How to update the memory blocks?

[wwfly93]

I am learning the sample code about  "tests/media_sender" in Rivermax SDK , and have some question about the data sending process. 

Within the "thread_data::init" function, a huge memory is allocated and filled with ycbcr video data, then assign the memory blocks to the output media stream by "rmx_output_media_assign_mem_blocks". But in the "thread_data::run_intrenal" function, the thread proc only "build_rtp_header" between "rmx_output_media_get_next_chunk" and "rmx_output_media_commit_chunk", it dosen't fill the packet payload. 
    1. does it mean that  Rivermax will use the previously assigned memory block, is it loop?
    2. Is it possible to set the reading order? 
    3. If I want to modify the sent data in real time, can I use the previous memory block pointer or should I use the pointer obtained from "rmx_output_media_get_chunk_strides"?
    4. Is there an internal synchronization mechanism for modifying and sending memory blocks?