cpp byte file reading

Question

Im trying to read a byte file with fstream in c++ (goal: binary data format deserialization). The dat file looks something like below in HxD Hex editor (bytes.dat):

bytefile

but something goes wrong when reading the binary file into a char array.. here is a mwe:

#include <iostream>
#include <fstream>
using namespace std;

int main (){
  ofstream outfile;
  outfile.open("bytes.dat", std::ios::binary);
  outfile << hex << (char) 0x77 << (char) 0x77 << (char) 0x77 << (char) 0x07 \
  << (char) 0x9C << (char) 0x04 << (char) 0x00 << (char) 0x00 << (char) 0x41 \
  << (char) 0x49 << (char) 0x44 << (char) 0x30 << (char) 0x00 << (char) 0x00 \
  << (char) 0x04 << (char) 0x9C;
  ifstream infile;
  infile.open("bytes.dat", ios::in | ios::binary);
  char bytes[16];
  for (int i = 0; i < 16; ++i)
  {
    infile.read(&bytes[i], 1);
    printf("%02X ", bytes[i]);
  }
}

but this shows in the cout (mingw compiled):

> g++ bytes.cpp -o bytes.exe

> bytes.exe 

6A 58 2E 76 FFFFFF9E 6A 2E 76 FFFFFFB0 1E 40 00 6C FFFFFFFF 28 00

im doing something wrong. How is it possible that there 4 bytes in some of the array entries?

Declare bytes as an unsigned char array or cast bytes[i] to unsigned char. — Captain Obvlious
tnx this solves the long FFFFFF problem. But the values are not what is in the hex editor — Walter
Use outfile.write() instead of the operator<< to store binary data. — πάντα ῥεῖ

NetVipeC NetVipeC · Accepted Answer · 2014-09-02T14:08:22

When working with binary data (binary file format and like), it's better to work with unsigned integer type to avoid sign extension conversions.
As recommended when reading and writing binary data, better use stream.read and stream.write functions (it's better read and write by block too).
If you need to storage fixed binary data use std::array or std::vector, if you need to load data from file (std::vector is the default)

Fixed code:

#include <iostream>
#include <fstream>
#include <vector>
using namespace std;

int main() {
    vector<unsigned char> bytes1{0x77, 0x77, 0x77, 0x07, 0x9C, 0x04, 0x00, 0x00,
                                0x41, 0x49, 0x44, 0x30, 0x00, 0x00, 0x04, 0x9C};
    ofstream outfile("bytes.dat", std::ios::binary);
    outfile.write((char*)&bytes1[0], bytes1.size());
    outfile.close();

    vector<unsigned char> bytes2(bytes1.size(), 0);
    ifstream infile("bytes.dat", ios::in | ios::binary);
    infile.read((char*)&bytes2[0], bytes2.size());
    for (int i = 0; i < bytes2.size(); ++i) {
        printf("%02X ", bytes2[i]);
    }
}

cpp byte file reading

1 Answers