libmad简介
MAD （libmad）是一个开源的高精度 MPEG 音频解码库，支持 MPEG-1（Layer I, Layer II 和 LayerIII（也就是 MP3）。LIBMAD 提供 24 -bit 的 PCM 输出，完全是定点计算，非常适合没有浮点支持的平台上使用。使用 libmad 提供的一系列 API，就可以非常简单地实现 MP3 数据解码工作。
移植涉及到的库： zlib-1.2.3.tar.gz libid3tag-0.15.1b.tar.gz libmad-0.15.1b.tar.gz --------------------------------------------------------------------------------------------------------------- madplay介绍： madplay基于libmad的基础上做了一个播放器，该播放器除了目前不支持网络播放以为，其余功能都支持。如快进播放，seek播放，暂停，恢复等 最后移植了一个基于libmad的应用madplay,可以直接用它来播放mp3. madplay-0.15.2b.tar.gz
介绍完了，当然移植不是一番风顺的，折腾了一天。中间遇到不少问题，还好，最后都一一解决了。
网上提供的代码。交叉编译，由于环境不一样，会出现各种问题。
这里记录一下过程。
首先是交叉编译zlib-1.2.3.tar.gz
用交叉编译工具编译zlib，并且把库生成到交叉编译环境的库目录下 ./configure --prefix=/home/ban/madplay/source 修改Makefile. CC=arm-linux-gnueabihf-gcc AR=arm-linux-gnueabihf-ar rc RANLIB=arm-linux-gnueabihf-ranlib make make install 安装完成后，在/home/ban/madplay/source/ 中将生产lib跟include2个文件夹。
这步一般不会有啥问题，但是，默认成功的是静态库啊， 虽然配置上是说默认配置生成动态库，但是确实没有。
实际是有的，指定--shared即可。
或者仔细查makefile,把相关的编译语句找出来，我手动调用gcc - shared -fPIC - $(OBJS)生成了.so
这里还需要注意的是,指定好自己的--prifix,因为后续的编译，好多是要依赖这个的。
接下来编译libid3tag-0.15.1b.tar.gz
这时候要注意了，如果上一步编译不过，或者没有指定--prifix, 这里就麻烦了。
由于我需要的是动态库，发现configer后，竟没带-FPIC参数，还要去改makefile才行。
./configure --host=arm-linux-gnueabihf --disable-debugging --prefix=/home/ban/madplay/source CPPFLAGS=-I/home/ban/madplay/source/include LDFLAGS=-L/home/ban/madplay/source/lib make make install
编译libmad ./configure --host=arm-linux-gnueabihf --disable-debugging --prefix=/home/ban/madplay/source CPPFLAGS=-I/home/ban/madplay/source/include LDFLAGS=-L/home/ban/madplay/source/lib make make install
出现错误： cc1: error: unrecognized command line option “-fforce-mem” 原因是高版本的gcc，已经将-fforce-mem去除了，解决方法： sed -i '/-fforce-mem/d' configure
再执行： ./configure --host=arm-linux-gnueabihf --prefix=/usr/local/libmad_arm --enable-shared --enable-static --enable-fpm=arm -- with-gnu-ld=arm-linux-gnueabihf-ld --build=arm 出现错误： /tmp/ccf2FxyW.s:1299: Error: selected processor does not support Thumb mode `rsc r0,r0,#0' /tmp/ccf2FxyW.s:1435: Error: selected processor does not support Thumb mode `rsc r8,r8,#0' /tmp/ccf2FxyW.s:1857: Error: selected processor does not support Thumb mode `rsc r0,r0,#0' /tmp/ccf2FxyW.s:1996: Error: selected processor does not support Thumb mode `rsc r0,r0,#0 网上搜了一下发现这是libmad的一个bug. 解决方法是： vim fixed.h 将 # define MAD_F_MLN(hi, lo) asm ("rsbs %0, %2, #0nt" "rsc %1, %3, #0" : "=r" (lo), "=r" (hi) : "0" (lo), "1" (hi) : "cc") 改为 #ifdef __thumb__ /* In Thumb-2, the RSB-immediate instruction is only allowed with a zero operand. If needed this code can also support Thumb-1 (simply append "s" to the end of the second two instructions). */ # define MAD_F_MLN(hi, lo) asm ("rsbs %0, %0, #0nt" " sbc %1, %1, %1nt" "sub %1, %1, %2" : " &r" (lo), "=&r" (hi) : "r" (hi) : "cc") #else /* ! __thumb__ */ # define MAD_F_MLN(hi, lo) asm ("rsbs %0, %2, #0nt" "rsc %1, %3, #0" : "=r" (lo), "=r" (hi) : "=&r" (lo), "=r" (hi) : "0" (lo), "1" (hi) : "cc") #endif /* __thumb__ */ 再make，编译通过了！
编译madplay ./configure --host=arm-linux-gnueabihf CC=arm-linux-gnueabihf-gcc --disable-debugging --with-alsa CPPFLAGS=-I/home/ban/madplay/source/include LDFLAGS=-L/home/ban/madplay/source/lib make make install 完成以后把生成的可执行文件madplay下载到开发板中 执行./madplay filename.mp3
这个需要注意的是，如果不指定--with-alsa，即便编译成功了，放到板子上也是跑不起的，提示找不到dev/dsp,这个让我折腾了好久，竟发现，配置上没启用alsa啊，
但板子上带的是alsa架构的linux音频驱动。
总体上操作是就这么几步，但是，你会发现，如果照这个步骤来，仍是有错。
具体细节。，根据编译提示的错误，基本都能定为到。比如，找不到上几步编译出的库，就去改makefile吧，添加进去路径
或者仍拷贝到 --prifix指定的目录中。
最后再说一点儿，编译网上这种开源库，最好设置下交叉工具链的环境变量为全局的，且用root权限。否则，坑真的好多。
附截图：
如果不用这个现成的播放器madplay,只测试下libmad是否成功，
可以编译测试下 libmad提供的一个简单demo,这个demo 不是播放mp3的，而是把mp3解码成 pcm文件 。
测试如下：
./testmad.out <demo1.mp3 >out1.pcm
显示出了信息，且在当前路径下产生了out1.pcm文件。
9522 frames decoded (0:04:08.7), 1.7 dB peak amplitude, 4202 clipped samples
代码语言：javascript复制/*
* 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 "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 main(int argc, char *argv[])
{
struct stat stat;
void *fdm;
if (argc != 1)
return 1;
if (fstat(STDIN_FILENO, &stat) == -1 ||
stat.st_size == 0)
return 2;
fdm = mmap(0, stat.st_size, PROT_READ, MAP_SHARED, STDIN_FILENO, 0);
if (fdm == MAP_FAILED)
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;
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);
}
/*
* 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;
/* pcm->samplerate contains the sampling frequency */
nchannels = pcm->channels;
nsamples = pcm->length;
left_ch = pcm->samples[0];
right_ch = pcm->samples[1];
while (nsamples--) {
signed int sample;
/* output sample(s) in 16-bit signed little-endian PCM */
sample = scale(*left_ch );
putchar((sample >> 0) & 0xff);
putchar((sample >> 8) & 0xff);
if (nchannels == 2) {
sample = scale(*right_ch );
putchar((sample >> 0) & 0xff);
putchar((sample >> 8) & 0xff);
}
}
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 0xx (%s) at byte offset %un",
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;
}
