Function vaMapBuffer 

pub unsafe extern "C" fn vaMapBuffer(
    dpy: VADisplay,
    buf_id: VABufferID,
    pbuf: *mut *mut c_void,
) -> VAStatus

Map data store of the buffer into the client’s address space vaCreateBuffer() needs to be called with “data” set to NULL before calling vaMapBuffer()

if buffer type is VAEncCodedBufferType, pbuf points to link-list of VACodedBufferSegment, and the list is terminated if “next” is NULL