diff --git a/include/boost/system/detail/error_code.ipp b/include/boost/system/detail/error_code.ipp
index 6187919..c5a2068 100644
--- a/include/boost/system/detail/error_code.ipp
+++ b/include/boost/system/detail/error_code.ipp
@@ -459,13 +459,19 @@ namespace
                                          //  address for comparison purposes
 # endif
 
-    BOOST_SYSTEM_DECL const error_category & system_category() BOOST_SYSTEM_NOEXCEPT
+# ifdef BOOST_ERROR_CODE_HEADER_ONLY
+#   define BOOST_SYSTEM_LINKAGE inline
+# else
+#   define BOOST_SYSTEM_LINKAGE BOOST_SYSTEM_DECL
+# endif
+
+    BOOST_SYSTEM_LINKAGE const error_category & system_category() BOOST_SYSTEM_NOEXCEPT
     {
       static const system_error_category  system_category_const;
       return system_category_const;
     }
 
-    BOOST_SYSTEM_DECL const error_category & generic_category() BOOST_SYSTEM_NOEXCEPT
+    BOOST_SYSTEM_LINKAGE const error_category & generic_category() BOOST_SYSTEM_NOEXCEPT
     {
       static const generic_error_category generic_category_const;
       return generic_category_const;