libmad学习进阶2----利用libmad将mp3转码成pcm

# include <stdio.h>
# include <unistd.h>
# include <sys/stat.h>
# include <sys/mman.h>

# include "mad.h"
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<stdlib.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.
*/
#define printf 
static Get_file_length(char *PATH);
static int decode(unsigned char const *, unsigned long);

int main(int argc, char *argv[])
{
printf("The main is start!\n");
struct stat stat;
void *fdm;

int fd;
//char buffer1[80000];
printf("###The input file is %s ! the arc=%d###\n",argv[1],argc);
if (argc == 1)
{
printf("The argc is wrong!\n");
return 1;
}

#if 0
if (fstat(STDIN_FILENO, &stat) == -1 ||
stat.st_size == 0)
return 2;
#endif

fd =open(argv[1],O_RDWR);
if(-1==fd)
{
printf("sorry,The file open is faild!\n");
}
else
{
printf("The file open is sucessed!\n");
}
//read(fd,buffer1,sizeof(buffer1));
//printf("%s", buffer1);
stat.st_size = Get_file_length(argv[1]);
printf("The file size is %d\n",stat.st_size );
printf("The Map is begin!\n");

fdm = mmap(0, stat.st_size, PROT_READ, MAP_SHARED, fd, 0);
if (fdm == MAP_FAILED)
{
printf("mmap is failed\n");
return 3;
}

decode(fdm, stat.st_size);

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

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;

if (!buffer->length)
return MAD_FLOW_STOP;

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

buffer->length = 0;
printf("1111");
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);
}

static int Get_file_length(char *PATH)
{ 
FILE *fp;
fp=fopen(PATH,"r");
if(!fp)
{
printf("sorry,The file open is faild!\n");
}
else
{
printf("The file open is sucessed!\n");
}
fseek(fp, 0L,SEEK_END);
return (ftell(fp));
}
/*
* 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;
mad_fixed_t const *left_ch, *right_ch;
static FILE *fdout;
char buf[1];

/* pcm->samplerate contains the sampling frequency */
fdout= fopen("mypcm.pcm","ab+");
if(!fdout)
{
printf("open is failed\n");
}
else
printf("out open is ok\n");
nchannels = pcm->channels;
nsamples = pcm->length;
left_ch = pcm->samples[0];
right_ch = pcm->samples[1];
//printf("channels=%d, samples=%d\n", nchannels,nsamples);
while (nsamples--) {
signed int sample;

/* output sample(s) in 16-bit signed little-endian PCM */
// printf("2222\n");
sample = scale(*left_ch++);

buf[0]=(sample >> 0) & 0xff;
printf("%d\t",buf[0]);
fwrite(buf,1,1,fdout);

buf[0]=(sample >> 8) & 0xff;
printf("%d\t",buf[0]);
fwrite(buf,1,1,fdout);
if (nchannels == 2) {
sample = scale(*right_ch++);

buf[0]=(sample >> 0) & 0xff;
fwrite(buf,1,1,fdout);

buf[0]=(sample >> 8) & 0xff;
fwrite(buf,1,1,fdout);
}
}
fclose(fdout);
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;

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;

}

liamad中的原始的minimad.c数据输入来自于标准 输入，数据 输出到标准输出。通过读写文件的方式，就可以 将一个mp3文件转换成pcm文件。pcm文件就可以直接通过工具播放或送如声音驱动设备中，不同系统不同平台部一样，后面将讲如何一个mp3文件解码成pcm直接送入声卡播放。