diff --git a/src/os/sys/unix.rs b/src/os/sys/unix.rs
index caa4c86..711b580 100644
--- a/src/os/sys/unix.rs
+++ b/src/os/sys/unix.rs
@@ -20,9 +20,15 @@ pub fn sys_page_size() -> usize {
 const GUARD_PROT:  c_int = libc::PROT_NONE;
 const STACK_PROT:  c_int = libc::PROT_READ
                          | libc::PROT_WRITE;
+#[cfg(not(any(target_os = "freebsd", target_os = "dragonfly")))]
 const STACK_FLAGS: c_int = libc::MAP_STACK
                          | libc::MAP_PRIVATE
                          | libc::MAP_ANON;
+// workaround for http://lists.freebsd.org/pipermail/freebsd-bugs/2011-July/044840.html
+// according to libgreen, DragonFlyBSD suffers from this too
+#[cfg(any(target_os = "freebsd", target_os = "dragonfly"))]
+const STACK_FLAGS: c_int = libc::MAP_PRIVATE
+                         | libc::MAP_ANON;
 
 pub unsafe fn map_stack(len: usize) -> Option<*mut u8> {
   let ptr = mmap(ptr::null_mut(), len as size_t,