--- a/Tests/CTestTestFdSetSize/sleep.c
+++ b/Tests/CTestTestFdSetSize/sleep.c
@@ -1,3 +1,6 @@
+#ifdef __linux__
+#  include <features.h>
+#endif
 #if defined(_WIN32)
 #  include <windows.h>
 #elif _XOPEN_SOURCE >= 500 || defined(_ALL_SOURCE)