函数 av_fifo_can_write 

pub unsafe extern "C" fn av_fifo_can_write(f: *const AVFifo) -> usize

@return Number of elements that can be written into the given FIFO without growing it.

    In other words, this number of elements or less is guaranteed to fit
    into the FIFO. More data may be written when the
    AV_FIFO_FLAG_AUTO_GROW flag was specified at FIFO creation, but this
    may involve memory allocation, which can fail.