cffi-posix/cffi-unistd

diff --git a/cffi-unistd.lisp b/cffi-unistd.lisp
index 5edb30e..b26355d 100644
--- a/cffi-unistd.lisp
+++ b/cffi-unistd.lisp
@@ -18,6 +18,9 @@
 
 (in-package :cffi-unistd)
 
+(deftype file-descriptor ()
+  '(unsigned-byte 31))
+
 (defcfun ("access" c-access) :int
   (name :string)
   (type :int))
diff --git a/package.lisp b/package.lisp
index 3bb20d5..6c652b9 100644
--- a/package.lisp
+++ b/package.lisp
@@ -132,6 +132,7 @@
    #:fd-set-filter
    #:fd-zero
    #:fds-bits
+   #:file-descriptor
    #:fork
    #:getcwd
    #:getegid