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 <stdio.h># include <unistd.h># include <sys/stat.h># include <sys/mman.h>#include <string.h>#include<fcntl.h>#include <stdlib.h>#include <sys/ioctl.h>#include <alsa/asoundlib.h># 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(&params); //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. */staticenum 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 inlinesigned 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. */staticenum 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. */staticenum 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). */staticint 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