diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index c7dbfc7ac..b7296bb5f 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,10 @@
+2011-03-28  Jon TURNEY  <jon.turney@dronecode.org.uk>
+
+	* thread.cc (semaphore::init, destroy, close): Standards conformance
+	fix.  On a failure, return -1 and set errno.
+	* thread.h (semaphore::terminate): Save errno since semaphore::close()
+	may now modify it.
+
 2011-03-27  Yaakov Selkowitz  <yselkowitz@users.sourceforge.net>
 
 	* cygwin.din (strchrnul): Export.
diff --git a/winsup/cygwin/thread.cc b/winsup/cygwin/thread.cc
index 628d1d13f..891b43fb9 100644
--- a/winsup/cygwin/thread.cc
+++ b/winsup/cygwin/thread.cc
@@ -3076,10 +3076,16 @@ semaphore::init (sem_t *sem, int pshared, unsigned int value)
 {
   /* opengroup calls this undefined */
   if (is_good_object (sem))
-    return EBUSY;
+    {
+      set_errno(EBUSY);
+      return -1;
+    }
 
   if (value > SEM_VALUE_MAX)
-    return EINVAL;
+    {
+      set_errno(EINVAL);
+      return -1;
+    }
 
   *sem = new semaphore (pshared, value);
 
@@ -3087,7 +3093,8 @@ semaphore::init (sem_t *sem, int pshared, unsigned int value)
     {
       delete (*sem);
       *sem = NULL;
-      return EAGAIN;
+      set_errno(EAGAIN);
+      return -1;
     }
   return 0;
 }
@@ -3096,11 +3103,17 @@ int
 semaphore::destroy (sem_t *sem)
 {
   if (!is_good_object (sem))
-    return EINVAL;
+    {
+      set_errno(EINVAL);
+      return -1;
+    }
 
   /* It's invalid to destroy a semaphore not opened with sem_init. */
   if ((*sem)->fd != -1)
-    return EINVAL;
+    {
+      set_errno(EINVAL);
+      return -1;
+    }
 
   /* FIXME - new feature - test for busy against threads... */
 
@@ -3113,11 +3126,17 @@ int
 semaphore::close (sem_t *sem)
 {
   if (!is_good_object (sem))
-    return EINVAL;
+    {
+      set_errno(EINVAL);
+      return -1;
+    }
 
   /* It's invalid to close a semaphore not opened with sem_open. */
   if ((*sem)->fd == -1)
-    return EINVAL;
+    {
+      set_errno(EINVAL);
+      return -1;
+    }
 
   delete (*sem);
   delete sem;
diff --git a/winsup/cygwin/thread.h b/winsup/cygwin/thread.h
index 62bd1b959..bea8e2aa3 100644
--- a/winsup/cygwin/thread.h
+++ b/winsup/cygwin/thread.h
@@ -21,6 +21,7 @@ details. */
 #include <limits.h>
 #include "security.h"
 #include <errno.h>
+#include "cygerrno.h"
 
 enum cw_sig_wait
 {
@@ -641,6 +642,7 @@ public:
   }
   static void terminate ()
   {
+    save_errno save;
     semaphores.for_each (&semaphore::_terminate);
   }