+#ifdef TCP_FASTOPEN
+static void
+tfo_out_check(int sock)
+{
+# if defined(TCP_INFO) && defined(EXIM_HAVE_TCPI_UNACKED)
+struct tcp_info tinfo;
+socklen_t len = sizeof(tinfo);
+
+if (getsockopt(sock, IPPROTO_TCP, TCP_INFO, &tinfo, &len) == 0)
+ {
+ /* This is a somewhat dubious detection method; totally undocumented so likely
+ to fail in future kernels. There seems to be no documented way. */
+
+ if (tinfo.tcpi_unacked > 1)
+ {
+ DEBUG(D_transport|D_v) debug_printf("TCP_FASTOPEN mode connection\n");
+ tcp_out_fastopen = TRUE;
+ }
+ }
+# endif
+}
+#endif
+
+