gistfile1.txt

static int process_command(AVFilterContext *ctx, const char *cmd, const char *args,
                           char *res, int res_len, int flags)
{
    InputSwapContext *inputswap = ctx->priv;
    int ret = AVERROR(ENOSYS);

    if (!strcmp(cmd, "swap")) {
        int nb_streams = 1, i;
        char name[16];
        char default_streams[16], *stream_specs, *spec, *cursor;
        AVStream *st;


        /* Close old input */
        for (i = 0; i < ctx->nb_outputs; i++) {
            av_freep(&ctx->output_pads[i].name);
            if (inputswap->st[i].st)
                avcodec_free_context(&inputswap->st[i].codec_ctx);
        }
        av_freep(&inputswap->st);
        av_freep(&inputswap->out_index);
        if (inputswap->format_ctx)
            avformat_close_input(&inputswap->format_ctx);

        /* Open new input */
        
        stream_specs = inputswap->stream_specs;
        if (!stream_specs) {
            snprintf(default_streams, sizeof(default_streams), "d%c%d",
                 !strcmp(ctx->filter->name, "ainputswap") ? 'a' : 'v',
                 inputswap->stream_index);
            stream_specs = default_streams;
        }
        for (cursor = stream_specs; *cursor; cursor++)
            if (*cursor == '+')
               nb_streams++;

        inputswap->format_ctx = NULL;
        ret = avformat_open_input(&inputswap->format_ctx, args, NULL, NULL);
        av_log(ctx, AV_LOG_WARNING, "Open input %d", ret);
        ret = avformat_find_stream_info(inputswap->format_ctx, NULL);
        av_log(ctx, AV_LOG_WARNING, "stream info %d", ret);
        for (i = 0; i < inputswap->format_ctx->nb_streams; i++)
            inputswap->format_ctx->streams[i]->discard = AVDISCARD_ALL;
        inputswap->st = av_calloc(1, sizeof(*inputswap->st));
        for (i = 0; i < nb_streams; i++) {
            spec = av_strtok(stream_specs, "+", &cursor);
            if (!spec)
                return AVERROR_BUG;
            stream_specs = NULL; 
            st = find_stream(ctx, inputswap->format_ctx, spec);
            if (!st)
                return AVERROR(EINVAL);
            st->discard = AVDISCARD_DEFAULT;
            inputswap->st[i].st = st;
            inputswap->max_stream_index = FFMAX(inputswap->max_stream_index, st->index);
            inputswap->st[i].discontinuity_threshold =
                av_rescale_q(inputswap->discontinuity_threshold, AV_TIME_BASE_Q, st->time_base);
        }
        inputswap->out_index = av_calloc(inputswap->max_stream_index + 1,
                                     sizeof(*inputswap->out_index));
        for (i = 0; i <= inputswap->max_stream_index; i++)
            inputswap->out_index[i] = -1;
        for (i = 0; i < nb_streams; i++) {
            AVFilterPad pad = { 0 };
            inputswap->out_index[inputswap->st[i].st->index] = i;
            snprintf(name, sizeof(name), "out%d", i);
            pad.type          = inputswap->st[i].st->codecpar->codec_type;
            pad.name          = av_strdup(name);
            if (!pad.name)
                return AVERROR(ENOMEM);
            pad.config_props  = inputswap_config_output_props;
            pad.request_frame = inputswap_request_frame;
            if ((ret = ff_insert_outpad(ctx, i, &pad)) < 0) {
                av_freep(&pad.name);
                return ret;
            }
            if ( inputswap->st[i].st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
                !inputswap->st[i].st->codecpar->channel_layout) {
                ret = guess_channel_layout(&inputswap->st[i], i, ctx);
                if (ret < 0)
                    return ret;
            }
            ret = open_stream(ctx, &inputswap->st[i]);
            if (ret < 0)
                return ret;
        }
        return 0;
    }

    return ret;
}