diff --git a/libc/upstream-netbsd/extern.h b/libc/upstream-netbsd/extern.h
index 942e237..616becd 100644
--- a/libc/upstream-netbsd/extern.h
+++ b/libc/upstream-netbsd/extern.h
@@ -17,6 +17,12 @@
 #ifndef _BIONIC_NETBSD_EXTERN_H_included
 #define _BIONIC_NETBSD_EXTERN_H_included
 
-// Placeholder.
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+const char* __strsignal(int, char*, size_t);
+
+__END_DECLS
 
 #endif