Use stderr for debug prints in DEBUG(); fix #3584

src/gc.c

@@ -701,7 +701,7 @@ mrb_gc_mark(mrb_state *mrb, struct RBasic *obj)
 static void
 obj_free(mrb_state *mrb, struct RBasic *obj, int end)
 {
-  DEBUG(printf("obj_free(%p,tt=%d)\n",obj,obj->tt));
+  DEBUG(fprintf(stderr, "obj_free(%p,tt=%d)\n",obj,obj->tt));
   switch (obj->tt) {
     /* immediate - no mark */
   case MRB_TT_TRUE:

src/vm.c

@@ -1809,7 +1809,7 @@ RETRY_TRY_BLOCK:
           return v;
         }
         pc = ci->pc;
-        DEBUG(printf("from :%s\n", mrb_sym2name(mrb, ci->mid)));
+        DEBUG(fprintf(stderr, "from :%s\n", mrb_sym2name(mrb, ci->mid)));
         proc = mrb->c->ci->proc;
         irep = proc->body.irep;
         pool = irep->pool;