diff --git a/lib/process_shared/mach.rb b/lib/process_shared/mach.rb
new file mode 100644
index 0000000..6ef87fc
--- /dev/null
+++ b/lib/process_shared/mach.rb
@@ -0,0 +1,93 @@
+require 'set'
+require 'mach'
+
+module ProcessShared
+  module Mach
+    include ::Mach
+
+    # The set of ports that should be shared to forked child
+    # processes.
+    #
+    # FIXME: protect with (original ruby) mutex?
+    def self.shared_ports
+      @shared_ports ||= Set.new
+    end
+
+    def self.after_fork_child
+      parent_port = Task.self.get_bootstrap_port
+
+      # give parent permission to send to child's task port
+      Task.self.copy_send(parent_port)
+
+      # create a second port and give the parent permission to send
+      port = Port.new
+      port.insert_right(:make_send)
+      port.copy_send(parent_port)
+
+      # parent copies sem, mutex port permissions directly to child
+      # task port
+
+      # wait for parent to send orig bootstrap port
+      orig_bootstrap = port.receive_right
+      Task.self.set_special_port(:bootstrap, orig_bootstrap)
+    end
+
+    def self.after_fork_parent(port)
+      child_task_port = port.receive_right
+      shared_ports.each do |p|
+        p.insert_right(:copy_send, :ipc_space => child_task_port)
+      end
+
+      child_port = port.receive_right
+      ::Mach::bootstrap_port.copy_send(child_port)
+    end
+  end
+end
+
+module Kernel
+  # Override to call Process::fork.
+  def fork(*args, &block)
+    Process.fork(*args, &block)
+  end
+end
+
+module Process
+  class << self
+    unless respond_to? :__mach_original_fork__
+      alias_method :__mach_original_fork__, :fork
+    end
+
+    # Override to first copy all shared ports (semaphores, etc.) from
+    # parent process to child process.
+    def fork
+      # make a port for receiving message from child
+      port = Mach::Port.new
+      port.insert_right(:make_send)
+      Mach::Task.self.set_bootstrap_port(port)
+      
+      if block_given?
+        pid = __mach_original_fork__ do
+          ProcessShared::Mach.after_fork_child
+          yield
+        end
+
+        ProcessShared::Mach.after_fork_parent(port)
+        pid
+      else
+        if pid = __mach_original_fork__
+          ProcessShared::Mach.after_fork_parent(port)
+          pid
+        else
+          ProcessShared::Mach.after_fork_child
+          nil
+        end
+      end
+    end
+  end
+end
+
+require 'mach/time_spec'
+require 'process_shared/time_spec'
+
+# Monkey patch to add #add_seconds! method
+Mach::TimeSpec.send(:include, ProcessShared::TimeSpec)
diff --git a/lib/process_shared/mach/semaphore.rb b/lib/process_shared/mach/semaphore.rb
new file mode 100644
index 0000000..a0fce59
--- /dev/null
+++ b/lib/process_shared/mach/semaphore.rb
@@ -0,0 +1,39 @@
+require 'mach'
+require 'mach/error'
+
+require 'process_shared/mach'
+
+module ProcessShared
+  module Mach
+    # Extends ::Mach::Semaphore to be compatible with ProcessShared::Semaphore
+    class Semaphore < ::Mach::Semaphore
+      include ProcessShared::Semaphore
+
+      def initialize(value = 1)
+        super(:value => value)
+        ProcessShared::Mach.shared_ports.add self
+      end
+
+      def try_wait(timeout = nil)
+        secs = timeout ? timeout : 0
+        begin
+          # TODO catch and convert exceptions...
+          timedwait(secs)
+        rescue Mach::Error::OPERATION_TIMED_OUT => e
+          klass = secs == 0 ? Errno::EAGAIN : Errno::ETIMEDOUT
+          raise klass, e.message
+        end
+      end
+
+      alias_method :post, :signal
+
+      def value
+        raise Errno::ENOTSUP
+      end
+
+      def close
+        # TODO
+      end
+    end
+  end
+end
diff --git a/spec/process_shared/semaphore_spec.rb b/spec/process_shared/semaphore_spec.rb
index 3e3fa1c..3332a8e 100644
--- a/spec/process_shared/semaphore_spec.rb
+++ b/spec/process_shared/semaphore_spec.rb
@@ -98,14 +98,13 @@ module ProcessShared
 
       it 'returns after waiting if another processes posts' do
         Semaphore.open(0) do |sem|
-          start = Time.now.to_f
-
           pid = fork do
             sleep 0.01
             sem.post
             Kernel.exit!
           end
 
+          start = Time.now.to_f
           sem.try_wait(0.1)
           (Time.now.to_f - start).must be_lt(0.1)