基于libmad编写mp3解码程序

最新推荐文章于 2025-05-17 14:41:51 发布

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本文介绍了一款基于libmad库实现的MP3文件解码和播放程序。该程序使用C++编写，通过mmap映射文件到内存，并利用/dev/dsp设备进行音频输出。文章提供了完整的源代码和Makefile配置。

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代码参考自：http://blog.youkuaiyun.com/scu_guzo/article/details/2942575

一. 首先，下载libmad：
http://www.linuxfromscratch.org/blfs/view/svn/multimedia/libmad.html

二. 编译安装libmad：
1) libmad-0.15.1b$ sudo ./configure
2) 修改Makefile，去掉-fforce-mem选项
3) libmad-0.15.1b$ sudo make
4) libmad-0.15.1b$ sudo make install

三. 编写解码程序：
源代码如下：
------------------------------------------------------------------------------------------------------------------------------------------

# 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;
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);
    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((const unsigned char*)fdm, stat.st_size);
    if (munmap(fdm, stat.st_size) == -1)
    return 4;
    ioctl(sfd, SNDCTL_DSP_RESET, 0);
    close(sfd);
    return 0;
}
static
enum mad_flow input(void *data, struct mad_stream *stream)
{
    struct buffer *buffer = (struct buffer*)data;
    if (!buffer->length)
    return MAD_FLOW_STOP;
    mad_stream_buffer(stream, buffer->start, buffer->length);
    buffer->length = 0;
    return MAD_FLOW_CONTINUE;
}

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;

    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;
    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;
    OutputPtr = Output;
    while (n) {
    wrote = write(sfd, OutputPtr, n);
    OutputPtr += wrote;
    n -= wrote;
    }
    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;
}

------------------------------------------------------------------------------------------------------------------------------------------
Makefile源码：
TARGET = mp3player

INC_DIRS := -I./libmad-0.15.1b

LIBS_DIR := \

LIBS := \
-lmad

OBJECTS := ./mp3player.o \

CFLAGS := \
$(INC_DIRS) -O0 -g

#all: $(TARGET)
#.PHONY: all

$(TARGET): $(OBJECTS)
    g++ -o $(TARGET) $(OBJECTS) $(LDFLAGS) $(LIBS)

$(OBJECTS): %.o:%.c
    g++ -c $(CFLAGS) $< -o $@

.PHONY: clean
clean:
    rm -rf $(OBJECTS) $(TARGET) *~

------------------------------------------------------------------------------------------------------------------------------------------

四. 编译运行：
$make
$./mp3player 1.mp3