libmad解码mp3生成pcm文件

在之前的一篇文章中介绍了如何在linux下，使用libmad解码mp3并且播放，将源码编译了，可以正常的播放MP3文件，但是在项目里面需要将mp3解码成pcm文件，所以需要将之前的代码进行修改一下，修改是在原来的基础上的，这样可以测试一下。

/*  * 本程序是从 minimad 改进而来，如要更详细的说明请参看 minimad.c * * MyMinimad.c , 2009/12/25 , SiChuan University , China * * 编译： gcc MyMinimad.c -o MyMinimad -lmad -lasound * 运行： ./MyMinimad filename.mp3 * * 修改: * 使用libmad解码mp3然后保存成pcm文件。 * 编译:gcc MyMinimad.c -o MyMinimad -lmad * 运行:./MyMinimad filename.mp3 outputfilename.pcm * */#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/stat.h>#include <sys/mman.h>//#include <sys/soundcard.h>#include <sys/ioctl.h>#include <sys/fcntl.h>#include <sys/types.h>#include <mad.h>//#include <alsa/asoundlib.h>#define SAMPLE_RATE 44100#define CHANNELS 2#define PCM_DEVICE "plughw:0,0"static snd_pcm_hw_params_t *hwparams = NULL;static snd_pcm_t *pcm_handle = NULL;struct buffer{    unsigned char const *start;    unsigned long length;};static int decode (unsigned char const *, unsigned long);//static int init_alsa ();FILE *outFile;int main (int argc, char *argv[]){    struct stat stat;    void *fdm;    char const *file;    int fd;if(argc != 3 )return -1;outFile = fopen(argv[2], "w+");    file = argv[1];    fd = open (file, O_RDONLY);    if (fstat (fd, &stat) == -1 || stat.st_size == 0)        return -1;    fdm = mmap (0, stat.st_size, PROT_READ, MAP_SHARED, fd, 0);//将文件内容拷贝到内存里面    if (fdm == MAP_FAILED)        return -1;//   if (init_alsa () == -1)//  {//      fprintf (stderr, "init_alsa() error/n");//     return -1;//  }    decode (fdm, stat.st_size);//进行解码    fclose(outFile);//关闭文件指针    if (munmap (fdm, stat.st_size) == -1)        return -1;    return 0;}//static int init_alsa ()//{//    snd_pcm_stream_t stream = SND_PCM_STREAM_PLAYBACK;//    char *pcm_name;//    int rate = SAMPLE_RATE;     /* Sample rate *///    int exact_rate;             /* Sample rate returned by *///    pcm_name = strdup (PCM_DEVICE);//    snd_pcm_hw_params_alloca (&hwparams);//    if (snd_pcm_open (&pcm_handle, pcm_name, stream, 0) < 0)//   {//        fprintf (stderr, "Error opening PCM device %s/n", pcm_name);//        return -1;//    }//    if (snd_pcm_hw_params_any (pcm_handle, hwparams) < 0)//    {//        fprintf (stderr, "Can not configure this PCM device./n");//        return -1;//   }//    if (snd_pcm_hw_params_set_access (pcm_handle, hwparams, SND_PCM_ACCESS_RW_INTERLEAVED) < 0)//    {//        fprintf (stderr, "Error setting access./n");//        return -1;//    }//    if (snd_pcm_hw_params_set_format (pcm_handle, hwparams, SND_PCM_FORMAT_S16_LE) < 0)//    {//        fprintf (stderr, "Error setting format./n");//        return -1;//    }//    exact_rate = rate;//    if (snd_pcm_hw_params_set_rate_near (pcm_handle, hwparams, &exact_rate, 0) < 0)//    {//        fprintf (stderr, "Error setting rate./n");//        return -1;//    }//    if (rate != exact_rate)//    {//        fprintf (stderr, "The rate %d Hz is not supported by your hardware./n==> Using %d Hz instead./n", rate, exact_rate);//    }//    if (snd_pcm_hw_params_set_channels (pcm_handle, hwparams, CHANNELS) < 0)//    {//        fprintf (stderr, "Error setting channels./n");//        return -1;//    }//    if (snd_pcm_hw_params (pcm_handle, hwparams) < 0)//    {//        fprintf (stderr, "Error setting HW params./n");//        return -1;//    }//    return 0;//}static enum mad_flow input (void *data, struct mad_stream *stream){    struct buffer *buffer = data;    if (!buffer->length)        return MAD_FLOW_STOP;    mad_stream_buffer (stream, buffer->start, buffer->length);    buffer->length = 0;    return MAD_FLOW_CONTINUE;}/*这一段是处理采样后的pcm音频 */static inline signed int scale (mad_fixed_t sample){    sample += (1L << (MAD_F_FRACBITS - 16));    if (sample >= MAD_F_ONE)        sample = MAD_F_ONE - 1;    else if (sample < -MAD_F_ONE)        sample = -MAD_F_ONE;    return sample >> (MAD_F_FRACBITS + 1 - 16);}static enum mad_flow output (void *data, struct mad_header const *header, struct mad_pcm *pcm){    unsigned int nchannels, nsamples, n;    mad_fixed_t const *left_ch, *right_ch;    unsigned char Output[6912], *OutputPtr;    int fmt, wrote, speed, exact_rate, err, dir;    nchannels = pcm->channels;    n = nsamples = pcm->length;    left_ch = pcm->samples[0];    right_ch = pcm->samples[1];//    fmt = AFMT_S16_LE;//    speed = pcm->samplerate * 2;        /*播放速度是采样率的两倍 */    OutputPtr = Output;//将OutputPtr指向Output    while (nsamples--)    {        signed int sample;        sample = scale (*left_ch++);        *(OutputPtr++) = sample >> 0;        *(OutputPtr++) = sample >> 8;        if (nchannels == 2)        {            sample = scale (*right_ch++);            *(OutputPtr++) = sample >> 0;            *(OutputPtr++) = sample >> 8;        }    }    OutputPtr = Output;//由于之前的操作将OutputPtr的指针指向了最后，这时需要将其指针移动到最前面fwrite(OutputPtr, 1, n*2*nchannels, outFile);//    snd_pcm_writei (pcm_handle, OutputPtr, n);    OutputPtr = Output;//写完文件后，OutputPtr的指针也移动到了最后，这时需要将其指针移动到最前面    return MAD_FLOW_CONTINUE;}static enum mad_flow error (void *data, struct mad_stream *stream, struct mad_frame *frame){    return MAD_FLOW_CONTINUE;}static int decode (unsigned char const *start, unsigned long length){    struct buffer buffer;    struct mad_decoder decoder;    int result;    buffer.start = start;    buffer.length = length;    mad_decoder_init (&decoder, &buffer, input, 0, 0, output, error, 0);    mad_decoder_options (&decoder, 0);    result = mad_decoder_run (&decoder, MAD_DECODER_MODE_SYNC);    mad_decoder_finish (&decoder);    return result;}

在终端运行gcc  MyMinimad.c -o MyMinimad -lmad就可以生成解码mp3文件生成pcm的工具.