diff -r 51627344fc58 Modules/signalmodule.c
--- a/Modules/signalmodule.c	Mon Aug 22 18:05:56 2016 +0300
+++ b/Modules/signalmodule.c	Wed Aug 24 00:04:12 2016 +0900
@@ -962,7 +962,11 @@
     PyStructSequence_SET_ITEM(result, 4, _PyLong_FromUid(si->si_uid));
     PyStructSequence_SET_ITEM(result, 5,
                                 PyLong_FromLong((long)(si->si_status)));
+#ifdef HAVE_SIGINFO_T_SI_BAND
     PyStructSequence_SET_ITEM(result, 6, PyLong_FromLong(si->si_band));
+#else
+    PyStructSequence_SET_ITEM(result, 6, PyLong_FromLong(0L));
+#endif
     if (PyErr_Occurred()) {
         Py_DECREF(result);
         return NULL;
diff -r 51627344fc58 configure
--- a/configure	Mon Aug 22 18:05:56 2016 +0300
+++ b/configure	Wed Aug 24 00:04:12 2016 +0900
@@ -12839,6 +12827,17 @@
 
 fi
 
+# Issue #21085: In Cygwin, siginfo_t does not have si_band field.
+ac_fn_c_check_member "$LINENO" "siginfo_t" "si_band" "ac_cv_member_siginfo_t_si_band" "$ac_includes_default"
+if test "x$ac_cv_member_siginfo_t_si_band" = xyes; then :
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_SIGINFO_T_SI_BAND 1
+_ACEOF
+
+
+fi
+
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for time.h that defines altzone" >&5
 $as_echo_n "checking for time.h that defines altzone... " >&6; }
diff -r 51627344fc58 configure.ac
--- a/configure.ac	Mon Aug 22 18:05:56 2016 +0300
+++ b/configure.ac	Wed Aug 24 00:04:12 2016 +0900
@@ -3753,6 +3753,8 @@
 AC_CHECK_MEMBERS([struct stat.st_gen])
 AC_CHECK_MEMBERS([struct stat.st_birthtime])
 AC_CHECK_MEMBERS([struct stat.st_blocks])
+# Issue #21085: In Cygwin, siginfo_t does not have si_band field.
+AC_CHECK_MEMBERS([siginfo_t.si_band])
 
 AC_MSG_CHECKING(for time.h that defines altzone)
 AC_CACHE_VAL(ac_cv_header_time_altzone,[
diff -r 51627344fc58 pyconfig.h.in
--- a/pyconfig.h.in	Mon Aug 22 18:05:56 2016 +0300
+++ b/pyconfig.h.in	Wed Aug 24 00:04:12 2016 +0900
@@ -837,6 +837,9 @@
 /* Define to 1 if you have the `sigaltstack' function. */
 #undef HAVE_SIGALTSTACK
 
+/* Define to 1 if `si_band' is a member of `siginfo_t'. */
+#undef HAVE_SIGINFO_T_SI_BAND
+
 /* Define to 1 if you have the `siginterrupt' function. */
 #undef HAVE_SIGINTERRUPT