diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 7484d9c6..5213446f 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -22,6 +22,7 @@ extern "C" {
 #endif
 #if SEXP_USE_GREEN_THREADS
 #include <sys/time.h>
+#include <sys/errno.h>
 #endif
 #endif
 
@@ -887,6 +888,8 @@ enum sexp_context_globals {
   SEXP_G_THREADS_LOCAL,
   SEXP_G_THREADS_SIGNALS,
   SEXP_G_THREADS_SIGNAL_RUNNER,
+  SEXP_G_THREADS_POLL_FDS,
+  SEXP_G_THREADS_BLOCKER,
 #endif
   SEXP_G_NUM_GLOBALS
 };
diff --git a/lib/srfi/18/threads.c b/lib/srfi/18/threads.c
index b84d59f4..88da3307 100644
--- a/lib/srfi/18/threads.c
+++ b/lib/srfi/18/threads.c
@@ -275,6 +275,10 @@ static sexp sexp_get_signal_handler (sexp ctx sexp_api_params(self, n), sexp sig
   return sexp_vector_ref(sexp_global(ctx, SEXP_G_SIGNAL_HANDLERS), signum);
 }
 
+static sexp sexp_blocker (sexp ctx sexp_api_params(self, n), sexp port) {
+  return SEXP_VOID;
+}
+
 sexp sexp_scheduler (sexp ctx sexp_api_params(self, n), sexp root_thread) {
   struct timeval tval;
   sexp res, ls1, ls2, runner, paused, front;
@@ -284,7 +288,7 @@ sexp sexp_scheduler (sexp ctx sexp_api_params(self, n), sexp root_thread) {
   front  = sexp_global(ctx, SEXP_G_THREADS_FRONT);
   paused = sexp_global(ctx, SEXP_G_THREADS_PAUSED);
 
-  /* check for signals */
+  /* check signals */
   if (sexp_global(ctx, SEXP_G_THREADS_SIGNALS) != SEXP_ZERO) {
     runner = sexp_global(ctx, SEXP_G_THREADS_SIGNAL_RUNNER);
     if (! sexp_contextp(runner)) { /* ensure the runner exists */
@@ -302,6 +306,10 @@ sexp sexp_scheduler (sexp ctx sexp_api_params(self, n), sexp root_thread) {
     }
   }
 
+  /* check blocked fds */
+  /* if () { */
+  /* } */
+
   /* if we've terminated, check threads joining us */
   if (sexp_context_refuel(ctx) <= 0) {
     for (ls1=SEXP_NULL, ls2=paused; sexp_pairp(ls2); ) {
@@ -411,7 +419,9 @@ sexp sexp_init_library (sexp ctx sexp_api_params(self, n), sexp env) {
   sexp_define_foreign(ctx, env, "get-signal-handler", 1, sexp_get_signal_handler);
 
   sexp_global(ctx, SEXP_G_THREADS_SCHEDULER)
-    = sexp_make_foreign(ctx, "scheduler", 0, 0, (sexp_proc1)sexp_scheduler, SEXP_FALSE);
+    = sexp_make_foreign(ctx, "scheduler", 1, 0, (sexp_proc1)sexp_scheduler, SEXP_FALSE);
+  sexp_global(ctx, SEXP_G_THREADS_BLOCKER)
+    = sexp_make_foreign(ctx, "blocker", 1, 0, (sexp_proc1)sexp_blocker, SEXP_FALSE);
 
   /* remember the env to lookup the runner later */
   sexp_global(ctx, SEXP_G_THREADS_SIGNAL_RUNNER) = env;
diff --git a/vm.c b/vm.c
index acbea8b2..f14a9650 100644
--- a/vm.c
+++ b/vm.c
@@ -1303,14 +1303,36 @@ sexp sexp_vm (sexp ctx, sexp proc) {
       _ARG1 = sexp_read_utf8_char(ctx, _ARG1, i);
     else
 #endif
-    _ARG1 = (i == EOF) ? SEXP_EOF : sexp_make_character(i);
+    if (i == EOF) {
+#if SEXP_USE_GREEN_THREADS
+      if ((errno == EAGAIN)
+          && sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER))) {
+        sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG1);
+        fuel = 0;
+        ip--;      /* try again */
+      } else
+#endif
+        _ARG1 = SEXP_EOF;
+    } else
+      _ARG1 = sexp_make_character(i);
     break;
   case SEXP_OP_PEEK_CHAR:
     if (! sexp_iportp(_ARG1))
       sexp_raise("peek-char: not an input-port", sexp_list1(ctx, _ARG1));
     i = sexp_read_char(ctx, _ARG1);
     sexp_push_char(ctx, i, _ARG1);
-    _ARG1 = (i == EOF) ? SEXP_EOF : sexp_make_character(i);
+    if (i == EOF) {
+#if SEXP_USE_GREEN_THREADS
+      if ((errno == EAGAIN)
+          && sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER))) {
+        sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG1);
+        fuel = 0;
+        ip--;      /* try again */
+      } else
+#endif
+        _ARG1 = SEXP_EOF;
+    } else
+      _ARG1 = sexp_make_character(i);
     break;
   case SEXP_OP_YIELD:
     fuel = 0;