diff -r 9b73f1c3e601 Modules/signalmodule.c
--- a/Modules/signalmodule.c	Sun Feb 22 23:55:39 2015 -0800
+++ b/Modules/signalmodule.c	Tue Mar 31 13:16:57 2015 +0900
@@ -708,21 +708,31 @@
     {"si_pid",          "sending process ID"},
     {"si_uid",          "real user ID of sending process"},
     {"si_status",       "exit value or signal"},
+#ifdef HAVE_SIGINFO_T_SI_BAND
     {"si_band",         "band event for SIGPOLL"},
+#endif
     {0}
 };
 
 PyDoc_STRVAR(struct_siginfo__doc__,
 "struct_siginfo: Result from sigwaitinfo or sigtimedwait.\n\n\
-This object may be accessed either as a tuple of\n\
-(si_signo, si_code, si_errno, si_pid, si_uid, si_status, si_band),\n\
-or via the attributes si_signo, si_code, and so on.");
+This object may be accessed either as a tuple of\n"
+#ifdef HAVE_SIGINFO_T_SI_BAND
+"(si_signo, si_code, si_errno, si_pid, si_uid, si_status, si_band),\n"
+#else
+"(si_signo, si_code, si_errno, si_pid, si_uid, si_status),\n"
+#endif
+"or via the attributes si_signo, si_code, and so on.");
 
 static PyStructSequence_Desc struct_siginfo_desc = {
     "signal.struct_siginfo",           /* name */
     struct_siginfo__doc__,       /* doc */
     struct_siginfo_fields,       /* fields */
+#ifdef HAVE_SIGINFO_T_SI_BAND
     7          /* n_in_sequence */
+#else
+    6
+#endif
 };
 
 static PyTypeObject SiginfoType;
@@ -741,7 +751,9 @@
     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));
+#endif
     if (PyErr_Occurred()) {
         Py_DECREF(result);
         return NULL;
diff -r 9b73f1c3e601 configure
--- a/configure	Sun Feb 22 23:55:39 2015 -0800
+++ b/configure	Tue Mar 31 13:16:57 2015 +0900
@@ -12314,6 +12314,17 @@
 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 9b73f1c3e601 configure.ac
--- a/configure.ac	Sun Feb 22 23:55:39 2015 -0800
+++ b/configure.ac	Tue Mar 31 13:16:57 2015 +0900
@@ -3427,6 +3427,8 @@
 AC_CHECK_MEMBERS([struct stat.st_gen])
 AC_CHECK_MEMBERS([struct stat.st_birthtime])
 AC_STRUCT_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 9b73f1c3e601 pyconfig.h.in
--- a/pyconfig.h.in	Sun Feb 22 23:55:39 2015 -0800
+++ b/pyconfig.h.in	Tue Mar 31 13:16:57 2015 +0900
@@ -813,6 +813,9 @@
 /* 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