Index: config/locale/dragonfly/c_locale.cc
===================================================================
RCS file: /cvsroot/src/external/gpl3/gcc/dist/libstdc++-v3/config/locale/dragonfly/c_locale.cc,v
retrieving revision 1.5
diff -p -u -r1.5 c_locale.cc
--- config/locale/dragonfly/c_locale.cc	11 Apr 2021 00:02:24 -0000	1.5
+++ config/locale/dragonfly/c_locale.cc	24 Apr 2021 04:41:34 -0000
@@ -28,6 +28,8 @@
 
 // Written by Benjamin Kosnik <bkoz@redhat.com>
 // Modified for DragonFly by John Marino <gnugcc@marino.st>
+// Modified for NetBSD by Christos Zoulas <christos@zoulas.com> and
+// matthew green <mrg@eterna.com.au>
 
 #include <cstdlib>
 #include <locale>
@@ -191,6 +193,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   const char* const* const locale::_S_categories = __gnu_cxx::category_names;
 
+#ifdef __NetBSD__
+  int
+  __convert_from_v(const __c_locale& __cloc, char* __out,
+		   const int __size __attribute__ ((__unused__)),
+		   const char* __fmt, ...)
+  {
+    __builtin_va_list __args;
+    __builtin_va_start(__args, __fmt);
+
+    const int __ret = vsnprintf_l(__out, __size, (struct _locale *)__cloc,
+      __fmt, __args);
+
+    __builtin_va_end(__args);
+
+    return __ret;
+  }
+#endif
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
Index: config/locale/dragonfly/c_locale.h
===================================================================
RCS file: /cvsroot/src/external/gpl3/gcc/dist/libstdc++-v3/config/locale/dragonfly/c_locale.h,v
retrieving revision 1.5
diff -p -u -r1.5 c_locale.h
--- config/locale/dragonfly/c_locale.h	11 Apr 2021 00:02:24 -0000	1.5
+++ config/locale/dragonfly/c_locale.h	24 Apr 2021 04:41:34 -0000
@@ -33,6 +33,8 @@
 
 // Written by Benjamin Kosnik <bkoz@redhat.com>
 // Modified for DragonFly by John Marino <gnugcc@marino.st>
+// Modified for NetBSD by Christos Zoulas <christos@zoulas.com> and
+// matthew green <mrg@eterna.com.au>
 
 #ifndef _GLIBCXX_CXX_LOCALE_H
 #define _GLIBCXX_CXX_LOCALE_H 1
@@ -54,6 +56,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   typedef int*			__c_locale;
 
+#ifndef __NetBSD__
   // Convert numeric value of type double and long double to string and
   // return length of string.  If vsnprintf is available use it, otherwise
   // fall back to the unsafe vsprintf which, in general, can be dangerous
@@ -63,30 +66,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		   const int __size __attribute__ ((__unused__)),
 		   const char* __fmt, ...)
   {
-#ifndef __NetBSD__
     __c_locale __old = (__c_locale)uselocale((locale_t)__cloc);
-#endif
 
     __builtin_va_list __args;
     __builtin_va_start(__args, __fmt);
 
-#ifndef __NetBSD__
 #if _GLIBCXX_USE_C99_STDIO
     const int __ret = __builtin_vsnprintf(__out, __size, __fmt, __args);
 #else
     const int __ret = __builtin_vsprintf(__out, __fmt, __args);
 #endif
 
-    uselocale((locale_t)__old);
-#else
-    const int __ret = vsnprintf_l(__out, __size, (locale_t)__cloc, __fmt,
-	__args);
-#endif
-
     __builtin_va_end(__args);
 
+    uselocale((locale_t)__old);
     return __ret;
   }
+#else
+  // NetBSD backend requires vasprintf_l() which may not be visible in
+  // all complation environments (eg, _XOPEN_SOURCE=600) so the backend
+  // lives in the library code, instead of this header.
+  int
+  __convert_from_v(const __c_locale& __cloc, char* __out,
+		   const int __size __attribute__ ((__unused__)),
+		   const char* __fmt, ...);
+#endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace