av_seek_frame实战--跳转到文件指定时间后开始推流或写入新文件

av_seek_frame用于跳转到指定之间点。
需要了解，当我们用avformat_open_input打开一个文件时，其实是将这个文件中的内容先读取到了内存中，当我们再去av_read_frame其实是去内存中读取文件，av_read_frame是怎么找到这块内存的呢，是AVFormatContext这个上下文标记了这个内存。
如果直接调用av_read_frame就是从内存起始位置开始读，当我们想从指定时间读取时，在调用av_read_frame前，需要要用av_seek_frame跳转到指定时间，此时AVFormatContext上下文就标记在指定时间，再用av_read_frame读时，就到这个指定时间读了。

#include 
#include 
#include 

static void log_packet(const AVFormatContext *fmt_ctx, const AVPacket *pkt, const char *tag)
{
    AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base;

    printf("%s: pts:%s pts_time:%s dts:%s dts_time:%s duration:%s duration_time:%s stream_index:%d\n",
           tag,
           av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, time_base),
           av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, time_base),
           av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, time_base),
           pkt->stream_index);
}

int cut_video(double from_seconds, double end_seconds, const char* in_filename, const char* out_filename) {
    AVOutputFormat *ofmt = NULL;
    AVFormatContext *ifmt_ctx = NULL, *ofmt_ctx = NULL;
    AVPacket pkt;
    int ret, i;

    av_register_all();

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

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

    av_dump_format(ifmt_ctx, 0, in_filename, 0);

    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;
    }

    ofmt = ofmt_ctx->oformat;

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        AVStream *in_stream = ifmt_ctx->streams[i];
        AVStream *out_stream = avformat_new_stream(ofmt_ctx, in_stream->codec->codec);
        if (!out_stream) {
            fprintf(stderr, "Failed allocating output stream\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        ret = avcodec_copy_context(out_stream->codec, in_stream->codec);
        if (ret < 0) {
            fprintf(stderr, "Failed to copy context from input to output stream codec context\n");
            goto end;
        }
        out_stream->codec->codec_tag = 0;
        if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
            out_stream->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
    }
    av_dump_format(ofmt_ctx, 0, out_filename, 1);

    if (!(ofmt->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;
        }
    }

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

    //    int indexs[8] = {0};


    //    int64_t start_from = 8*AV_TIME_BASE;
    ret = av_seek_frame(ifmt_ctx, -1, from_seconds*AV_TIME_BASE, AVSEEK_FLAG_ANY);
    if (ret < 0) {
        fprintf(stderr, "Error seek\n");
        goto end;
    }

    int64_t *dts_start_from = malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);
    memset(dts_start_from, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);
    int64_t *pts_start_from = malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);
    memset(pts_start_from, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);

    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];

        log_packet(ifmt_ctx, &pkt, "in");

        if (av_q2d(in_stream->time_base) * pkt.pts > end_seconds) {
            av_free_packet(&pkt);
            break;
        }

        if (dts_start_from[pkt.stream_index] == 0) {
            dts_start_from[pkt.stream_index] = pkt.dts;
            printf("dts_start_from: %s\n", av_ts2str(dts_start_from[pkt.stream_index]));
        }
        if (pts_start_from[pkt.stream_index] == 0) {
            pts_start_from[pkt.stream_index] = pkt.pts;
            printf("pts_start_from: %s\n", av_ts2str(pts_start_from[pkt.stream_index]));
        }

        /* copy packet */
        pkt.pts = av_rescale_q_rnd(pkt.pts - pts_start_from[pkt.stream_index], in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
        pkt.dts = av_rescale_q_rnd(pkt.dts - dts_start_from[pkt.stream_index], in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
        if (pkt.pts < 0) {
            pkt.pts = 0;
        }
        if (pkt.dts < 0) {
            pkt.dts = 0;
        }
        pkt.duration = (int)av_rescale_q((int64_t)pkt.duration, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;
        log_packet(ofmt_ctx, &pkt, "out");
        printf("\n");

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

    av_write_trailer(ofmt_ctx);
end:

    avformat_close_input(&ifmt_ctx);

    /* close output */
    if (ofmt_ctx && !(ofmt->flags & AVFMT_NOFILE))
        avio_closep(&ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);

    if (ret < 0 && ret != AVERROR_EOF) {
        fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));
        return 1;
    }

    return 0;
}

int main(int argc, char *argv[]){
    if(argc < 5){
        fprintf(stderr, "Usage: \
                command startime, endtime, srcfile, outfile");
        return -1;
    }

    double startime = atoi(argv[1]);
    double endtime = atoi(argv[2]);
    cut_video(startime, endtime, argv[3], argv[4]);

    return 0;
}