函数 av_get_packet 

pub unsafe extern "C" fn av_get_packet(
    s: *mut AVIOContext,
    pkt: *mut AVPacket,
    size: c_int,
) -> c_int

Allocate and read the payload of a packet and initialize its fields with default values.

@param s associated IO context @param pkt packet @param size desired payload size @return >0 (read size) if OK, AVERROR_xxx otherwise