diff --git a/src/os.c b/src/os.c
index 8079e5a0..24252c85 100644
--- a/src/os.c
+++ b/src/os.c
@@ -8,6 +8,14 @@ terms of the MIT license. A copy of the license can be found in the file
 #define _DEFAULT_SOURCE   // ensure mmap flags are defined
 #endif
 
+#if defined(__sun)
+// illumos provides new mman.h api when any of these are defined
+// otherwise the old api based on caddr_t which predates the void pointers one.
+// stock solaris provides only the former, chose to atomically to discard those
+// flags only here rather than project wide tough.
+#undef _XOPEN_SOURCE
+#undef _POSIX_C_SOURCE
+#endif
 #include "mimalloc.h"
 #include "mimalloc-internal.h"
 #include "mimalloc-atomic.h"
diff --git a/src/static.c b/src/static.c
index bf86166d..4fafb4f5 100644
--- a/src/static.c
+++ b/src/static.c
@@ -5,6 +5,11 @@ terms of the MIT license. A copy of the license can be found in the file
 "LICENSE" at the root of this distribution.
 -----------------------------------------------------------------------------*/
 #define _DEFAULT_SOURCE
+#if defined(__sun)
+// same remarks as os.c for the static's context.
+#undef _XOPEN_SOURCE
+#undef _POSIX_C_SOURCE
+#endif
 
 #include "mimalloc.h"
 #include "mimalloc-internal.h"