libx264 encoded video lags by few seconds in comparison to audio

Below is the code i am using for reading a .3gp file encoding it using libx264 and writing it to .mp4 container. Code is working but when i play .mp4 file then video is not in sync with audio. Video is few second slow in comparison to audio.

// Read all packet from context
while( av_read_frame( fmt_ctx , pAVPacket ) >= 0 )
{ int decoded = pAVPacket->size; if(pAVPacket->stream_index == video_stream_idx) { //decode packet value = avcodec_decode_video2(video_dec_ctx , pAVFrame , &frameFinished , pAVPacket ); if( value < 0) { av_free_packet(pAVPacket); return -1; } if(frameFinished)// Frame successfully decoded :) { sws_scale(swsCtx_, (const uint8_t *const *) pAVFrame->data, pAVFrame->linesize, 0, video_dec_ctx->height, outFrame->data, outFrame->linesize); av_init_packet(&outPacket); outPacket.data = NULL; // packet data will be allocated by the encoder outPacket.size = 0; outFrame->pts=z; //encode frame avcodec_encode_video2(outAVCodecContext , &outPacket ,outFrame , &got_picture); if(got_picture) { if(outPacket.pts != AV_NOPTS_VALUE) outPacket.pts = av_rescale_q(outPacket.pts, video_st- >codec->time_base, video_st->time_base); if(outPacket.dts != AV_NOPTS_VALUE) outPacket.dts = av_rescale_q(outPacket.dts, video_st- >codec->time_base, video_st->time_base); //Write encoded packet to file. if(av_write_frame(outAVFormatContext , &outPacket) != 0) { av_free_packet(pAVPacket); av_free_packet(&outPacket); return -1; } z++; av_free_packet(&outPacket); } // got_picture } // got_picture } // If packet is from audio stream else if (pAVPacket->stream_index == audio_stream_idx) { //Write to file without decoding and encoding it if(av_write_frame(outAVFormatContext , pAVPacket) != 0) { } } av_free_packet(pAVPacket); //z++;
}// End of while-loop
value = av_write_trailer(outAVFormatContext);
if( value < 0)
{ return -1;
}

Can you please help in finding out what i am doing wrong?