Index: include/ruby/intern.h
===================================================================
--- include/ruby/intern.h	(revision 23101)
+++ include/ruby/intern.h	(working copy)
@@ -213,6 +213,7 @@
 void rb_fd_clr(int, rb_fdset_t *);
 int rb_fd_isset(int, const rb_fdset_t *);
 void rb_fd_copy(rb_fdset_t *, const fd_set *, int);
+void rb_fd_resize(int, rb_fdset_t *);
 int rb_fd_select(int, rb_fdset_t *, rb_fdset_t *, rb_fdset_t *, struct timeval *);
 
 #define rb_fd_ptr(f)	((f)->fdset)
@@ -231,6 +232,7 @@
 void rb_fd_set(int, rb_fdset_t *);
 #define rb_fd_clr(n, f)		rb_w32_fdclr(n, (f)->fdset)
 #define rb_fd_isset(n, f)	rb_w32_fdisset(n, (f)->fdset)
+#define rb_fd_resize(n, f)	(void)(f)
 #define rb_fd_select(n, rfds, wfds, efds, timeout)	rb_w32_select(n, (rfds) ? ((rb_fdset_t*)rfds)->fdset : NULL, (wfds) ? ((rb_fdset_t*)wfds)->fdset : NULL, (efds) ? ((rb_fdset_t*)efds)->fdset: NULL, timeout)
 
 #define rb_fd_ptr(f)	((f)->fdset)
@@ -244,6 +246,7 @@
 #define rb_fd_clr(n, f)	FD_CLR(n, f)
 #define rb_fd_isset(n, f) FD_ISSET(n, f)
 #define rb_fd_copy(d, s, n) (*(d) = *(s))
+#define rb_fd_resize(n, f)	(void)(f)
 #define rb_fd_ptr(f)	(f)
 #define rb_fd_init(f)	FD_ZERO(f)
 #define rb_fd_term(f)	(void)(f)
Index: io.c
===================================================================
--- io.c	(revision 23101)
+++ io.c	(working copy)
@@ -6696,6 +6696,10 @@
 	ep = 0;
     }
 
+    if (rp) { rb_fd_resize(max, &fds[0]); rp = rb_fd_ptr(&fds[0]); }
+    if (wp) { rb_fd_resize(max, &fds[1]); wp = rb_fd_ptr(&fds[1]); }
+    if (ep) { rb_fd_resize(max, &fds[2]); ep = rb_fd_ptr(&fds[2]); }
+
     max++;
 
     n = rb_thread_select(max, rp, wp, ep, tp);
Index: thread.c
===================================================================
--- thread.c	(revision 23101)
+++ thread.c	(working copy)
@@ -2174,7 +2174,7 @@
     }
 }
 
-static void
+void
 rb_fd_resize(int n, rb_fdset_t *fds)
 {
     int m = howmany(n + 1, NFDBITS) * sizeof(fd_mask);