static int camera_viewfinder(Camera *camera, GPContext *context, int onoff)
{
CameraWidget *widget = NULL, *child = NULL;
CameraWidgetType type;
int ret,val;

ret = gp_camera_get_config (camera, &widget, context);
if (ret < GP_OK) {
	fprintf (stderr, "camera_get_config failed: %d\n", ret);
	return ret;
}
ret = _lookup_widget2 (widget, "viewfinder", &child);
if (ret < GP_OK) {
	fprintf (stderr, "lookup 'viewfinder' failed: %d\n", ret);
	goto out;
}

/* check that this is a toggle */
ret = gp_widget_get_type (child, &type);
if (ret < GP_OK) {
	fprintf (stderr, "widget get type failed: %d\n", ret);
	goto out;
}
switch (type) {
	case GP_WIDGET_TOGGLE:
		break;
	default:
		fprintf (stderr, "widget has bad type %d\n", type);
		ret = GP_ERROR_BAD_PARAMETERS;
		goto out;
}

ret = gp_widget_get_value (child, &val);
if (ret < GP_OK) {
	fprintf (stderr, "could not get widget value: %d\n", ret);
	goto out;
}
val = onoff;
ret = gp_widget_set_value (child, &val);
if (ret < GP_OK) {
	fprintf (stderr, "could not set widget value to 1: %d\n", ret);
	goto out;
}

ret = gp_camera_set_config (camera, widget, context);
if (ret < GP_OK) {
	fprintf (stderr, "could not set config tree to eosviewfinder: %d\n", ret);
	goto out;
}
out:
gp_widget_free (widget);
return ret;

}

// init camera ....

/*

• start use

• Can I set the size of the live preview？
  */
  while(true) {

  int retval = gp_file_new(&file);
  std::this_thread::sleep_for(std::chrono::milliseconds(7));//Device busy
  if (retval != GP_OK) {

    fprintf(stderr,"gp_file_new: %d\n", retval);
    assert(false);
  

  }

  int retval = gp_camera_capture_preview(mCanon, file, mCanonContext);// success
  std::this_thread::sleep_for(std::chrono::milliseconds(2));//Device busy
  retval = gp_file_save(file, output_file.c_str());// jpg size is 960x640
  }

problem:
How can I set the camera's live preview size?

gphoto2 --list-config
/main/actions/syncdatetimeutc
/main/actions/syncdatetime
/main/actions/uilock
/main/actions/autofocusdrive
/main/actions/manualfocusdrive
/main/actions/cancelautofocus
/main/actions/eoszoom
/main/actions/eoszoomposition
/main/actions/viewfinder
/main/actions/eosremoterelease
/main/actions/opcode
/main/settings/datetimeutc
/main/settings/datetime
/main/settings/reviewtime
/main/settings/output
/main/settings/movierecordtarget
/main/settings/evfmode
/main/settings/ownername
/main/settings/artist
/main/settings/copyright
/main/settings/customfuncex
/main/settings/focusinfo
/main/settings/focusarea
/main/settings/autopoweroff
/main/settings/depthoffield
/main/settings/capturetarget
/main/settings/capture
/main/status/serialnumber
/main/status/manufacturer
/main/status/cameramodel
/main/status/deviceversion
/main/status/vendorextension
/main/status/model
/main/status/ptpversion
/main/status/Battery Level
/main/status/batterylevel
/main/status/lensname
/main/status/eosserialnumber
/main/status/shuttercounter
/main/status/availableshots
/main/imgsettings/imageformat
/main/imgsettings/imageformatsd
/main/imgsettings/imageformatcf
/main/imgsettings/iso
/main/imgsettings/whitebalance
/main/imgsettings/colortemperature
/main/imgsettings/whitebalanceadjusta
/main/imgsettings/whitebalanceadjustb
/main/imgsettings/whitebalancexa
/main/imgsettings/whitebalancexb
/main/imgsettings/colorspace
/main/capturesettings/exposurecompensation
/main/capturesettings/focusmode
/main/capturesettings/aspectratio
/main/capturesettings/storageid
/main/capturesettings/autoexposuremode
/main/capturesettings/drivemode
/main/capturesettings/picturestyle
/main/capturesettings/aperture
/main/capturesettings/shutterspeed
/main/capturesettings/bracketmode
/main/capturesettings/aeb
/main/other/d402
/main/other/d407
/main/other/d406
/main/other/d303
/main/other/5001