int avfilter_process_command(AVFilterContext *filter, const char *cmd, const char *arg, char *res, int res_len, int flags)
{
if(!strcmp(cmd, "ping")){
+ char local_res[256] = {0};
+
+ if (!res) {
+ res = local_res;
+ res_len = sizeof(local_res);
+ }
av_strlcatf(res, res_len, "pong from:%s %s\n", filter->filter->name, filter->name);
+ if (res == local_res)
+ av_log(filter, AV_LOG_INFO, "%s", res);
return 0;
}else if(!strcmp(cmd, "enable")) {
return set_enable_expr(filter, arg);