diff --git a/src/alloc.c b/src/alloc.c
index 8acff783..5e45e76f 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4,6 +4,10 @@ This is free software; you can redistribute it and/or modify it under the
 terms of the MIT license. A copy of the license can be found in the file
 "LICENSE" at the root of this distribution.
 -----------------------------------------------------------------------------*/
+#ifndef _DEFAULT_SOURCE
+#define _DEFAULT_SOURCE   // for realpath() on Linux
+#endif
+
 #include "mimalloc.h"
 #include "mimalloc-internal.h"
 #include "mimalloc-atomic.h"
diff --git a/src/random.c b/src/random.c
index 255bede4..0e2331bd 100644
--- a/src/random.c
+++ b/src/random.c
@@ -4,6 +4,10 @@ This is free software; you can redistribute it and/or modify it under the
 terms of the MIT license. A copy of the license can be found in the file
 "LICENSE" at the root of this distribution.
 -----------------------------------------------------------------------------*/
+#ifndef _DEFAULT_SOURCE
+#define _DEFAULT_SOURCE   // for syscall() on Linux
+#endif
+
 #include "mimalloc.h"
 #include "mimalloc-internal.h"