h264解码成yuv与aac解码成pcm原理是一样的。下面是流程图

aac转pcm                                                                                                  h264转yuv

从流程图看，步骤是完全一样的。

下面我们上完整的代码

#include <stdio.h>
#include <stdlib.h>
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libavutil/imgutils.h>
#include <libswscale/swscale.h>

int main(int argc, char *argv[])
{
    char* in_file = NULL;
    char* out_file = NULL;
    av_log_set_level(AV_LOG_INFO);
    if(argc < 3){
        av_log(NULL, AV_LOG_ERROR, "缺少输入文件和输出文件参数");
        return -1;
    }
    in_file = argv[1];
    out_file = argv[2];

    AVFormatContext *fmt_ctx = NULL;
    AVCodecContext *cod_ctx = NULL;
    AVCodec *cod = NULL;
    struct SwsContext *img_convert_ctx = NULL;
    int ret = 0;
    AVPacket packet;

    //第一步创建输入文件AVFormatContext
    fmt_ctx = avformat_alloc_context();
    if (fmt_ctx == NULL)
    {
        ret = -1;
        av_log(NULL, AV_LOG_ERROR,"alloc fail");
        goto __ERROR;
    }
    if (avformat_open_input(&fmt_ctx, in_file, NULL, NULL) != 0)
    {
        ret = -1;
        av_log(NULL, AV_LOG_ERROR,"open fail");
        goto __ERROR;
    }

    //第二步 查找文件相关流，并初始化AVFormatContext中的流信息
    if (avformat_find_stream_info(fmt_ctx, NULL) < 0)
    {
        ret = -1;
        av_log(NULL, AV_LOG_ERROR,"find stream fail");
        goto __ERROR;
    }

    av_dump_format(fmt_ctx, 0, in_file, 0);

    //第三步查找视频流索引和解码器
    int stream_index = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, &cod, -1);

    //第四步设置解码器上下文并打开解码器
    AVCodecParameters *codecpar = fmt_ctx->streams[stream_index]->codecpar;
    if (!cod)
    {
        ret = -1;
        av_log(NULL, AV_LOG_ERROR,"find codec fail");
        goto __ERROR;
    }
    cod_ctx = avcodec_alloc_context3(cod);
    avcodec_parameters_to_context(cod_ctx, codecpar);
    ret = avcodec_open2(cod_ctx, cod, NULL);
    if (ret < 0)
    {
        av_log(NULL, AV_LOG_ERROR,"can't open codec");
        goto __ERROR;
    }

    //第五步打开输出文件
    FILE *out_fb = NULL;
    out_fb = fopen(out_file, "wb");
    if (!out_fb)
    {
        av_log(NULL, AV_LOG_ERROR,"can't open file");
        goto __ERROR;
    }

    //创建packet,用于存储解码前的数据
    av_init_packet(&packet);


    //第六步创建Frame，用于存储解码后的数据
    AVFrame *frame = av_frame_alloc();
    frame->width = codecpar->width;
    frame->height = codecpar->height;
    frame->format = codecpar->format;
    av_frame_get_buffer(frame, 32);

    AVFrame *yuv_frame = av_frame_alloc();
    yuv_frame->width = codecpar->width;
    yuv_frame->height = codecpar->height;
    yuv_frame->format = AV_PIX_FMT_YUV420P;
    av_frame_get_buffer(yuv_frame, 32);

    // size_t writesize = av_image_get_buffer_size(frame->format, frame->width,frame->height, 32);
    //第七步重采样初始化与设置参数
    // uint8_t **data = (uint8_t **)av_calloc((size_t)out_channels, sizeof(*data))
                                      
    img_convert_ctx = sws_getContext(codecpar->width, 
                                    codecpar->height, 
                                    codecpar->format, 
                                    codecpar->width, 
                                    codecpar->height, 
                                    AV_PIX_FMT_YUV420P, 
                                    SWS_BICUBIC, 
                                    NULL, NULL, NULL);

    //while循环，每次读取一帧，并转码
    //第八步 读取数据并解码,重采样进行保存
    int count = 0;
    while (av_read_frame(fmt_ctx, &packet) >= 0)
    {
        if (packet.stream_index != stream_index)
        {
            av_packet_unref(&packet);
            continue;
        }

        
        ret = avcodec_send_packet(cod_ctx, &packet);
        if (ret < 0)
        {
            ret = -1;
            av_log(NULL, AV_LOG_ERROR,"decode error");
            goto __ERROR;
        }

        while (avcodec_receive_frame(cod_ctx, frame) >= 0)
        {
            av_log(NULL, AV_LOG_INFO,"decode frame count = %d\n" , count++);
            sws_scale(img_convert_ctx,
                         (const uint8_t **)frame->data, 
                        frame->linesize,
                        0,
                        codecpar->height, 
                        yuv_frame->data,
                         yuv_frame->linesize);
            int y_size = cod_ctx->width * cod_ctx->height;
            fwrite(yuv_frame->data[0], 1, y_size, out_fb);
            fwrite(yuv_frame->data[1], 1, y_size/4, out_fb);
            fwrite(yuv_frame->data[2], 1, y_size/4, out_fb);
        }

        av_packet_unref(&packet);
    }

     while(1)
    {
        av_log(NULL, AV_LOG_INFO,"flush decode \n");
        ret = avcodec_send_packet(cod_ctx, NULL);
        if (ret < 0)
        {
            ret = -1;
            av_log(NULL, AV_LOG_ERROR,"flush decode error\n");
            goto __ERROR;
        }

        while (avcodec_receive_frame(cod_ctx, frame) >= 0)
        {
           av_log(NULL, AV_LOG_INFO,"flush decode frame count = %d" , count++);
           sws_scale(img_convert_ctx,
                         (const uint8_t **)frame->data, 
                        frame->linesize,
                        0,
                        codecpar->height, 
                        yuv_frame->data,
                         yuv_frame->linesize);
            int y_size = cod_ctx->width * cod_ctx->height;
            fwrite(yuv_frame->data[0], 1, y_size, out_fb);
            fwrite(yuv_frame->data[1], 1, y_size/4, out_fb);
            fwrite(yuv_frame->data[2], 1, y_size/4, out_fb);
        }
    
    }



__ERROR:
    if (fmt_ctx)
    {
        avformat_close_input(&fmt_ctx);
        avformat_free_context(fmt_ctx);
    }

    if (cod_ctx)
    {
        avcodec_close(cod_ctx);
        avcodec_free_context(&cod_ctx);
    }

    if (out_fb)
    {
        fclose(out_fb);
    }

    if (frame)
    {
        av_frame_free(&frame);
    }

    if (yuv_frame)
    {
        av_frame_free(&yuv_frame);
    }

    if(img_convert_ctx)
    {
        sws_freeContext(img_convert_ctx);
    }


    return ret;
}