diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index 3c4d0d6..c5d8e85 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -435,6 +435,9 @@
   /* True means return pragmas as tokens rather than processing
      them directly. */
   bool defer_pragmas;
+
+  /* True means error callback should be used for diagnostics.  */
+  bool client_diagnostic;
 };
 
 /* Callback for header lookup for HEADER, which is the name of a
@@ -467,6 +470,11 @@
   int (*valid_pch) (cpp_reader *, const char *, int);
   void (*read_pch) (cpp_reader *, const char *, int, const char *);
   missing_header_cb missing_header;
+
+  /* Called to emit a diagnostic if client_diagnostic option is true.
+     This callback receives the translated message.  */
+  void (*error) (cpp_reader *, int, const char *, va_list)
+       ATTRIBUTE_PRINTF(3,0);
 };
 
 /* Chain of directories to look for include files in.  */