diff --git a/Misc/NEWS b/Misc/NEWS
index 0dff2313319..1bd8a5f1385 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -1328,7 +1328,8 @@ Extension Modules
 Build
 -----
 
-- Issue #1759169: Drop _XOPEN_SOURCE on Solaris.
+- Issue #1759169, #8864: Drop _XOPEN_SOURCE on Solaris, define it for
+  multiprocessing only.
 
 - Issue #8625: Turn off optimization in --with-pydebug builds with
   gcc.  (Optimization was unintentionally turned on in gcc
diff --git a/Modules/_multiprocessing/multiprocessing.h b/Modules/_multiprocessing/multiprocessing.h
index c1496343c39..14425de8218 100644
--- a/Modules/_multiprocessing/multiprocessing.h
+++ b/Modules/_multiprocessing/multiprocessing.h
@@ -3,6 +3,12 @@
 
 #define PY_SSIZE_T_CLEAN
 
+#ifdef __sun
+/* The control message API is only available on Solaris 
+   if XPG 4.2 or later is requested. */
+#define _XOPEN_SOURCE 500
+#endif
+
 #include "Python.h"
 #include "structmember.h"
 #include "pythread.h"