diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index 31b429ba1..01bb3eb68 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,7 @@
+2015-06-30  Corinna Vinschen  <corinna@vinschen.de>
+
+	* signal.cc (sigaltstack): Add comment.
+
 2015-06-27  Corinna Vinschen  <corinna@vinschen.de>
 
 	* exceptions.cc (_cygtls::call_signal_handler): Drop manipulating
diff --git a/winsup/cygwin/signal.cc b/winsup/cygwin/signal.cc
index 467c919c3..2b9cfda3e 100644
--- a/winsup/cygwin/signal.cc
+++ b/winsup/cygwin/signal.cc
@@ -670,6 +670,10 @@ sigaltstack (const stack_t *ss, stack_t *oss)
 	{
 	  char stack_marker;
 	  memcpy (oss, &me.altstack, sizeof *oss);
+	  /* Check if the current stack is the alternate signal stack.  If so,
+	     set ss_flags accordingly.  We do this here rather than setting
+	     ss_flags in _cygtls::call_signal_handler since the signal handler
+	     calls longjmp, so we never return to reset the flag. */
 	  if (!me.altstack.ss_flags && me.altstack.ss_sp)
 	    {
 	      if (&stack_marker >= (char *) me.altstack.ss_sp