本来不是什么难搞的问题，代码写完了，音频流信息中的详细信息，具体代码表现为

format_ctx->streams[audio_stream_index]->codecpar是空指针。

这个查了一圈也没人给出正确答案，实际上是由于我自己编译的ffmpeg时候，开启的选项的导致的。把音频解码器相关的给禁掉了。重新开启相关编译选项，编译ffmpeg后，一切正常。

具体的选项为：

ffmpeg 交叉编译

./configure --prefix=../arm-ffmpegbuild \
--enable-shared \
--enable-libmp3lame \
 --enable-libx264 \
 --enable-gpl \
 --disable-asm \
 --enable-version3 \
 --enable-libmp3lame \
 --enable-libx264 \
 --enable-libvpx \
 --enable-nonfree \
 --cross-prefix=aarch64-linux- \
 --target-os=linux \
 --extra-cflags="-I /opt/ffmpeg_test_make/lame-3.100/lamebuild/include" \
 --extra-ldflags="-L /opt/ffmpeg_test_make/lame-3.100/lamebuild/lib" \
 --enable-cross-compile \
 --enable-small \
 --arch=arm64 \
 --enable-decoder=h264 \
 --enable-parser=h264 \
 --enable-demuxer=rtsp \
 --extra-ldflags="-L ../x264build/lib" \
 --extra-cflags="-I ../x264build/include"
 
 lame交叉编译
 ./configure \
 --host=aarch64-linux \
 --prefix=/opt/ffmpeg_test_make/lame-3.100/lamebuild \
 cc=aarch64-linux-gcc 

话不多说上代码：

bool FfpDecoderWav::dump_wav(std::string rtsp_url, std::string file_path) {
    AVDictionary *format_options = NULL;
    av_dict_set(&format_options, "rtsp_transport", "tcp", 0); // 以tcp的方式打开,
    av_register_all();
    avformat_network_init();
    // 打开 RTSP 流
    int reconnect_times = 3;
    AVFormatContext *format_ctx = NULL;
    bool online = false;
    while (reconnect_times-- > 0) {
        if (format_ctx != NULL) {
            avformat_close_input(&format_ctx);
            format_ctx = NULL;
        }
        format_ctx = avformat_alloc_context();
        if (avformat_open_input(&format_ctx, rtsp_url.c_str(), NULL, &format_options) != 0) {
            Logger::error("open rtsp url:{} faile", rtsp_url);
            // std::this_thread::sleep_for(std::chrono::milliseconds(500));
            usleep(100000);
        } else {
            online = true;
            break;
        }
    }
    av_dict_free(&format_options); // 释放 format_options
    if (!online) {
        return false;
    }
    Logger::info("open rtsp url:{} for wav success", rtsp_url);
    // 查找音频流
    int audio_stream_index = -1;

    if (avformat_find_stream_info(format_ctx, NULL) < 0) {
        Logger::info("can not avformat_find_stream_info url:{}", rtsp_url);
        return false;
    }
    AVCodec *codec = NULL;
    audio_stream_index = av_find_best_stream(format_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, &codec, 0);

    std::cout << "codec name :" << codec->name << std::endl;
    std::cout << "codec long_name :" << codec->long_name << std::endl;
    std::cout << "codec AVMediaType :" << (int)codec->type << std::endl;
    std::cout << "codec AVCodecID :" << (int)codec->id << std::endl;

    if (audio_stream_index < 0 || codec == NULL) {
        Logger::info("can not find sound stream rtsp url:{}", rtsp_url);
        return false;
    }
    Logger::info("find sound stream success index:{}", audio_stream_index);

    av_dump_format(format_ctx, 0, rtsp_url.c_str(), 0);
    bool had_audio_code = true;
    SwrContext *swr_ctx = NULL;
    AVCodecContext *codec_ctx = avcodec_alloc_context3(codec);

    if (format_ctx->streams[audio_stream_index]->codecpar) {
        Logger::info("avcodec_alloc_context3 success channels={}", codec_ctx->channels);
        Logger::info("avcodec_alloc_context3 success sample_rate={}", codec_ctx->sample_rate);
        // std::cout << "had codecpar inf" << std::endl;
        // printf("had codecpar inf\n");
        avcodec_parameters_to_context(codec_ctx, format_ctx->streams[audio_stream_index]->codecpar);
        if (avcodec_open2(codec_ctx, codec, NULL) < 0) {
            Logger::info("avcodec_open2 error rtsp url:{}", rtsp_url);
            return false;
        }
        Logger::info("avcodec_open2 success channels={}", codec_ctx->channels);
        Logger::info("avcodec_open2 success sample_rate={}", codec_ctx->sample_rate);
        // 创建重采样上下文

        swr_ctx = swr_alloc_set_opts(NULL, NUM_CHANNELS, AV_SAMPLE_FMT_S16, SAMPLE_RATE, codec_ctx->channels,
                                     codec_ctx->sample_fmt, codec_ctx->sample_rate, 0, NULL);
        Logger::info("swr_alloc_set_opts success");
        if (!swr_ctx || swr_init(swr_ctx) < 0) {
            // Logger::info("swr_init error rtsp url:{}", rtsp_url);
            return false;
        }
    } else {
        printf("cdecpar is nullodecpar is nullodecpar is nullodecpar is null\n");
        std::cout << "codecpar is null" << std::endl;
        had_audio_code = false;
    }

    // 创建输出 WAV 文件
    std::ofstream wav_file(file_path.c_str(), std::ios::binary);
    if (!wav_file) {
        // Logger::info("fopen local_path save wav failed path:{}", file_path);
        return false;
    }
    // Logger::info("open wav_file success");
    //  写入 WAV 文件头
    WAVHeader wav_header;
    unsigned int file_size = sizeof(wav_header);
    // Logger::info("wav_header size:{}", file_size);
    wav_file.write((const char *)&wav_header, file_size);
    time_t start_time = time(NULL);
    AVPacket packet;
    int ret = 0;
    int count = 1000;
    while (true) {
        if (ret = av_read_frame(format_ctx, &packet) < 0) {
            // Logger::info("av_read_frame failed: {}", ret);
            break;
        }
        time_t current_time = time(NULL);
        time_t duration = current_time - start_time;
        if (duration > 60) {
            // Logger::info("save sound end by 20 s time");
            break;
        }
        if (packet.stream_index == audio_stream_index) {

            if (!had_audio_code) {
                wav_file.write((char *)packet.data, packet.size);
                std::cout << "write sws data codecpar inf insfsjfjaslkjfas" << std::endl;
                printf(" wav_file.write((char *)packet.data, packet.size);\n");
                continue;
            }
            AVFrame *frame = av_frame_alloc();
            if (avcodec_send_packet(codec_ctx, &packet) >= 0 && avcodec_receive_frame(codec_ctx, frame) >= 0) {
                uint8_t *out_buffer[NUM_CHANNELS];
                int out_samples = 0;
                int out_size = 0;
                for (int i = 0; i < NUM_CHANNELS; i++) {
                    out_buffer[i] = (uint8_t *)malloc(frame->nb_samples * 2 * sizeof(uint8_t));
                }
                out_samples = swr_convert(swr_ctx, out_buffer, frame->nb_samples, (const uint8_t **)frame->data,
                                          frame->nb_samples);
                out_size = out_samples * NUM_CHANNELS * 2;
                wav_file.write(reinterpret_cast<char *>(out_buffer[0]), out_size);
                // std::cout << "write sws data codecpar inf" << std::endl;
                // printf(" wav_file.write(reinterpret_cast<char *>(out_buffer[0]), out_siz22;\n");
                for (int i = 0; i < NUM_CHANNELS; i++) {
                    free(out_buffer[i]);
                }
            }
            av_frame_free(&frame);
        }
        av_packet_unref(&packet);
    }
    // 更新 WAV 文件头中的数据大小
    uint32_t subchunk2Size = static_cast<unsigned int>(wav_file.tellp()) - 44;
    uint32_t chunkSize = subchunk2Size + 36;
    wav_file.seekp(4, std::ios::beg);
    wav_file.write(reinterpret_cast<char *>(&chunkSize), 4);

    wav_file.seekp(40, std::ios::beg);
    wav_file.write(reinterpret_cast<char *>(&subchunk2Size), 4);
    // 关闭文件
    wav_file.close();
    // 释放资源
    avcodec_close(codec_ctx);
    avcodec_free_context(&codec_ctx);
    avformat_close_input(&format_ctx);
    swr_free(&swr_ctx);

    // Logger::info("save local_path  wav success path:{}", file_path);

    return true;
}

wav格式的数据头文件：

struct WAVHeader {
    char chunkID[4] = {'R', 'I', 'F', 'F'};
    uint32_t chunkSize = 0;
    char format[4] = {'W', 'A', 'V', 'E'};
    char subchunk1ID[4] = {'f', 'm', 't', ' '};
    uint32_t subchunk1Size = 16;
    uint16_t audioFormat = 1;
    uint16_t numChannels = NUM_CHANNELS;
    uint32_t sampleRate = SAMPLE_RATE;
    uint32_t byteRate = SAMPLE_RATE * NUM_CHANNELS * 16 / 8;
    uint16_t blockAlign = 4;
    uint16_t bitsPerSample = 16;
    char subchunk2ID[4] = {'d', 'a', 't', 'a'};
    uint32_t subchunk2Size = 4;
};

最后，就是wav注意的地方，一共是两个值：

chunkSize 和subchunk2Size

// 更新 WAV 文件头中的数据大小

也就是说：subchunk2Size是出去wav文件头部数据意外的数据长度。

即文件总长度减去头部长度44个字节。

chunkSize=subchunk2Size+36

具体为什么，可以查看wav格式的说明。