diff --git a/klippy/reactor.py b/klippy/reactor.py
index 9bce6a9b..76e7d65e 100644
--- a/klippy/reactor.py
+++ b/klippy/reactor.py
@@ -1,17 +1,27 @@
 # File descriptor and timer event helper
 #
-# Copyright (C) 2016,2017  Kevin O'Connor <kevin@koconnor.net>
+# Copyright (C) 2016-2018  Kevin O'Connor <kevin@koconnor.net>
 #
 # This file may be distributed under the terms of the GNU GPLv3 license.
-import select, math, time
+import os, select, math, time, Queue
 import greenlet
-import chelper
+import chelper, util
 
 class ReactorTimer:
     def __init__(self, callback, waketime):
         self.callback = callback
         self.waketime = waketime
 
+class ReactorCallback:
+    def __init__(self, reactor, callback):
+        self.reactor = reactor
+        self.timer = reactor.register_timer(self.invoke, reactor.NOW)
+        self.callback = callback
+    def invoke(self, eventtime):
+        self.reactor.unregister_timer(self.timer)
+        self.callback(eventtime)
+        return self.reactor.NEVER
+
 class ReactorFileHandler:
     def __init__(self, fd, callback):
         self.fd = fd
@@ -28,13 +38,20 @@ class SelectReactor:
     NOW = 0.
     NEVER = 9999999999999999.
     def __init__(self):
-        self._fds = []
+        # Main code
+        self._process = False
+        self.monotonic = chelper.get_ffi()[1].get_monotonic
+        # Timers
         self._timers = []
         self._next_timer = self.NEVER
-        self._process = False
+        # Callbacks
+        self._pipe_fds = None
+        self._async_queue = Queue.Queue()
+        # File descriptors
+        self._fds = []
+        # Greenlets
         self._g_dispatch = None
         self._greenlets = []
-        self.monotonic = chelper.get_ffi()[1].get_monotonic
     # Timers
     def _note_time(self, t):
         nexttime = t.waketime
@@ -70,6 +87,36 @@ class SelectReactor:
         if eventtime >= self._next_timer:
             return 0.
         return min(1., max(.001, self._next_timer - self.monotonic()))
+    # Callbacks
+    def register_callback(self, callback):
+        ReactorCallback(self, callback)
+    def register_async_callback(self, callback):
+        self._async_queue.put_nowait(callback)
+        try:
+            os.write(self._pipe_fds[1], '.')
+        except os.error:
+            pass
+    def _got_pipe_signal(self, eventtime):
+        try:
+            os.read(self._pipe_fds[0], 4096)
+        except os.error:
+            pass
+        while 1:
+            try:
+                callback = self._async_queue.get_nowait()
+            except Queue.Empty:
+                break
+            ReactorCallback(self, callback)
+    def _setup_async_callbacks(self):
+        self._pipe_fds = os.pipe()
+        util.set_nonblock(self._pipe_fds[0])
+        util.set_nonblock(self._pipe_fds[1])
+        self.register_fd(self._pipe_fds[0], self._got_pipe_signal)
+    def __del__(self):
+        if self._pipe_fds is not None:
+            os.close(self._pipe_fds[0])
+            os.close(self._pipe_fds[1])
+            self._pipe_fds = None
     # Greenlets
     def _sys_pause(self, waketime):
         # Pause using system sleep for when reactor not running
@@ -91,10 +138,13 @@ class SelectReactor:
         g.timer = self.register_timer(g.switch, waketime)
         return g_next.switch()
     def _end_greenlet(self, g_old):
+        # Cache this greenlet for later use
         self._greenlets.append(g_old)
         self.unregister_timer(g_old.timer)
         g_old.timer = None
+        # Switch to existing dispatch
         self._g_dispatch.switch(self.NEVER)
+        # This greenlet was reactivated - prepare for main processing loop
         self._g_dispatch = g_old
     # File descriptors
     def register_fd(self, fd, callback):
@@ -119,6 +169,8 @@ class SelectReactor:
                     break
         self._g_dispatch = None
     def run(self):
+        if self._pipe_fds is None:
+            self._setup_async_callbacks()
         self._process = True
         g_next = ReactorGreenlet(run=self._dispatch_loop)
         g_next.switch()