Prefer C linkage
diff --git a/src/hb-ot-layout-gsub-private.hh b/src/hb-ot-layout-gsub-private.hh
index 9af5b63..f09b4d2 100644
--- a/src/hb-ot-layout-gsub-private.hh
+++ b/src/hb-ot-layout-gsub-private.hh
@@ -29,6 +29,8 @@
 
 #include "hb-ot-layout-gsubgpos-private.hh"
 
+HB_BEGIN_DECLS
+
 
 struct SingleSubstFormat1
 {
@@ -541,8 +543,9 @@
 };
 
 
-
+HB_BEGIN_DECLS
 static inline bool substitute_lookup (hb_apply_context_t *c, unsigned int lookup_index);
+HB_END_DECLS
 
 struct ContextSubst : Context
 {
@@ -936,4 +939,6 @@
 }
 
 
+HB_END_DECLS
+
 #endif /* HB_OT_LAYOUT_GSUB_PRIVATE_HH */