int ffio_init_context(AVIOContext *s,
                  unsigned char *buffer,
                  int buffer_size,
                  int write_flag,
                  void *opaque,
                  int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int64_t (*seek)(void *opaque, int64_t offset, int whence))
{
    s->buffer = buffer;
    s->buffer_size = buffer_size;
    s->buf_ptr = buffer;
    s->opaque = opaque;
    s->direct = 0;
    url_resetbuf(s, write_flag ? AVIO_FLAG_WRITE : AVIO_FLAG_READ);
    s->write_packet = write_packet;
    s->read_packet = read_packet;
    s->seek = seek;
    s->pos = 0;
    s->must_flush = 0;
    s->eof_reached = 0;
    s->error = 0;
    s->seekable = AVIO_SEEKABLE_NORMAL;
    s->max_packet_size = 0;
    s->update_checksum= NULL;
    if(!read_packet && !write_flag){
        s->pos = buffer_size;
        s->buf_end = s->buffer + buffer_size;
    }
    s->read_pause = NULL;
    s->read_seek  = NULL;
    return 0;
}