函数 av_fifo_read_to_cb 

pub unsafe extern "C" fn av_fifo_read_to_cb(
    f: *mut AVFifo,
    write_cb: AVFifoCB,
    opaque: *mut c_void,
    nb_elems: *mut usize,
) -> c_int

Feed data from a FIFO into a user-provided callback.

@param f the FIFO buffer @param write_cb Callback the data will be supplied to. May be called multiple times. @param opaque opaque user data to be provided to write_cb @param nb_elems Should point to the maximum number of elements that can be read. Will be updated to contain the total number of elements actually sent to the callback.

@return non-negative number on success, a negative error code on failure