$OpenBSD$

Index: runtime/druntime/src/core/sys/openbsd/sys/cdefs.d
--- runtime/druntime/src/core/sys/openbsd/sys/cdefs.d.orig
+++ runtime/druntime/src/core/sys/openbsd/sys/cdefs.d
@@ -0,0 +1,18 @@
+/**
+  * D header file for OpenBSD
+  *
+  * Authors: Ryan Boggs
+  */
+module core.sys.openbsd.sys.cdefs;
+
+version (OpenBSD):
+
+    public import core.sys.posix.config;
+
+    // Used core/sys/freebsd/sys/cdefs.d as base
+    // Values retrieved from: https://cvsweb.openbsd.org/src/sys/sys/cdefs.h
+    // with the assumption that OpenBSD is compiled with _XOPEN_SOURCE = 600
+    enum __POSIX_VISIBLE = 200112;
+    enum __XPG_VISIBLE = 600;
+    enum __BSD_VISIBLE = true;
+    enum __ISO_C_VISIBLE = 1999;
