ffmpeg中stream_loop参数不生效原因分析

问题

使用ffmpeg把一个视频文件发布成一个rtmp流，并设置成循环推流，此时需要使用参数stream_loop，命令如下:

ffmpeg.exe -stream_loop -1 -re -i D:\tools\ffmpeg-5.1.2\bin\sei.h264 -c copy -f flv -safe 0 rtmp://localhost:1935/live/test

但是发现一个问题，stream_loop参数没有生效，循环推流失败，报错如下：

[flv @ 0000024c65c266c0] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
Seek to start failed.=-1.0 size= 21954kB time=00:00:46.38 bitrate=3877.7kbits/s speed= 1x
D:\tools\ffmpeg-5.1.2-full_build\bin\sei.h264: Operation not permitted
[flv @ 0000024c65c266c0] Failed to update header with correct duration.
[flv @ 0000024c65c266c0] Failed to update header with correct filesize.
frame= 1116 fps= 24 q=-1.0 Lsize= 21954kB time=00:00:46.50 bitrate=3867.3kbits/s speed=0.999x
video:21932kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.100925%

截图：

解决

通过分析日志可以发现，报错前出现了一个报警信息：Seek to start failed. 个人猜测循环推流没有成功就是因为Seek操作失败了，首先看一下ffmpeg源码：

    if (ret < 0 && ifile->loop) {
        AVCodecContext *avctx;
        for (i = 0; i < ifile->nb_streams; i++) {
            ist = input_streams[ifile->ist_index + i];
            avctx = ist->dec_ctx;
            if (ist->processing_needed) {
                ret = process_input_packet(ist, NULL, 1);
                if (ret>0)
                    return 0;
                if (ist->decoding_needed)
                    avcodec_flush_buffers(avctx);
            }
        }
#if HAVE_THREADS
        free_input_thread(file_index);
#endif
        ret = seek_to_start(ifile, is);
#if HAVE_THREADS
        thread_ret = init_input_thread(file_index);
        if (thread_ret < 0)
            return thread_ret;
#endif
        if (ret < 0)
            av_log(NULL, AV_LOG_WARNING, "Seek to start failed.\n");
        else
            ret = get_input_packet(ifile, &pkt);
        if (ret == AVERROR(EAGAIN)) {
            ifile->eagain = 1;
            return ret;
        }
    }

报警是seek_too_start方法触发的，传入的参数是文件名，同时，接下来的报错又是文件读取没有权限：Operation not permitted。所以自己大胆猜测可能是文件格式不支持。

于是，自己又将sei.h264封装成mp4，命令如下：