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;

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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。