diff --git a/include/spawn.h b/include/spawn.h
index 29c799e..f3e9e23 100644
--- a/include/spawn.h
+++ b/include/spawn.h
@@ -21,6 +21,7 @@
 #define POSIX_SPAWN_SETSIGMASK 8
 #define POSIX_SPAWN_SETSCHEDPARAM 16
 #define POSIX_SPAWN_SETSCHEDULER 32
+#define POSIX_SPAWN_SETSID 128
 
 typedef struct {
 	int __flags;