diff --git a/include/boost/system/detail/generic_category.hpp b/include/boost/system/detail/generic_category.hpp
index bad6085..d9ee1b5 100644
--- a/include/boost/system/detail/generic_category.hpp
+++ b/include/boost/system/detail/generic_category.hpp
@@ -11,6 +11,7 @@
 //  See library home page at http://www.boost.org/libs/system
 
 #include <boost/system/detail/error_category.hpp>
+#include <boost/system/detail/generic_category_impl.hpp>
 #include <boost/system/detail/config.hpp>
 #include <boost/cstdint.hpp>
 #include <boost/config.hpp>
@@ -56,82 +57,16 @@ public:
 
 // generic_error_category::message
 
-#if defined(__GLIBC__)
-
-// glibc has two incompatible strerror_r definitions
-
-inline char const * strerror_r_helper( char const * r, char const * ) BOOST_NOEXCEPT
-{
-    return r;
-}
-
-inline char const * strerror_r_helper( int r, char const * buffer ) BOOST_NOEXCEPT
-{
-    return r == 0? buffer: "Unknown error";
-}
-
 inline char const * generic_error_category::message( int ev, char * buffer, std::size_t len ) const BOOST_NOEXCEPT
 {
-    return strerror_r_helper( strerror_r( ev, buffer, len ), buffer );
+    return generic_error_category_message( ev, buffer, len );
 }
 
 inline std::string generic_error_category::message( int ev ) const
 {
-    char buffer[ 128 ];
-    return generic_error_category_message( ev, buffer, sizeof( buffer ) );
+    return generic_error_category_message( ev );
 }
 
-#else // #if defined(__GLIBC__)
-
-// std::strerror is thread-safe on everything else, incl. Windows
-
-# if defined( BOOST_MSVC )
-#  pragma warning( push )
-#  pragma warning( disable: 4996 )
-# elif defined(__clang__) && defined(__has_warning)
-#  pragma clang diagnostic push
-#  if __has_warning("-Wdeprecated-declarations")
-#   pragma clang diagnostic ignored "-Wdeprecated-declarations"
-#  endif
-# endif
-
-inline std::string generic_error_category::message( int ev ) const
-{
-    char const * m = std::strerror( ev );
-    return m? m: "Unknown error";
-}
-
-inline char const * generic_error_category::message( int ev, char * buffer, std::size_t len ) const BOOST_NOEXCEPT
-{
-    if( len == 0 )
-    {
-        return buffer;
-    }
-
-    if( len == 1 )
-    {
-        buffer[0] = 0;
-        return buffer;
-    }
-
-    char const * m = std::strerror( ev );
-
-    if( m == 0 ) return "Unknown error";
-
-    std::strncpy( buffer, m, len - 1 );
-    buffer[ len-1 ] = 0;
-
-    return buffer;
-}
-
-# if defined( BOOST_MSVC )
-#  pragma warning( pop )
-# elif defined(__clang__) && defined(__has_warning)
-#  pragma clang diagnostic pop
-# endif
-
-#endif // #if defined(__GLIBC__)
-
 } // namespace detail
 
 // generic_category()
diff --git a/include/boost/system/detail/generic_category_impl.hpp b/include/boost/system/detail/generic_category_impl.hpp
new file mode 100644
index 0000000..5b968ad
--- /dev/null
+++ b/include/boost/system/detail/generic_category_impl.hpp
@@ -0,0 +1,108 @@
+#ifndef BOOST_SYSTEM_DETAIL_GENERIC_CATEGORY_IMPL_HPP_INCLUDED
+#define BOOST_SYSTEM_DETAIL_GENERIC_CATEGORY_IMPL_HPP_INCLUDED
+
+// Implementation details of generic_error_category
+//
+// Copyright 2018 Peter Dimov
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// See library home page at http://www.boost.org/libs/system
+
+#include <boost/config.hpp>
+#include <string>
+#include <cstring>
+
+namespace boost
+{
+
+namespace system
+{
+
+namespace detail
+{
+
+#if defined(__GLIBC__)
+
+// glibc has two incompatible strerror_r definitions
+
+inline char const * strerror_r_helper( char const * r, char const * ) BOOST_NOEXCEPT
+{
+    return r;
+}
+
+inline char const * strerror_r_helper( int r, char const * buffer ) BOOST_NOEXCEPT
+{
+    return r == 0? buffer: "Unknown error";
+}
+
+inline char const * generic_error_category_message( int ev, char * buffer, std::size_t len ) BOOST_NOEXCEPT
+{
+    return strerror_r_helper( strerror_r( ev, buffer, len ), buffer );
+}
+
+inline std::string generic_error_category_message( int ev )
+{
+    char buffer[ 128 ];
+    return generic_error_category_message( ev, buffer, sizeof( buffer ) );
+}
+
+#else // #if defined(__GLIBC__)
+
+// std::strerror is thread-safe on everything else, incl. Windows
+
+# if defined( BOOST_MSVC )
+#  pragma warning( push )
+#  pragma warning( disable: 4996 )
+# elif defined(__clang__) && defined(__has_warning)
+#  pragma clang diagnostic push
+#  if __has_warning("-Wdeprecated-declarations")
+#   pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#  endif
+# endif
+
+inline std::string generic_error_category_message( int ev )
+{
+    char const * m = std::strerror( ev );
+    return m? m: "Unknown error";
+}
+
+inline char const * generic_error_category_message( int ev, char * buffer, std::size_t len ) BOOST_NOEXCEPT
+{
+    if( len == 0 )
+    {
+        return buffer;
+    }
+
+    if( len == 1 )
+    {
+        buffer[0] = 0;
+        return buffer;
+    }
+
+    char const * m = std::strerror( ev );
+
+    if( m == 0 ) return "Unknown error";
+
+    std::strncpy( buffer, m, len - 1 );
+    buffer[ len-1 ] = 0;
+
+    return buffer;
+}
+
+# if defined( BOOST_MSVC )
+#  pragma warning( pop )
+# elif defined(__clang__) && defined(__has_warning)
+#  pragma clang diagnostic pop
+# endif
+
+#endif // #if defined(__GLIBC__)
+
+} // namespace detail
+
+} // namespace system
+
+} // namespace boost
+
+#endif // #ifndef BOOST_SYSTEM_DETAIL_GENERIC_CATEGORY_IMPL_HPP_INCLUDED