linux 使用 ALSA lib 播放 aac 檔

先安裝開發程式庫

        sudo   apt   install   libfdk-acc-dev   libasound-dev

下載一些測試 aac 檔案: https://espressif-docs.readthedocs-hosted.com/projects/esp-adf/en/latest/design-guide/audio-samples.html

// 主程式: aacplay.cpp
#include <sys/mman.h>
#include <fdk-aac/aacdecoder_lib.h>
#include <alsa/asoundlib.h>
struct PCM16 {
    int16_t *data;
    int length;
    int channel;
    int fs;
};
const PCM16 zeroPCM16 = {.data = (int16_t *)0, .length = 0, .channel = 0, .fs = 0};
const char *path_to_close = "./close!";
auto stepDecoder = [](const void *path = nullptr, int repeate = 0) {
    static void *mmapFile = MAP_FAILED; // to management mmap file
    static HANDLE_AACDECODER decoder = nullptr;
    static int32_t length = 0;// mmap file length
    static int32_t position = 0; // trace file position
    static int32_t remainRepeate = 0;
    if (path == path_to_close) { // to close decoder
        if (decoder) {
            aacDecoder_Close(decoder);
            decoder = nullptr;
            length = 0;
            position = 0;
            remainRepeate = 0;
        }
    } else {
        if (path) {// use non-null path to initialize mmap
            const char *filename = (const char *)path;
            int fd = open(filename, O_RDONLY);
            if (mmapFile != MAP_FAILED) munmap(mmapFile, length);
            if (fd < 0) {
                printf("%s => not found.\n", filename);
            } else {
                int fileLength = lseek(fd, 0l, SEEK_END);
                if (fileLength >= 512) {
                    mmapFile = mmap(0, fileLength, PROT_READ, MAP_PRIVATE, fd, 0);
                    if (mmapFile == MAP_FAILED)  {
                        printf("%s mmap fail.\n", filename);
                    } else {
                        if (decoder) aacDecoder_Close(decoder);
                        decoder = aacDecoder_Open(TT_MP4_ADTS, 1);
                        printf("%s file length = %d\n", filename, fileLength);
                        length = fileLength;// mmap sucess
                        position = 0;
                        remainRepeate = repeate;
                    }
                }
                close(fd);// After the mmap() call, fd can be closed immediately.
            }
        }
        if (remainRepeate > 0 && (position >= length)) { // when repeate enable
            printf("End of file, position = %d, wrap arond to repeate again. remainRepeate = %d\n", position, remainRepeate);
            remainRepeate --;
            position = 0;
        }
        if (position < length) {
            unsigned char *src[] = { (unsigned char *)mmapFile + position };
            static int16_t pcmdist[1152 * 5];// distinct PCM16 buffer for 5 channels
            const uint32_t maxSteps = length - position;
            uint32_t tempSteps = maxSteps;// tempSteps will be updated by decoder
            aacDecoder_Fill(decoder, src, &maxSteps, &tempSteps);
            int result = (int)aacDecoder_DecodeFrame(decoder, pcmdist, sizeof(pcmdist), 0);
            position += maxSteps - tempSteps; // go ahead, and back off by tempSteps
            printf("maxSteps = %8d, tempSteps = %8d, position = %8d, steps = %8d, err = %4x:\t\t\n",
                maxSteps,
                tempSteps,
                position,
                maxSteps - tempSteps,
                (unsigned)result
            );          
            if (result == AAC_DEC_OK) {               
                CStreamInfo *info = aacDecoder_GetStreamInfo(decoder);                         
                return PCM16 {
                    .data = pcmdist,
                    .length = info->frameSize,
                    .channel = info->numChannels,
                    .fs = info->sampleRate
                };
            }
        }
    }
    return zeroPCM16;
};

int main(int argc, char const *argv[]) {    
    PCM16 frame = stepDecoder((argc > 1) ? argv[1] : "test.aac");
     if (frame.length==0) return 0;
    snd_pcm_t *handle;
    if (snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0) == 0) {
        if (snd_pcm_set_params(handle, SND_PCM_FORMAT_S16_LE, SND_PCM_ACCESS_RW_INTERLEAVED, frame.channel, frame.fs, 1, frame.fs / 4) == 0) {    
            do {
                 int err = snd_pcm_writei(handle, frame.data, frame.length);
                 if (err < 0) {// try to recover
                     if (snd_pcm_recover(handle, err, 0) < 0) {
                        printf("alsa can't recover\n");
                        break;
                    }
                 }
                frame = stepDecoder();
            } while (frame.length);
        }
        snd_pcm_close(handle);
    }   
    stepDecoder(path_to_close);
    return 0;
}

編譯並執行:

    g++   aacplay.cpp   -lfdk-aac   -lasound   &&  ./a.out