diff --git a/gcc/ada/cstreams.c b/gcc/ada/cstreams.c
index 4e00dedbbd68..8072320ae10c 100644
--- a/gcc/ada/cstreams.c
+++ b/gcc/ada/cstreams.c
@@ -37,6 +37,11 @@
 #define _FILE_OFFSET_BITS 64
 /* the define above will make off_t a 64bit type on GNU/Linux */
 
+/* Tell Cygwin's <stdio.h> to expose fileno_unlocked() */
+#if defined(__CYGWIN__) && !defined(__CYGWIN32__) && !defined(_GNU_SOURCE)
+#define _GNU_SOURCE
+#endif
+
 #include <stdio.h>
 #include <sys/types.h>
 #include <sys/stat.h>