Hi,
The callback function will be called when encoded slice is ready. Please try the commands to show the cases with/without –sle:
01_video_encode$ gst-launch-1.0 nvarguscamerasrc num-buffers=10 ! nvvidconv ! video/x-raw,width=640,height=480,format=I420 ! filesink location= a.yuv
01_video_encode$ ./video_encode a.yuv 640 480 H264 a.h264 -slt 1 -slen 300 --report-metadata
01_video_encode$ ./video_encode a.yuv 640 480 H264 a.h264 -slt 1 -slen 300 --report-metadata --sle