diff --git a/libvncclient/tls_openssl.c b/libvncclient/tls_openssl.c
index 00d031d..a531778 100644
--- a/libvncclient/tls_openssl.c
+++ b/libvncclient/tls_openssl.c
@@ -18,6 +18,10 @@
  *  USA.
  */
 
+#ifndef _MSC_VER
+#define _XOPEN_SOURCE 500
+#endif
+
 #include <rfb/rfbclient.h>
 #include <errno.h>