diff --git a/src/runtime/cgo/dragonfly.c b/src/runtime/cgo/dragonfly.c
index 3c95ff354e7..c233c8ba9a3 100644
--- a/src/runtime/cgo/dragonfly.c
+++ b/src/runtime/cgo/dragonfly.c
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build dragonfly
+
 #include "textflag.h"
 
 // Supply environ and __progname, because we don't
diff --git a/src/runtime/cgo/freebsd.c b/src/runtime/cgo/freebsd.c
index aefc481e64c..4876b2abe47 100644
--- a/src/runtime/cgo/freebsd.c
+++ b/src/runtime/cgo/freebsd.c
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build freebsd
+
 #include "textflag.h"
 
 // Supply environ and __progname, because we don't
diff --git a/src/runtime/cgo/netbsd.c b/src/runtime/cgo/netbsd.c
index de38bb77078..076cc87f12a 100644
--- a/src/runtime/cgo/netbsd.c
+++ b/src/runtime/cgo/netbsd.c
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build netbsd
+
 #include "textflag.h"
 
 // Supply environ and __progname, because we don't
diff --git a/src/runtime/cgo/openbsd.c b/src/runtime/cgo/openbsd.c
index 7c2b6c17378..476649544df 100644
--- a/src/runtime/cgo/openbsd.c
+++ b/src/runtime/cgo/openbsd.c
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build openbsd
+
 #include "textflag.h"
 
 // Supply environ, __progname and __guard_local, because