mp3 파일을 뜯어보는 C++ 코드를 작성했는데, 파일의 대부분이 0으로 출력됩니다.

#include <iostream>
#include <fstream>
#include <exception>
#include <sstream>
#include <cstring>
#include <iomanip>
 
//////////////////////////////////////////////////////////////////
// 
// Title: Edit MP3 with metadata's indexes
// Date: 2015. 04. 16. 18:19
// Author: HDNua (DoYeong Han, from Republic of Korea)
// E-mail: <a href="mailto:rbfwmqwntm@naver.com" rel="nofollow">rbfwmqwntm@naver.com</a>, <a href="http://blog.naver.com/rbfwmqwntm
//" rel="nofollow">http://blog.naver.com/rbfwmqwntm
//</a> 
//////////////////////////////////////////////////////////////////
 
// definitions
typedef char byte;
typedef std::string Exception;
 
// constants & definition
const int MAX_CACHE_SIZ = 20 * 1024 * 1024;
 
// fields
byte cache[MAX_CACHE_SIZ] = "";
int cache_size = 0;
 
// methods
std::string get_metadata_str(byte *bin, int start, int len=1); // print out to os from bin[start] to bin[end]
void set_metadata_str(const char *str, byte *bin, int start, int range); // replace byte array's data with str
 
int read_bin(byte *dst_bin, const char *filename); // read file's binary data, save to byte array and returns read byte size
bool is_id3v2(const byte *bin); // check whether the binary data is ID3v2 format
 
// bench
int main(void) {
	try {
		const std::string emptystr = "";
		std::string dir = "song/";
		std::string name = "Kalimba";
		std::string ext = ".mp3";
		std::string filename = dir + name + ext; //dir + idx + ext;
 
		read_bin(cache, filename.c_str());
		if (is_id3v2(cache) == false) {
			throw Exception("invalid mp3 file data");
		}
 
		std::stringstream out_hexa;
		std::stringstream out_byte;
		std::stringstream out;
		std::ostream &os = out;
 
		// get default mp3 file's data
		os<<filename<<": "<<std::endl;
		os<<"- mp3 size: "<<(cache_size / 1024.0)<<"KB ("<<cache_size<<" bytes)"<<std::endl;
 
		// show binary data map
		const int width = 40;
		const int width_i = 8;
 
		int unit = 150000;
		int start = unit * 0;
		int end = unit * 1;
		for (int i=start; i<end; ++i) {
			if (i % width == 0) {
				os<<out_hexa.str()<<"| "<<out_byte.str()<<"|"<<std::endl;
				out_hexa.str(emptystr); out_byte.str(emptystr);
				os.width(width_i);
				os<<std::hex<<i<<"(";
				os.width(width_i);
				os<<std::dec<<i<<"): ";
			}
			int value = (int)(cache[i] & 0xff);
			char ch = cache[i] ? cache[i] : '.';
			out_hexa.width(2); out_hexa.fill('0');
			out_hexa<<std::hex<<value<<' ';
			out_byte<<ch;
		}
		std::cout<<out.str();
 
		/*
		// get mp3 file's metadata
		os<<"- TRACK: ["<<get_metadata_str(cache, 62)<<']'<<std::endl;
		os<<"- YEAR: ["<<get_metadata_str(cache, 74, 4)<<']'<<std::endl;
		os<<"- ALBUM: ["<<get_metadata_str(cache, 89, 3)<<']'<<std::endl;
		os<<"- GENRE: ["<<get_metadata_str(cache, 103, 6)<<']'<<std::endl;
		os<<"- ALBUMARTIST: ["<<get_metadata_str(cache, 120, 18)<<']'<<std::endl;
		os<<"- TITLE: ["<<get_metadata_str(cache, 149, 8)<<']'<<std::endl;
		os<<"- ARTIST: ["<<get_metadata_str(cache, 168, 6)<<']'<<std::endl;
 
		// change mp3 files metadata
		set_metadata_str("6", cache, 62, 1);
		set_metadata_str("1999", cache, 74, 4);
		set_metadata_str("하!", cache, 89, 3);
		set_metadata_str("새장르", cache, 103, 6);
		set_metadata_str("앨범가수", cache, 120, 18);
		set_metadata_str("타이틀", cache, 149, 8);
		set_metadata_str("가수", cache, 168, 6);
 
		// create new file and write modified metadata to it
		os<<"Creating new file and writing modified metadata: ";
		std::string newFileIdx = "6";
		std::string newFileName = dir + newFileIdx + ext;
		std::ofstream fout(newFileName.c_str());
		fout.write(cache, cache_size);
		fout.close();
		os<<"Complete"<<std::endl;
		*/
 
	} catch (Exception &ex) {
		std::cerr<<ex<<std::endl;
	}
	return 0;
}
 
// implementations
void set_metadata_str(const char *str, byte *bin, int start, int range) {
	strncpy(&bin[start], str, range);
}
std::string get_metadata_str(byte *bin, int start, int len) { // print out to os from bin[start] to bin[end]
	std::stringstream ss;
	for (int i=start, end=start+len; i<end; ++i) {
		char ch = bin[i];
		ss<<ch;
	}
	return ss.str();
}
int read_bin(byte *dst_bin, const char *filename) { // read file's binary data, save to byte array and returns read byte size
	std::ifstream fin(filename, std::ios::binary);
	if (fin.fail()) {
		throw Exception("cannot find file: " + std::string(filename));
	}
	fin.get(dst_bin, MAX_CACHE_SIZ);
	fin.seekg(0, fin.end);
	cache_size = fin.tellg();
	fin.seekg(0, fin.beg);
	return cache_size;
}
bool is_id3v2(const byte *bin) { // check whether the binary data is ID3v2 format
	// check binary's first 3 bytes are equals to string 'ID3'
	return (bin[0] == 'I' && bin[1] == 'D' && bin[2] == '3');
}