也说alsa音乐播放(mp3文件从零开始)

首先也是最重要的是:我说大话了,本文也就是一个思路而已,如果你是音乐播放小白也想弄明白原理可以往下看,否则,return -1;

现有的音乐格式千差万别,于是就有了ffmpeg话说这个框架真是够牛逼,音频视频转码都不在话下,我是个死脑筋,遇到不懂的老是想弄明白,对于音乐播放感觉是一头雾水,经过几天的研究以后大致想明白了,对一个mp3文件要播放出来需要经过几个步骤:

1.转码,其实也就是提取文件中有用的音频信息

2.经过第一步得到的wav波形文件

3.调用声卡来播放wav文件,对于linux 3.0以后的内核一般为alsa驱动,windows对wav也是天然支持的

关于转码可以见大婶写的转码系列教程http://lfp001.iteye.com/blog/739585

转码命令:

ffmepg -i 1.mp3 -f wav 1.wav

编译: gcc -o alsa alsa.c -lasound

播放命令

./alsa 1.wav

//alsa.c 编写者未知,纯属引用
#include<stdio.h>
#include<stdlib.h>
#include <string.h>
#include <alsa/asoundlib.h>

struct WAV_HEADER
{
    char rld[4];    //riff 标志符号
    int rLen;
    char wld[4];    //格式类型（wave）
    char fld[4];    //"fmt"

    int fLen;   //sizeof(wave format matex)

    short wFormatTag;   //编码格式
    short wChannels;    //声道数
    int   nSamplesPersec ;  //采样频率
    int   nAvgBitsPerSample;//WAVE文件采样大小
    short  wBlockAlign; //块对齐
    short wBitsPerSample;   //WAVE文件采样大小

    char dld[4];        //”data“
    int wSampleLength;  //音频数据的大小

} wav_header;

int set_pcm_play(FILE *fp);

int main(int argc,char *argv[])
{

    if(argc!=2)
    {
        printf("Usage:wav-player+wav file name\n");
        exit(1);
    }

    int nread;
    FILE *fp;
    fp=fopen(argv[1],"rb");
    if(fp==NULL)
    {
        perror("open file failed:\n");
        exit(1);
    }

    nread=fread(&wav_header,1,sizeof(wav_header),fp);
    printf("nread=%d\n",nread);

    //printf("RIFF 标志%s\n",wav_header.rld);
    printf("文件大小rLen：%d\n",wav_header.rLen);
    //printf("wld=%s\n",wav_header.wld);
    //printf("fld=%s\n",wav_header.fld);

   // printf("fLen=%d\n",wav_header.fLen);

    //printf("wFormatTag=%d\n",wav_header.wFormatTag);
    printf("声道数：%d\n",wav_header.wChannels);
    printf("采样频率：%d\n",wav_header.nSamplesPersec);
    //printf("nAvgBitsPerSample=%d\n",wav_header.nAvgBitsPerSample);
    //printf("wBlockAlign=%d\n",wav_header.wBlockAlign);
    printf("采样的位数：%d\n",wav_header.wBitsPerSample);

   // printf("data=%s\n",wav_header.dld);
    printf("wSampleLength=%d\n",wav_header.wSampleLength);

    set_pcm_play(fp);
    return 0;
}

int set_pcm_play(FILE *fp)
{
        int    rc;
        int    ret;
        int    size;
        snd_pcm_t*       handle;        //PCI设备句柄
        snd_pcm_hw_params_t*      params;//硬件信息和PCM流配置
        unsigned int       val;
        int                dir=0;
        snd_pcm_uframes_t  frames;
        char   *buffer;
        int channels=wav_header.wChannels;
        int frequency=wav_header.nSamplesPersec;
        int bit=wav_header.wBitsPerSample;
        int datablock=wav_header.wBlockAlign;
        unsigned char ch[100];  //用来存储wav文件的头信息

        rc=snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
        if(rc<0)
        {
                perror("\nopen PCM device failed:");
                exit(1);
        }

        snd_pcm_hw_params_alloca(¶ms); //分配params结构体
        if(rc<0)
        {
                perror("\nsnd_pcm_hw_params_alloca:");
                exit(1);
        }
         rc=snd_pcm_hw_params_any(handle, params);//初始化params
        if(rc<0)
        {
                perror("\nsnd_pcm_hw_params_any:");
                exit(1);
        }
        rc=snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);                                 //初始化访问权限
        if(rc<0)
        {
                perror("\nsed_pcm_hw_set_access:");
                exit(1);

        }

        //采样位数
        switch(bit/8)
        {
        case 1:snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_U8);
                break ;
        case 2:snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
                break ;
        case 3:snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S24_LE);
                break ;

        }
        rc=snd_pcm_hw_params_set_channels(handle, params, channels);  //设置声道,1表示单声>道，2表示立体声
        if(rc<0)
        {
                perror("\nsnd_pcm_hw_params_set_channels:");
                exit(1);
        }
        val = frequency;
        rc=snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);  //设置>频率
        if(rc<0)
        {
                perror("\nsnd_pcm_hw_params_set_rate_near:");
                exit(1);
        }

        rc = snd_pcm_hw_params(handle, params);
        if(rc<0)
        {
        perror("\nsnd_pcm_hw_params: ");
        exit(1);
        }

        rc=snd_pcm_hw_params_get_period_size(params, &frames, &dir);  /*获取周期长度*/
        if(rc<0)
        {
                perror("\nsnd_pcm_hw_params_get_period_size:");
                exit(1);
        }

        size = frames * datablock;   /*4 代表数据快长度*/

        buffer =(char*)malloc(size);
    fseek(fp,58,SEEK_SET);  //定位歌曲到数据区

    while (1)
        {
                memset(buffer,0,sizeof(buffer));
                ret = fread(buffer, 1, size, fp);
                if(ret == 0)
                {
                        printf("歌曲写入结束\n");
                        break;
                }
                 else if (ret != size)
                {
                 }
                // 写音频数据到PCM设备
        while(ret = snd_pcm_writei(handle, buffer, frames)<0)
           {
                 usleep(2000);
                 if (ret == -EPIPE)
                {
                  /* EPIPE means underrun */
                  fprintf(stderr, "underrun occurred\n");
                  //完成硬件参数设置，使设备准备好
                  snd_pcm_prepare(handle);
                 }
                 else if (ret < 0)
                 {
                          fprintf(stderr,
                      "error from writei: %s\n",
                      snd_strerror(ret));
                 }
            }

    }

        snd_pcm_drain(handle);
        snd_pcm_close(handle);
        free(buffer);
        return 0;
}