linux 基于alsa 使用libmad 解码库实现MP3文件的播放

本设计根据libmad库中minimad.c改写成的，保留了原始的英文注释，minimad.c实现了MP3的解码成PCM音频数据，打印到屏幕上。本设计添加了alsa的播放设置函数，以及在解码output的函数中，将输出写入到声卡中，实现了MP3 文件的解码播放。
注意：本设计编译之前需要编译libmad库， 编译时需要连上 -lmad -lasound 的选项。
使用方法为在终端:./mp3-player + mp3 file.

mp3-player.c:

/*
 * libmad - MPEG audio decoder library
 * Copyright (C) 2000-2004 Underbit Technologies, Inc.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 * $Id: minimad.c,v 1.4 2004/01/23 09:41:32 rob Exp $
 */

# include 
# include 
# include 
# include 


#include 
#include
#include 
#include 
#include 

# include "mad.h"



/*
 * This is perhaps the simplest example use of the MAD high-level API.
 * Standard input is mapped into memory via mmap(), then the high-level API
 * is invoked with three callbacks: input, output, and error. The output
 * callback converts MAD's high-resolution PCM samples to 16 bits, then
 * writes them to standard output in little-endian, stereo-interleaved
 * format.
 */

static int decode(unsigned char const *, unsigned long);
int set_pcm();
snd_pcm_t*             handle=NULL;        //PCI设备句柄
snd_pcm_hw_params_t*   params=NULL;//硬件信息和PCM流配置

int main(int argc, char *argv[])
{
  struct stat stat;
  void *fdm;

  if (argc != 2)
    {
    printf("Usage: minimad + mp3 file name");
    return 1;
    }
  int fd;
  fd=open(argv[1],O_RDWR);
  if(fd<0)
  {
    perror("open file failed:");
    return 1;
  }    

  if (fstat(fd, &stat) == -1 ||stat.st_size == 0)
  {
    printf("fstat failed:\n");
    return 2;
  }
   //printf("stat.st_size=%d\n",stat.st_size);

  fdm = mmap(0, stat.st_size, PROT_READ, MAP_SHARED, fd, 0);
  if (fdm == MAP_FAILED)
    return 3;


  if(set_pcm()!=0)                 //设置pcm 参数
    {
        printf("set_pcm fialed:\n");
        return 1;   
    }
  decode(fdm, stat.st_size);

  if (munmap(fdm, stat.st_size) == -1)
    return 4;

    snd_pcm_drain(handle);
    snd_pcm_close(handle);

  return 0;
}


int set_pcm()
{
    int    rc;     
    int  dir=0;
    int rate = 44100;;                /* 采样频率 44.1KHz*/
    int format = SND_PCM_FORMAT_S16_LE; /*     量化位数 16      */
    int channels = 2;                 /*     声道数 2           */

    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

    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);

        }

    rc=snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);             //设置16位采样精度  
        if(rc<0)
       {
            perror("snd_pcm_hw_params_set_format failed:");
            exit(1);
        } 

    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);
        }    

     rc=snd_pcm_hw_params_set_rate_near(handle, params, &rate, &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);
        } 

    return 0;              
}

/*
 * This is a private message structure. A generic pointer to this structure
 * is passed to each of the callback functions. Put here any data you need
 * to access from within the callbacks.
 */

struct buffer {
  unsigned char const *start;
  unsigned long length;
};

/*
 * This is the input callback. The purpose of this callback is to (re)fill
 * the stream buffer which is to be decoded. In this example, an entire file
 * has been mapped into memory, so we just call mad_stream_buffer() with the
 * address and length of the mapping. When this callback is called a second
 * time, we are finished decoding.
 */

static
enum mad_flow input(void *data,
            struct mad_stream *stream)
{
  struct buffer *buffer = data;

 printf("this is input\n");
  if (!buffer->length)
    return MAD_FLOW_STOP;

  mad_stream_buffer(stream, buffer->start, buffer->length);

  buffer->length = 0;

  return MAD_FLOW_CONTINUE;
}

/*
 * The following utility routine performs simple rounding, clipping, and
 * scaling of MAD's high-resolution samples down to 16 bits. It does not
 * perform any dithering or noise shaping, which would be recommended to
 * obtain any exceptional audio quality. It is therefore not recommended to
 * use this routine if high-quality output is desired.
 */

static inline
signed int scale(mad_fixed_t sample)
{
  /* round */
  sample += (1L << (MAD_F_FRACBITS - 16));

  /* clip */
  if (sample >= MAD_F_ONE)
    sample = MAD_F_ONE - 1;
  else if (sample < -MAD_F_ONE)
    sample = -MAD_F_ONE;

  /* quantize */
  return sample >> (MAD_F_FRACBITS + 1 - 16);
}

/*
 * This is the output callback function. It is called after each frame of
 * MPEG audio data has been completely decoded. The purpose of this callback
 * is to output (or play) the decoded PCM audio.
 */

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;

  /* pcm->samplerate contains the sampling frequency */

  nchannels = pcm->channels;
  n=nsamples  = pcm->length;
  left_ch   = pcm->samples[0];
  right_ch  = pcm->samples[1];

  unsigned char Output[6912], *OutputPtr;  
  int fmt, wrote, speed, exact_rate, err, dir; 


//   printf("This is output\n");



   OutputPtr = Output;  

   while (nsamples--) 
   {
    signed int sample;

    /* output sample(s) in 16-bit signed little-endian PCM */

    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;  
    snd_pcm_writei (handle, OutputPtr, n);  
    OutputPtr = Output;     

  return MAD_FLOW_CONTINUE;
}

/*
 * This is the error callback function. It is called whenever a decoding
 * error occurs. The error is indicated by stream->error; the list of
 * possible MAD_ERROR_* errors can be found in the mad.h (or stream.h)
 * header file.
 */

static
enum mad_flow error(void *data,
            struct mad_stream *stream,
            struct mad_frame *frame)
{
  struct buffer *buffer = data;
  printf("this is mad_flow error\n");
  fprintf(stderr, "decoding error 0x%04x (%s) at byte offset %u\n",
      stream->error, mad_stream_errorstr(stream),
      stream->this_frame - buffer->start);

  /* return MAD_FLOW_BREAK here to stop decoding (and propagate an error) */

  return MAD_FLOW_CONTINUE;
}

/*
 * This is the function called by main() above to perform all the decoding.
 * It instantiates a decoder object and configures it with the input,
 * output, and error callback functions above. A single call to
 * mad_decoder_run() continues until a callback function returns
 * MAD_FLOW_STOP (to stop decoding) or MAD_FLOW_BREAK (to stop decoding and
 * signal an error).
 */

static
int decode(unsigned char const *start, unsigned long length)
{
  struct buffer buffer;
  struct mad_decoder decoder;
  int result;

  /* initialize our private message structure */

  buffer.start  = start;
  buffer.length = length;

  /* configure input, output, and error functions */

  mad_decoder_init(&decoder, &buffer,
           input, 0 /* header */, 0 /* filter */, output,
           error, 0 /* message */);

  /* start decoding */

  result = mad_decoder_run(&decoder, MAD_DECODER_MODE_SYNC);

  /* release the decoder */

  mad_decoder_finish(&decoder);

  return result;
}

转自：http://blog.chinaunix.net/uid-27106528-id-3334746.html