Convert a video from a format to another on MAC OSx using FFMPEG API

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ffmpeg

Hello Guys,
I am working on ffmpeg using its libraries on mac OSx. I am trying to transcode a video. For this I am using this :

const char* inFileName = "C:\\abc.avi"; 
av_register_all(); 
AVFormatContext* inContainer = NULL; 
if(avformat_open_input(&inContainer, inFileName, NULL, NULL) < 0)       
exit(1);   
// Find video stream 
int videoStreamIndex = -1; 
for (unsigned int i = 0; i < inContainer->nb_streams; ++i) 
{ 
      if (inContainer->streams[i] && inContainer->streams[i]->codec && 
            inContainer->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) 
            { 
                   videoStreamIndex = i; 
                   break; 
            } 
}   
if (videoStreamIndex == -1) 
    exit(1);     
av_dump_format(inContainer, 0, inFileName,0); 

//Write header to ouput container 
avformat_write_header(outContainer, NULL); 

AVPacket decodePacket, encodedPacket; 
int got_frame, len; 
while(av_read_frame(inContainer, &decodePacket)>=0) 
{ 
     if (decodePacket.stream_index == videoStreamIndex) 
     { 
           AVFrame *decodedFrame = avcodec_alloc_frame();
     } 
}     

this avcodec_alloc_frame() functions returns AVFrame. But here i am not getting the proper frame. When i try to debug the code, it returns me frame but i think its empty. it shows no width, height and data in it. Can anyone please tell me what is the mistake in it. Your help will really appreciate.

Thanks.
Sam