Large file extensions are not exposed to funopen.c unless two macros are defined.
--- a/src/funopen.c
+++ b/src/funopen.c
@@ -24,6 +24,9 @@
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

+#define _LARGEFILE64_SOURCE
+#define _FILE_OFFSET_BITS 64
+
 #include <sys/cdefs.h>
 #include <sys/types.h>
 #include <errno.h>