MyMinimad ── Linux下用libmad写的mp3解码播放程序

 首先这里有篇介绍libmad的技术文章：http://www.ibm.com/developerworks/cn/linux/l-cn-libmadmp3player/index.html,介绍了大部分libmad技术。

/*****************************************************************/

平台：ubuntu-8.04.1-amd64-desktop
声卡驱动：oss
安装依赖软件包： sudo apt-get install libmad0 libmad0-dev libid3tag0 libid3tag0-dev

libmad的官网上没有文档，有两个例子：minimad(high-level api),madlld(low-level api)。low-level api用起来太复杂，所以选择minimad为本程序的蓝本。

打开音频设备：

  static int sfd;
  if((sfd=open("/dev/dsp",O_WRONLY))<0)
  {
    printf("can not open device!!!/n");
    return 5;
  }

在output函数中设置音频设备。在mp3字节流解码后得到pcm音频的采样率，声道和音频格式。

一般mp3文件都是立体声（有2个声道），由于要把pcm采样后并处理的数据放入一个char型的数组，而并行的左右声道的每个采样要在字符数组中处理成2个，所以字符数组中的数据的个数应该是pcm音频采样数的4倍。又因为把左右声道的数据合在一个字符数组里串行处理，所以播放的速度应该是pcm音频采样率的两倍。代码如下：

  unsigned char Output[6912],*OutputPtr;
  int fmt,wrote,speed;
    fmt=AFMT_S16_LE;
  speed=pcm->samplerate*2;
  ioctl(sfd, SNDCTL_DSP_SPEED, &(speed));
  ioctl(sfd, SNDCTL_DSP_SETFMT, &fmt);
  ioctl(sfd, SNDCTL_DSP_CHANNELS, &(pcm->channels));
 
  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;
    }
  }
  n*=4;

然后，就是把数据写入/dev/dsp中播放了：
 
  OutputPtr=Output;

  while(n)
  {
    wrote=write(sfd,OutputPtr,n);
    if(wrote==-1) printf("wrote ====  -1");
    OutputPtr+=wrote;
    n-=wrote;
    
  }

minimad是用的从标准输入得到mp3文件字节流，感觉不爽，改了下，用mmap把文件映射到内存

  struct stat stat;
  void *fdm;
  char const *file;
  int fd;
 
  file=argv[1];
  fd=open(file,O_RDONLY);
  if (fstat(fd, &stat) == -1 ||
      stat.st_size == 0)
    return 2;

  fdm = mmap(0, stat.st_size, PROT_READ, MAP_SHARED, fd, 0);

/************************  end  *************************************/

完整源代码

/* * 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 , 2008/09/17 , SiChuan University , China * * 编译： gcc MyMinimad.c -o MyMinimad -lmad -g -Wall * 运行： ./MyMinimad filename.mp3 */# 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>struct buffer { unsigned char const *start; unsigned long length;};static int sfd;/*声音设备的描述符 */static int decode(unsigned char const *, unsigned long);int main(int argc, char *argv[]){ struct stat stat; void *fdm; char const *file; int fd; file = argv[1]; fd = open(file, O_RDONLY); if ((sfd = open("/dev/dsp", O_WRONLY)) < 0) {printf("can not open device!!!/n");return 5; } ioctl(sfd, SNDCTL_DSP_SYNC, 0);/*此句可以不要 */ if (fstat(fd, &stat) == -1 || stat.st_size == 0)return 2; fdm = mmap(0, stat.st_size, PROT_READ, MAP_SHARED, fd, 0); if (fdm == MAP_FAILED)return 3; decode(fdm, stat.st_size); if (munmap(fdm, stat.st_size) == -1)return 4; ioctl(sfd, SNDCTL_DSP_RESET, 0); close(sfd); return 0;}staticenum 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);}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; unsigned char Output[6912], *OutputPtr; int fmt, wrote, speed; 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 */ ioctl(sfd, SNDCTL_DSP_SPEED, &(speed)); ioctl(sfd, SNDCTL_DSP_SETFMT, &fmt); ioctl(sfd, SNDCTL_DSP_CHANNELS, &(pcm->channels)); OutputPtr = 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;} } n *= 4;/*数据长度为pcm音频采样的4倍 */ OutputPtr = Output; while (n) {wrote = write(sfd, OutputPtr, n);OutputPtr += wrote;n -= wrote; } OutputPtr = Output; return MAD_FLOW_CONTINUE;}staticenum mad_flow error(void *data, struct mad_stream *stream, struct mad_frame *frame){ return MAD_FLOW_CONTINUE;}staticint 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;}