diff --git a/racket/src/rktio/README.txt b/racket/src/rktio/README.txt
new file mode 100644
index 0000000000..9432621fa7
--- /dev/null
+++ b/racket/src/rktio/README.txt
@@ -0,0 +1,18 @@
+The "rktio" library (for "Racket I/O") is a layer just above the OS
+layer to provide a portable interface to filesystem, networking, etc.
+facilities.
+
+The library is meant to be
+
+ * easily embeddable;
+
+ * always non-blocking;
+
+ * independent of global state (except on Windows, where internal
+   global state is managed appropriately with locks), so that it works
+   with or without threads; and
+
+ * easily callable though a FFI.
+
+Many such libraries exist already. This one happens to have exactly
+the things that a Racket implementation needs.
diff --git a/racket/src/rktio/rktio_poll_set.c b/racket/src/rktio/rktio_poll_set.c
index 3c897c8490..70cae53fda 100644
--- a/racket/src/rktio/rktio_poll_set.c
+++ b/racket/src/rktio/rktio_poll_set.c
@@ -824,12 +824,20 @@ int rktio_get_fd_limit(rktio_poll_set_t *fds)
   int actual_limit;
 
 # ifdef STORED_ACTUAL_FDSET_LIMIT
-  actual_limit = FDSET_LIMIT(rd);
-  if (FDSET_LIMIT(wr) > actual_limit)
-    actual_limit = FDSET_LIMIT(wr);
-  if (FDSET_LIMIT(ex) > actual_limit)
-    actual_limit = FDSET_LIMIT(ex);
-  actual_limit++;
+  {
+    fd_set *rd, *wr, *ex;
+
+    rd = RKTIO_FDS(fds);
+    wr = RKTIO_FDS(RKTIO_GET_FDSET(fds, 1));
+    ex = RKTIO_FDS(RKTIO_GET_FDSET(fds, 2));
+
+    actual_limit = FDSET_LIMIT(rd);
+    if (FDSET_LIMIT(wr) > actual_limit)
+      actual_limit = FDSET_LIMIT(wr);
+    if (FDSET_LIMIT(ex) > actual_limit)
+      actual_limit = FDSET_LIMIT(ex);
+    actual_limit++;
+  }
 # elif defined (USE_ULIMIT)
   actual_limit = ulimit(4, 0);
 #elif defined(FIXED_FD_LIMIT)
diff --git a/racket/src/rktio/rktio_private.h b/racket/src/rktio/rktio_private.h
index 07c83d0d5d..98b5521bf1 100644
--- a/racket/src/rktio/rktio_private.h
+++ b/racket/src/rktio/rktio_private.h
@@ -16,6 +16,13 @@
 # include <pthread.h>
 #endif
 
+#if defined(__linux__) || defined(OS_X) || defined(__NetBSD__) \
+    || defined(__NetBSD__) || defined(__OpenBSD__) \
+    || defined(__FreeBSD__) || defined(__FreeBSD_kernel__) \
+    || defined(__DragonFly__) || defined(__QNX__)
+# define USE_DYNAMIC_FDSET_SIZE
+#endif
+
 #if RKTIO_SYSTEM_WINDOWS
 # define USE_FAR_RKTIO_FDCALLS
 #endif
@@ -85,7 +92,7 @@ void rktio_fdset(rktio_poll_set_t *fd, int n);
 void rktio_fdclr(rktio_poll_set_t *fd, int n);
 int rktio_fdisset(rktio_poll_set_t *fd, int n);
   
-# define DECL_FDSET(n, c) fd_set *n
+# define DECL_FDSET(n, c) rktio_poll_set_t *n
 # define INIT_DECL_FDSET(r, w, e) { \
    r = RKTIO_GET_FDSET(rktio->rktio_global_poll_set, 0 ); \
    w = RKTIO_GET_FDSET(rktio->rktio_global_poll_set, 1 ); \
@@ -102,7 +109,7 @@ int rktio_fdisset(rktio_poll_set_t *fd, int n);
 # define RKTIO_FD_ISSET(n, p) rktio_fdisset(p, n)
 
 # if !defined(HAVE_POLL_SYSCALL) && !defined(RKTIO_SYSTEM_WINDOWS)
-#  define RKTIO_FDS(p) ((fd_set *)fds)
+#  define RKTIO_FDS(p) ((fd_set *)p)
 # endif
 
 #else