diff --git a/sysinfo/os/darwin.mk b/sysinfo/os/darwin.mk
index 7145f5a..f16b1f1 100644
--- a/sysinfo/os/darwin.mk
+++ b/sysinfo/os/darwin.mk
@@ -7,4 +7,6 @@ OS_IMPLIB_EXT	= .invalid
 OS_LIBDEF_EXT	= .invalid
 OS_ARCHIVE_EXT	= .a
 
+CFLAGS_OS	+= -D_DARWIN_C_SOURCE
+
 .PHONY:		$(SHARED_IMPLIB)