-size input-buffer-size
                    :output-buffer-size output-buffer-size)))