gctl_set_param_err
void gctl_set_param_err(struct gctl_req *req, const char *param, void const *ptr, int len);
gctl_set_param_err(req, "output", sbuf_data(sb), sbuf_len(sb) + 1);
gctl_set_param_err(req, "output", sbuf_data(sb), sbuf_len(sb) + 1);
gctl_set_param_err(req, "output", sbuf_data(sb), sbuf_len(sb) + 1);
gctl_set_param_err(req, "output", buf, strlen(buf) + 1);