程序說明：其實本來應(yīng)該是在output函數(shù)中設(shè)置采樣率和聲道數(shù)的，但有莫名奇妙的問題。

所以定了個一般化的

#define SAMPLE_RATE 44100
#define CHANNELS 2
#define PCM_DEVICE "plughw:0,0"

即：mp3的采樣率為44100Hz，聲道數(shù)為2（立體聲）

源代碼：

/* 
 * 本程序是從 minimad 改進而來，如要更詳細的說明請參看 minimad.c
 *
 * MyMinimad.c , 2009/12/25 , SiChuan University , China
 *
 * 編譯： gcc MyMinimad.c -o MyMinimad -lmad -lasound
 * 運行： ./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>
#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 ();
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 (fstat (fd, &stat) == -1 || stat.st_size == 0)
        return -1;
    fdm = mmap (0, stat.st_size, PROT_READ, MAP_SHARED, fd, 0);
    if (fdm == MAP_FAILED)
        return -1;
    if (init_alsa () == -1)
    {
        fprintf (stderr, "init_alsa() error/n");
        return -1;
    }
    decode (fdm, stat.st_size);
    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;
}
/*這一段是處理采樣后的pcm音頻 */
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;        /*播放速度是采樣率的兩倍 */
    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;
        }
    }
    OutputPtr = Output;
    snd_pcm_writei (pcm_handle, OutputPtr, n);
    OutputPtr = Output;
    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;
}