diff --git a/lib/mach/functions.rb b/lib/mach/functions.rb
index d5cd8e7..eda7628 100644
--- a/lib/mach/functions.rb
+++ b/lib/mach/functions.rb
@@ -72,8 +72,8 @@ module Mach
 
     # Attach a function as with +attach_function+, but check the
     # return value and raise an exception on errors.
-    def self.attach_mach_function(sym, argtypes, rettype)
-      attach_function(sym, argtypes, rettype)
+    def self.attach_mach_function(sym, argtypes, rettype, options = nil)
+      attach_function(sym, argtypes, rettype, options)
       error_check(sym)
     end
 
@@ -208,10 +208,12 @@ module Mach
                          :kern_return_t)
     attach_mach_function(:semaphore_wait,
                          [:semaphore_t],
-                         :kern_return_t)
+                         :kern_return_t,
+                         :blocking => true)
     attach_mach_function(:semaphore_timedwait,
                          [:semaphore_t, TimeSpec.val],
-                         :kern_return_t)
+                         :kern_return_t,
+                         :blocking => true)
 
   end
 end
diff --git a/lib/process_shared/posix/semaphore.rb b/lib/process_shared/posix/semaphore.rb
index e75028f..d1fe59e 100644
--- a/lib/process_shared/posix/semaphore.rb
+++ b/lib/process_shared/posix/semaphore.rb
@@ -26,9 +26,12 @@ module ProcessShared
 
         attach_function :sem_getvalue, [:sem_p, :pointer], :int
         attach_function :sem_post, [:sem_p], :int
-        attach_function :sem_wait, [:sem_p], :int
-        attach_function :sem_trywait, [:sem_p], :int
-        attach_function :sem_timedwait, [:sem_p, TimeSpec], :int
+        attach_function :sem_wait, [:sem_p], :int, :blocking => true
+        attach_function :sem_trywait, [:sem_p], :int, :blocking => true
+
+        # Workaround bug which only appears to affect Ruby 1.8.7 and REE
+        BLOCKING_SEM_TIMEDWAIT = (RUBY_VERSION != '1.8.7')
+        attach_function :sem_timedwait, [:sem_p, TimeSpec], :int, :blocking => BLOCKING_SEM_TIMEDWAIT
 
         error_check(:sem_close, :sem_unlink, :sem_init, :sem_destroy,
                     :sem_getvalue, :sem_post, :sem_wait, :sem_trywait,
diff --git a/spec/process_shared/lock_behavior.rb b/spec/process_shared/lock_behavior.rb
index 77f97bc..7e49869 100644
--- a/spec/process_shared/lock_behavior.rb
+++ b/spec/process_shared/lock_behavior.rb
@@ -55,6 +55,5 @@ module ProcessShared
       mem.get_char(0).must_equal(0)
     end
 
-
   end
 end
diff --git a/spec/process_shared/semaphore_spec.rb b/spec/process_shared/semaphore_spec.rb
index c6afad1..a0a844c 100644
--- a/spec/process_shared/semaphore_spec.rb
+++ b/spec/process_shared/semaphore_spec.rb
@@ -91,6 +91,32 @@ module ProcessShared
           end
         end
       end
+
+      it 'allows other threads in a process to continue while waiting' do
+        sem = Semaphore.new
+        was_set = false
+        t2 = nil
+
+        sem.synchronize do
+          t1 = Thread.new do
+            # give t2 a chance to wait on the lock, then set the flag
+            sleep 0.01
+            was_set = true
+          end
+
+          t2 = Thread.new do
+            sem.synchronize { }
+          end
+
+          # t1 should set the flag and die while t2 is still waiting on the lock
+          t1.join
+        end
+
+        was_set.must_equal true
+
+        t2.join
+      end
+
     end
 
     describe '#try_wait' do
@@ -131,6 +157,39 @@ module ProcessShared
           ::Process.wait(pid)
         end
       end
+
+      unless RUBY_VERSION == '1.8.7'
+        it 'allows other threads in a process to continue while waiting' do
+          start = Time.now.to_f
+          sem = Semaphore.new
+          was_set = false
+          t2 = nil
+
+          sem.synchronize do
+            t1 = Thread.new do
+              # give t2 a chance to wait on the lock, then set the flag
+              sleep 0.01
+              was_set = true
+            end
+
+            t2 = Thread.new do
+              begin
+                sem.try_wait(10.0)
+              rescue Errno::ETIMEDOUT
+                # success
+              end
+            end
+
+            # t1 should set the flag and die while t2 is still waiting on the lock
+            t1.join
+          end
+
+          was_set.must_equal true
+          (Time.now.to_f - start).must be_lt(0.1)
+
+          t2.join
+        end
+      end
     end
   end
 end