[FFmpeg-devel] [PATCH] examples/demuxing_decoding: Fix invalidd API usage

[wm4]

On Thu,  6 Feb 2014 13:03:44 +0000
Derek Buitenhuis <derek.buitenhuis at gmail.com> wrote:

> We are not technically allowed to reuse the codec context
> found inside the AVStream in the format context.
> 
> Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
> ---
>  doc/examples/demuxing_decoding.c | 37 ++++++++++++++++++++++++++-----------
>  1 file changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/doc/examples/demuxing_decoding.c b/doc/examples/demuxing_decoding.c
> index 0ef5df7..c69e07b 100644
> --- a/doc/examples/demuxing_decoding.c
> +++ b/doc/examples/demuxing_decoding.c
> @@ -135,12 +135,11 @@ static int decode_packet(int *got_frame, int cached)
>      return decoded;
>  }
>  
> -static int open_codec_context(int *stream_idx,
> -                              AVFormatContext *fmt_ctx, enum AVMediaType type)
> +static int open_codec_context(int *stream_idx, AVFormatContext *fmt_ctx,
> +                              enum AVMediaType type, AVCodecContext **dec_ctx)
>  {
>      int ret;
>      AVStream *st;
> -    AVCodecContext *dec_ctx = NULL;
>      AVCodec *dec = NULL;
>      AVDictionary *opts = NULL;
>  
> @@ -153,19 +152,37 @@ static int open_codec_context(int *stream_idx,
>          *stream_idx = ret;
>          st = fmt_ctx->streams[*stream_idx];
>  


> -        /* find decoder for the stream */
> -        dec_ctx = st->codec;
> -        dec = avcodec_find_decoder(dec_ctx->codec_id);
> +        /* find decoder for the stream as a sanity check */
> +        dec = avcodec_find_decoder(st->codec->codec_id);
>          if (!dec) {
>              fprintf(stderr, "Failed to find %s codec\n",
>                      av_get_media_type_string(type));
>              return AVERROR(EINVAL);
>          }

What exactly does this sanity check? What happens if this code is
removed, and there's no codec? Shouldn't one of the functions below
simply fail?

>  
> +        /*
> +         * Allocate our decoding context, since we are not allowed
> +         * to reuse the one from the AVStream
> +         */
> +        *dec_ctx = avcodec_alloc_context3(NULL);
> +        if (!*dec_ctx) {
> +            fprintf(stderr, "Failed to allocate %s codec context.\n",
> +                    av_get_media_type_string(type));
> +            return AVERROR(ENOMEM);
> +        }
> +
> +        /* Copy the properties over */
> +        ret = avcodec_copy_context(*dec_ctx, st->codec);
> +        if (ret < 0) {
> +            fprintf(stderr, "Failed to copy %s codec context.\n",
> +                    av_get_media_type_string(type));
> +            return AVERROR(EINVAL);
> +        }
> +
>          /* Init the decoders, with or without reference counting */
>          if (api_mode == API_MODE_NEW_API_REF_COUNT)
>              av_dict_set(&opts, "refcounted_frames", "1", 0);
> -        if ((ret = avcodec_open2(dec_ctx, dec, &opts)) < 0) {
> +        if ((ret = avcodec_open2(*dec_ctx, dec, &opts)) < 0) {
>              fprintf(stderr, "Failed to open %s codec\n",
>                      av_get_media_type_string(type));
>              return ret;
> @@ -252,9 +269,8 @@ int main (int argc, char **argv)
>          exit(1);
>      }
>  
> -    if (open_codec_context(&video_stream_idx, fmt_ctx, AVMEDIA_TYPE_VIDEO) >= 0) {
> +    if (open_codec_context(&video_stream_idx, fmt_ctx, AVMEDIA_TYPE_VIDEO, &video_dec_ctx) >= 0) {
>          video_stream = fmt_ctx->streams[video_stream_idx];
> -        video_dec_ctx = video_stream->codec;
>  
>          video_dst_file = fopen(video_dst_filename, "wb");
>          if (!video_dst_file) {
> @@ -274,9 +290,8 @@ int main (int argc, char **argv)
>          video_dst_bufsize = ret;
>      }
>  
> -    if (open_codec_context(&audio_stream_idx, fmt_ctx, AVMEDIA_TYPE_AUDIO) >= 0) {
> +    if (open_codec_context(&audio_stream_idx, fmt_ctx, AVMEDIA_TYPE_AUDIO, &audio_dec_ctx) >= 0) {
>          audio_stream = fmt_ctx->streams[audio_stream_idx];
> -        audio_dec_ctx = audio_stream->codec;
>          audio_dst_file = fopen(audio_dst_filename, "wb");
>          if (!audio_dst_file) {
>              fprintf(stderr, "Could not open destination file %s\n", video_dst_filename);