int avdevice_list_devices(AVFormatContext *s, AVDeviceInfoList **device_list)
    {
  189     int ret;
  190     av_assert0(s);
  191     av_assert0(device_list);
  192     av_assert0(s->oformat || s->iformat);
  193     if ((s->oformat && !s->oformat->get_device_list) ||
  194         (s->iformat && !s->iformat->get_device_list)) {
  195         *device_list = NULL;
  196         return AVERROR(ENOSYS);
  197     }
  198     *device_list = av_mallocz(sizeof(AVDeviceInfoList));
  199     if (!(*device_list))
  200         return AVERROR(ENOMEM);
  201     /* no default device by default */
  202     (*device_list)->default_device = -1;
  203     if (s->oformat)
  204         ret = s->oformat->get_device_list(s, *device_list);
  205     else
  206         ret = s->iformat->get_device_list(s, *device_list);
  207     if (ret < 0)
  208         avdevice_free_list_devices(device_list);
  209     return ret;
  210 }