Change the callback function to use void**.

NOTE: This change breaks backwards-compatibility by default.
If you have old callback functions, you can define PB_OLD_CALLBACK_STYLE
to retain the old behaviour.

If you want to convert your old callbacks to new signature, you need
to do the following:

1) Change decode callback argument to   void **arg
      and encode callback argument to   void * const *arg.

2) Change any reference to arg into *arg.

The rationale for making the new behaviour the default is that it
simplifies the common case of "allocate some memory in decode callback".

Update issue 69
Status: FixedInGit

pb_encode.c

@@ -198,9 +198,16 @@ bool checkreturn encode_static_field(pb_ostream_t *stream, const pb_field_t *fie
 bool checkreturn encode_callback_field(pb_ostream_t *stream, const pb_field_t *field, const void *pData)
 {
     const pb_callback_t *callback = (const pb_callback_t*)pData;
+    
+#ifdef PB_OLD_CALLBACK_STYLE
+    const void *arg = callback->arg;
+#else
+    void * const *arg = &(callback->arg);
+#endif    
+    
     if (callback->funcs.encode != NULL)
     {
-        if (!callback->funcs.encode(stream, field, callback->arg))
+        if (!callback->funcs.encode(stream, field, arg))
             PB_RETURN_ERROR(stream, "callback error");
     }
     return true;