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.h

@@ -203,10 +203,19 @@ typedef struct _pb_istream_t pb_istream_t;
 typedef struct _pb_ostream_t pb_ostream_t;
 typedef struct _pb_callback_t pb_callback_t;
 struct _pb_callback_t {
+#ifdef PB_OLD_CALLBACK_STYLE
+    /* Deprecated since nanopb-0.2.1 */
     union {
         bool (*decode)(pb_istream_t *stream, const pb_field_t *field, void *arg);
         bool (*encode)(pb_ostream_t *stream, const pb_field_t *field, const void *arg);
     } funcs;
+#else
+    /* New function signature, which allows modifying arg contents in callback. */
+    union {
+        bool (*decode)(pb_istream_t *stream, const pb_field_t *field, void **arg);
+        bool (*encode)(pb_ostream_t *stream, const pb_field_t *field, void * const *arg);
+    } funcs;
+#endif    
     
     /* Free arg for use by callback */
     void *arg;