只要流程图如下：

注意事项：
1. 里面的gy_log_info和gy_log_error为我自己编写的日志系统，需要自己替换成printf。
2. 文件路径请自行修改，内部使用的是追加，所以一开始要用fopen来清除文件的内容。

源码如下：

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
#include <libavutil/log.h>

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define INBUF_SIZE 4096

uint64_t fileSize = 0;

static void decode(AVCodecContext *dec_ctx, AVFrame *frame, AVPacket *pkt,
                   const char *filename)
{
  char buf[1024];
  int ret;
  FILE* fp;
  int i;

  // 将包含h264数据的packet传入AVCodecContext里面，该过程包含解码过程，接完的数据放置于AVCodecContext 里面。
  ret = avcodec_send_packet(dec_ctx, pkt);
  if (ret < 0) {
    gy_log_error(__FILE__, __LINE__, "Error sending a packet for decoding.\n");
    exit(1);
  }

  fp = fopen(filename, "a+");
  if(fp == NULL)
  {
    gy_log_error(__FILE__, __LINE__, "Open %s error.\n", filename);
    exit(-1);
  }

  while (ret >= 0) 
  {
    // 从AVCodecContext 里面获取数据
    ret = avcodec_receive_frame(dec_ctx, frame);
    if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
    {
      fclose(fp);
      return;
    }
    else if (ret < 0) 
    {
      gy_log_error(__FILE__, __LINE__, "Error during decoding.\n");
      exit(1);
    }

    gy_log_info(__FILE__, __LINE__, "Saving frame %d.\n", dec_ctx->frame_number);
    fflush(stdout);

    /* the picture is allocated by the decoder. no need to
        free it */
    gy_log_info(__FILE__, __LINE__, "P5:\n\t(%d %d) %d %d %d\n", frame->width, frame->height, frame->linesize[0], frame->linesize[1], frame->linesize[2]);
    // 写入文件，由于我这里是yuv420，所以是这么编写的，其他格式请自行参考。
    fwrite(frame->data[0], frame->height, frame->width, fp);
    fwrite(frame->data[1], frame->height/2, frame->width/2, fp);
    fwrite(frame->data[2], frame->height/2, frame->width/2, fp);
  }

  fclose(fp);
  return;
}

void decode_avc(const char* inputFilePath, const char* outputFilePath)
{
  FILE *inFp;
  const AVCodec *codec;
  AVCodecParserContext *parser;
  AVCodecContext *c = NULL;
  AVFrame *frame;
  uint8_t inbuf[INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
  uint8_t *data;
  size_t  size = 0;
  size_t  data_size;
  int ret;
  AVPacket *pkt;

  pkt = av_packet_alloc();
  if(!pkt)
  {
    gy_log_error(__FILE__, __LINE__, "Packet alloc error.\n");
    exit(1);
  }

  memset(inbuf + INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);

  codec = avcodec_find_decoder(AV_CODEC_ID_H264);
  if(!codec)
  {
    gy_log_error(__FILE__, __LINE__, "Codec not found.\n");
    exit(-1);
  }

  parser = av_parser_init(codec->id);
  if(!parser)
  {
    gy_log_error(__FILE__, __LINE__, "parser not found.\n");
    exit(-1);
  }

  c = avcodec_alloc_context3(codec);
  if(!c)
  {
    gy_log_error(__FILE__, __LINE__, "Could not allocate video codec context.\n");
    exit(-1);
  }

  if(avcodec_open2(c, codec, NULL) < 0)
  {
    gy_log_error(__FILE__, __LINE__, "Could not open codec.\n");
    exit(-1);
  }

  inFp = fopen(inputFilePath, "rb");
  if(!inFp)
  {
    gy_log_error(__FILE__, __LINE__, "Could not open %s.\n", inputFilePath);
    exit(-1);
  }

  frame = av_frame_alloc();
  if(!frame)
  {
    gy_log_error(__FILE__, __LINE__, "Could not allocate video frame.\n");
    exit(-1);
  }

  while(!feof(inFp))
  {
    data_size = fread(inbuf, 1, INBUF_SIZE, inFp);
    if(!data_size)
    {
      gy_log_info(__FILE__, __LINE__, "File is end. The sum of size is %d\n", size);
      break;
    }
    size += data_size;

    data = inbuf;
    while(data_size > 0)
    {
      // 获取data数据，并将其转化成packet。
      ret = av_parser_parse2(parser, c, &pkt->data, &pkt->size, data,
                             data_size, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);

      if(ret < 0)
      {
        gy_log_error(__FILE__, __LINE__, "Error while parsing.\n");
        exit(-1);
      }

      data      += ret;
      data_size -= ret;

      if(pkt->size)
        decode(c, frame, pkt, outputFilePath);
    }
  }

  /* flush the decoder */
  decode(c, frame, NULL, outputFilePath);
  
  fclose(inFp);

  av_parser_close(parser);
  avcodec_free_context(&c);
  av_frame_free(&frame);
  av_packet_free(&pkt);
}

int main()
{
  FILE* fp = fopen("/home/user/TestFfmpeg/out10.yuv", "w");
  fclose(fp);
  decode_avc("/home/user/TestFfmpeg/out.h264", "/home/user/TestFfmpeg/out10.yuv");

  return 0;
}