函数 avfilter_process_command 

pub unsafe extern "C" fn avfilter_process_command(
    filter: *mut AVFilterContext,
    cmd: *const c_char,
    arg: *const c_char,
    res: *mut c_char,
    res_len: c_int,
    flags: c_int,
) -> c_int

Make the filter instance process a command. It is recommended to use avfilter_graph_send_command().