在FFmpeg的libavformat中修改RTMP

#include <libavformat/avformat.h>
#include <libavutil/timestamp.h>
#include <libavutil/mathematics.h>

int main(int argc, char *argv[]) {
    AVFormatContext *ifmt_ctx = NULL, *ofmt_ctx = NULL;
    AVPacket pkt;
    const char *in_filename, *out_filename;
    int ret, video_stream_index = -1;
    AVStream *out_vid_strm;

    if (argc < 3) {
        fprintf(stderr, "Usage: %s <input rtmp url> <output file>\n", argv[0]);
        return -1;
    }

    in_filename  = argv[1];
    out_filename = argv[2];

    av_register_all();

    // 打开输入流
    if ((ret = avformat_open_input(&ifmt_ctx, in_filename, 0, 0)) < 0) {
        fprintf(stderr, "Could not open input file '%s'", in_filename);
        goto end;
    }

    // 获取流信息
    if ((ret = avformat_find_stream_info(ifmt_ctx, 0)) < 0) {
        fprintf(stderr, "Failed to retrieve input stream information");
        goto end;
    }

    // 查找视频流
    for (int i = 0; i < ifmt_ctx->nb_streams; i++) {
        if (ifmt_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");
        ret = -1;
        goto end;
    }

    // 打开输出文件
    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);
    if (!ofmt_ctx) {
        fprintf(stderr, "Could not create output context\n");
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    // 复制流到输出上下文
    out_vid_strm = avformat_new_stream(ofmt_ctx, NULL);
    if (!out_vid_strm) {
        fprintf(stderr, "Failed allocating output stream\n");
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    // 复制编解码器参数
    if ((ret = avcodec_parameters_copy(out_vid_strm->codecpar, ifmt_ctx->streams[video_stream_index]->codecpar)) < 0) {
        fprintf(stderr, "Failed to copy codec parameters\n");
        goto end;
    }
    out_vid_strm->codecpar->codec_tag = 0;

    // 打开输出文件
    if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            fprintf(stderr, "Could not open output file '%s'", out_filename);
            goto end;
        }
    }

    // 写文件头
    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        fprintf(stderr, "Error occurred when opening output file\n");
        goto end;
    }

    // 读取并写入数据包
    while (1) {
        AVStream *in_stream, *out_stream;

        ret = av_read_frame(ifmt_ctx, &pkt);
        if (ret < 0)
            break;

        in_stream  = ifmt_ctx->streams[pkt.stream_index];
        out_stream = ofmt_ctx->streams[pkt.stream_index];

        /* copy packet */
        pkt.pts = av_rescale_q_rnd(pkt.pts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
        pkt.dts = av_rescale_q_rnd(pkt.dts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
        pkt.duration = av_rescale_q(pkt.duration, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;

        ret = av_interleaved_write_frame(ofmt_ctx, &pkt);
        if (ret < 0) {
            fprintf(stderr, "Error muxing packet\n");
            break;
        }
        av_packet_unref(&pkt);
    }

    // 写文件尾
    av_write_trailer(ofmt_ctx);

end:
    if (ifmt_ctx) {
        avformat_close_input(&ifmt_ctx);
    }
    if (ofmt_ctx && !(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        avio_closep(&ofmt_ctx->pb);
    }
    if (ofmt_ctx) {
        avformat_free_context(ofmt_ctx);
    }

    return ret;
}