如何在H264中从ffmpeg中读取sei未注册用户数据？

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>

void process_sei(AVCodecContext *avctx, const uint8_t *buf, int size) {
    const uint8_t *sei = buf;
    while (sei < buf + size) {
        int payload_type = 0;
        int last_payload_type_size = 0;
        int payload_size = 0;

        // 解析SEI头
        if (sei + 1 > buf + size)
            break;
        payload_type = sei[0];
        last_payload_type_size = sei[1];
        payload_size = last_payload_type_size >> 4;
        sei += 2;

        // 如果是未注册用户数据
        if (payload_type == 5) {
            // 处理未注册用户数据
            printf("Unregistered User Data: ");
            for (int i = 0; i < payload_size; i++) {
                printf("%02x ", sei[i]);
            }
            printf("\n");
        }

        // 移动到下一个SEI单元
        sei += payload_size + last_payload_type_size & 0x0F;
    }
}

int main(int argc, char **argv) {
    AVFormatContext *fmt_ctx = NULL;
    AVCodecContext *codec_ctx = NULL;
    const AVCodec *codec = NULL;

    av_register_all();

    if (avformat_open_input(&fmt_ctx, "input.mp4", NULL, NULL) < 0) {
        fprintf(stderr, "Could not open input file\n");
        return -1;
    }

    if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {
        fprintf(stderr, "Failed to retrieve input stream information\n");
        return -1;
    }

    int video_stream_index = -1;
    for (int i = 0; i < fmt_ctx->nb_streams; i++) {
        if (fmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
            video_stream_index = i;
            break;
        }
    }

    if (video_stream_index == -1) {
        fprintf(stderr, "Could not find video stream in the input, aborting\n");
        return -1;
    }

    codec = avcodec_find_decoder(fmt_ctx->streams[video_stream_index]->codecpar->codec_id);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        return -1;
    }

    codec_ctx = avcodec_alloc_context3(codec);
    if (!codec_ctx) {
        fprintf(stderr, "Could not allocate video codec context\n");
        return -1;
    }

    if (avcodec_parameters_to_context(codec_ctx, fmt_ctx->streams[video_stream_index]->codecpar) < 0) {
        fprintf(stderr, "Failed to copy codec parameters to decoder context\n");
        return -1;
    }

    if (avcodec_open2(codec_ctx, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        return -1;
    }

    AVPacket pkt;
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    while (av_read_frame(fmt_ctx, &pkt) >= 0) {
        if (pkt.stream_index == video_stream_index) {
            int ret;
            AVFrame *frame = av_frame_alloc();
            if (!frame) {
                fprintf(stderr, "Could not allocate video frame\n");
                return -1;
            }

            ret = avcodec_send_packet(codec_ctx, &pkt);
            if (ret < 0) {
                fprintf(stderr, "Error sending a packet for decoding\n");
                return -1;
            }

            while (ret >= 0) {
                ret = avcodec_receive_frame(codec_ctx, frame);
                if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
                    break;
                else if (ret < 0) {
                    fprintf(stderr, "Error during decoding\n");
                    return -1;
                }

                // 处理SEI信息
                process_sei(codec_ctx, frame->data[5], frame->linesize[5]);
            }

            av_frame_free(&frame);
        }
        av_packet_unref(&pkt);
    }

    avcodec_free_context(&codec_ctx);
    avformat_close_input(&fmt_ctx);

    return 0;
}