CameraFile* file;

res = gp_camera_capture_preview(camera, file, context);
if(res!=GP_OK) {syslog(LOG_DAEMON|LOG_INFO, "14 - Code: %d - %s\n", res, gp_result_as_string(res));}

camera and context are the initialized Camera and GPContext objects I use for main capture (that works well)

the result is:
May 15 13:29:07 OpenWrt daemon.info syslog: 14 - Code: -2 - Bad parameters