函数 av_fifo_read 

pub unsafe extern "C" fn av_fifo_read(
    f: *mut AVFifo,
    buf: *mut c_void,
    nb_elems: usize,
) -> c_int

Read data from a FIFO.

In case nb_elems > av_fifo_can_read(f), nothing is read and an error is returned.

@param f the FIFO buffer @param buf Buffer to store the data. nb_elems * av_fifo_elem_size(f) bytes will be written into buf on success. @param nb_elems number of elements to read from FIFO

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