diff --git a/lib/spdylay_frame.h b/lib/spdylay_frame.h
index e06f7da3..2d02f2ca 100644
--- a/lib/spdylay_frame.h
+++ b/lib/spdylay_frame.h
@@ -683,7 +683,8 @@ void spdylay_frame_window_update_free(spdylay_window_update *frame);
 
 /*
  * Initializes SETTINGS frame |frame| with given values. |frame| takes
- * ownership of |iv|, so caller must not free it.
+ * ownership of |iv|, so caller must not free it. The |flags| are
+ * bitwise-OR of one or more of spdylay_settings_flag.
  */
 void spdylay_frame_settings_init(spdylay_settings *frame,
                                  uint16_t version, uint8_t flags,
diff --git a/lib/spdylay_map.c b/lib/spdylay_map.c
index eef9165e..cfddd197 100644
--- a/lib/spdylay_map.c
+++ b/lib/spdylay_map.c
@@ -197,20 +197,24 @@ size_t spdylay_map_size(spdylay_map *map)
   return map->size;
 }
 
-static void for_each(spdylay_map_entry *entry,
-                     void (*func)(key_type key, void *val, void *ptr),
-                     void *ptr)
+static int for_each(spdylay_map_entry *entry,
+                    int (*func)(key_type key, void *val, void *ptr),
+                    void *ptr)
 {
-  if(entry != NULL) {
-    for_each(entry->left, func, ptr);
-    func(entry->key, entry->val, ptr);
-    for_each(entry->right, func, ptr);
+  if(entry) {
+    int rv;
+    if((rv = for_each(entry->left, func, ptr)) != 0 ||
+       (rv = func(entry->key, entry->val, ptr)) != 0 ||
+       (rv = for_each(entry->right, func, ptr)) != 0) {
+      return rv;
+    }
   }
+  return 0;
 }
 
-void spdylay_map_each(spdylay_map *map,
-                      void (*func)(key_type key, void *val, void *ptr),
-                      void *ptr)
+int spdylay_map_each(spdylay_map *map,
+                     int (*func)(key_type key, void *val, void *ptr),
+                     void *ptr)
 {
-  for_each(map->root, func, ptr);
+  return for_each(map->root, func, ptr);
 }
diff --git a/lib/spdylay_map.h b/lib/spdylay_map.h
index 6237c612..87a1c9e0 100644
--- a/lib/spdylay_map.h
+++ b/lib/spdylay_map.h
@@ -102,9 +102,16 @@ size_t spdylay_map_size(spdylay_map *map);
  * Applies the function |func| to each key/item pair in the map |map|
  * with the optional user supplied pointer |ptr|.  This function is
  * useful to free item in the map.
+ *
+ * If the |func| returns 0, this function calls the |func| with the
+ * next key and value pair. If the |func| returns nonzero, it will not
+ * call the |func| for further key and value pair and return the
+ * return value of the |func| immediately.  Thus, this function
+ * returns 0 if all the invocations of the |func| return 0, or nonzero
+ * value which the last invocation of |func| returns.
  */
-void spdylay_map_each(spdylay_map *map,
-                      void (*func)(key_type key, void *val, void *ptr),
-                      void *ptr);
+int spdylay_map_each(spdylay_map *map,
+                     int (*func)(key_type key, void *val, void *ptr),
+                     void *ptr);
 
 #endif /* SPDYLAY_MAP_H */
diff --git a/lib/spdylay_session.c b/lib/spdylay_session.c
index edd4d3ee..e1b593b2 100644
--- a/lib/spdylay_session.c
+++ b/lib/spdylay_session.c
@@ -274,10 +274,11 @@ int spdylay_session_server_new(spdylay_session **session_ptr,
   return r;
 }
 
-static void spdylay_free_streams(key_type key, void *val, void *ptr)
+static int spdylay_free_streams(key_type key, void *val, void *ptr)
 {
   spdylay_stream_free((spdylay_stream*)val);
   free(val);
+  return 0;
 }
 
 static void spdylay_session_ob_pq_free(spdylay_pq *pq)
@@ -1783,28 +1784,56 @@ int spdylay_session_on_rst_stream_received(spdylay_session *session,
   return 0;
 }
 
-static void spdylay_update_initial_window_size_func(key_type key, void *value,
-                                                    void *ptr)
+static int spdylay_update_initial_window_size_func(key_type key, void *value,
+                                                   void *ptr)
 {
-  int32_t *vals;
-  vals = (int32_t*)ptr;
-  spdylay_stream_update_initial_window_size((spdylay_stream*)value,
-                                            vals[0], vals[1]);
+  spdylay_update_window_size_arg *arg;
+  spdylay_stream *stream;
+  arg = (spdylay_update_window_size_arg*)ptr;
+  stream = (spdylay_stream*)value;
+  spdylay_stream_update_initial_window_size(stream,
+                                            arg->new_window_size,
+                                            arg->old_window_size);
+  /* If window size gets positive, push deferred DATA frame to
+     outbound queue. */
+  if(stream->window_size > 0 &&
+     stream->deferred_data &&
+     (stream->deferred_flags & SPDYLAY_DEFERRED_FLOW_CONTROL)) {
+    int rv;
+    rv = spdylay_pq_push(&arg->session->ob_pq, stream->deferred_data);
+    if(rv == 0) {
+      spdylay_stream_detach_deferred_data(stream);
+    } else {
+      /* FATAL */
+      assert(rv < SPDYLAY_ERR_FATAL);
+      return rv;
+    }
+  }
+  return 0;
 }
 
 /*
  * Updates the initial window size of all active streams.
+ * If error occurs, all streams may not be updated.
+ *
+ * This function returns 0 if it succeeds, or one of the following
+ * negative error codes:
+ *
+ * SPDYLAY_ERR_NOMEM
+ *     Out of memory.
  */
-static void spdylay_session_update_initial_window_size
+static int spdylay_session_update_initial_window_size
 (spdylay_session *session,
  int32_t new_initial_window_size)
 {
-  int32_t vals[2];
-  vals[0] = new_initial_window_size;
-  vals[1] = session->remote_settings[SPDYLAY_SETTINGS_INITIAL_WINDOW_SIZE];
-  spdylay_map_each(&session->streams,
-                   spdylay_update_initial_window_size_func,
-                   vals);
+  spdylay_update_window_size_arg arg;
+  arg.session = session;
+  arg.new_window_size = new_initial_window_size;
+  arg.old_window_size =
+    session->remote_settings[SPDYLAY_SETTINGS_INITIAL_WINDOW_SIZE];
+  return spdylay_map_each(&session->streams,
+                          spdylay_update_initial_window_size_func,
+                          &arg);
 }
 
 void spdylay_session_update_local_settings(spdylay_session *session,
@@ -1821,6 +1850,7 @@ void spdylay_session_update_local_settings(spdylay_session *session,
 int spdylay_session_on_settings_received(spdylay_session *session,
                                          spdylay_frame *frame)
 {
+  int rv;
   size_t i;
   int check[SPDYLAY_SETTINGS_MAX+1];
   if(!spdylay_session_check_version(session, frame->settings.hd.version)) {
@@ -1842,12 +1872,14 @@ int spdylay_session_on_settings_received(spdylay_session *session,
       /* Update the initial window size of the all active streams */
       /* Check that initial_window_size < (1u << 31) */
       if(entry->value < (1u << 31)) {
-        spdylay_session_update_initial_window_size(session, entry->value);
+        rv = spdylay_session_update_initial_window_size(session, entry->value);
+        if(rv != 0) {
+          return rv;
+        }
       }
     } else if(entry->settings_id ==
               SPDYLAY_SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE) {
       if(!session->server) {
-        int rv;
         /* Limit certificate vector length in the reasonable size. */
         entry->value = spdylay_min(entry->value,
                                    SPDYLAY_MAX_CLIENT_CERT_VECTOR_LENGTH);
diff --git a/lib/spdylay_session.h b/lib/spdylay_session.h
index c5f8f263..8551023f 100644
--- a/lib/spdylay_session.h
+++ b/lib/spdylay_session.h
@@ -197,6 +197,13 @@ struct spdylay_session {
   void *user_data;
 };
 
+/* Struct used when updating initial window size of each active
+   stream. */
+typedef struct {
+  spdylay_session *session;
+  int32_t new_window_size, old_window_size;
+} spdylay_update_window_size_arg;
+
 /* TODO stream timeout etc */
 
 /*
diff --git a/tests/spdylay_session_test.c b/tests/spdylay_session_test.c
index 24d2fc84..bc4422a2 100644
--- a/tests/spdylay_session_test.c
+++ b/tests/spdylay_session_test.c
@@ -1736,6 +1736,8 @@ void test_spdylay_session_flow_control(void)
   spdylay_frame frame;
   spdylay_stream *stream;
   int32_t new_initial_window_size;
+  spdylay_settings_entry iv[1];
+  spdylay_frame settings_frame;
 
   memset(&callbacks, 0, sizeof(spdylay_session_callbacks));
   callbacks.send_callback = null_send_callback;
@@ -1769,6 +1771,8 @@ void test_spdylay_session_flow_control(void)
   stream->window_size = new_initial_window_size-
     (session->remote_settings[SPDYLAY_SETTINGS_INITIAL_WINDOW_SIZE]
      -stream->window_size);
+  session->remote_settings[SPDYLAY_SETTINGS_INITIAL_WINDOW_SIZE] =
+    new_initial_window_size;
   CU_ASSERT(-48*1024 == stream->window_size);
 
   /* Back 48KiB */
@@ -1787,9 +1791,15 @@ void test_spdylay_session_flow_control(void)
   CU_ASSERT(0 == spdylay_session_send(session));
   CU_ASSERT(16*1024 == ud.data_source_length);
 
-  /* Back 16KiB */
-  frame.window_update.delta_window_size = 16*1024;
-  spdylay_session_on_window_update_received(session, &frame);
+  /* Increase initial window size to 32KiB */
+  iv[0].settings_id = SPDYLAY_SETTINGS_INITIAL_WINDOW_SIZE;
+  iv[0].flags = SPDYLAY_ID_FLAG_SETTINGS_NONE;
+  iv[0].value = 32*1024;
+
+  spdylay_frame_settings_init(&settings_frame.settings, SPDYLAY_PROTO_SPDY3,
+                              SPDYLAY_FLAG_SETTINGS_NONE, dup_iv(iv, 1), 1);
+  spdylay_session_on_settings_received(session, &settings_frame);
+  spdylay_frame_settings_free(&settings_frame.settings);
 
   /* Sends another 16KiB data */
   CU_ASSERT(0 == spdylay_session_send(session));