-XBT_PUBLIC_DATA std::ifstream* action_fs;
-XBT_PUBLIC int replay_runner(const char* actor_name, const char* trace_filename);
+/** Launch a replaying actor of the given name.
+ *
+ * If trace_filename is nullptr, then the tracefile is shared between all instances, and was passed using
+ * xbt_replay_set_tracefile(). If trace_filename is not nullptr, then it's not shared and this trace file is for this
+ * actor only */
+XBT_PUBLIC int replay_runner(const char* actor_name, const char* trace_filename = nullptr);