--- a/tmux.c
+++ b/tmux.c
@@ -16,6 +16,10 @@
  * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <sys/types.h>
 #include <sys/stat.h>
 
