Binary input/output in C++

Question

I am trying a reasonably simple program to test binary input/output. I am basically writing a file with a header (string) and some data (doubles). The code is as follows:

#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <vector>
#include <iterator>
#include <algorithm>


int main() {
   typedef std::ostream_iterator<double> oi_t;
   typedef std::istream_iterator<double> ii_t;
   std::ofstream ofs("data.bin", std::ios::in);
   //-If file doesn't exist, create a new one now
   if(!ofs) {
      ofs.open("data.bin", std::ios::out|std::ios::binary|std::ios::app);
   }
   else {
      ofs.close();
      ofs.open("data.bin", std::ios::out|std::ios::binary|std::ios::app);
   }

   //-Write a header consisting of length of grid subdomain and its name
   ///*
   const std::string grid = "Header";
   unsigned int olen = grid.size();
   ofs.write(reinterpret_cast<const char*>(&olen), sizeof(olen));
   ofs.write(grid.c_str(), olen);
   //*/

   //-Now write the data
   ///*
   std::vector<double> data_out;
   //std::vector<std::pair<int, int> > cell_ids;
   for(int i=0; i<100; ++i) {
      data_out.push_back(5.0*double(i) + 100.0);
   }
   ofs << std::setprecision(4);
   std::copy(data_out.begin(), data_out.end(), oi_t(ofs, " "));
   //*/
   ofs.close();

   //-Now read the binary file; first header then data
   std::ifstream ifs("data.bin", std::ios::binary);
   ///*
   unsigned int ilen;
   ifs.read(reinterpret_cast<char*>(&ilen), sizeof(ilen));
   std::string header;
   if(ilen > 0) {
      char* buf = new char[ilen];
      ifs.read(buf,ilen);
      header.append(buf,ilen);
      delete[] buf;
   }
   std::cout << "Read header: " << header << "\n";
   //*/

   ///*
   std::vector<double> data_in;
   ii_t ii(ifs);
   std::copy(ii, ii_t(), std::back_inserter(data_in));
   std::cout << "Read data size: " << data_in.size() << "\n";
   //*/
   ifs.close();

   //-Check the result
   ///*
   for(int i=0; i < data_out.size(); ++i) {
      std::cout << "Testing input/output element #" << i << " : "
                << data_out[i] << "  " << data_in[i] << "\n";
   }
   std::cout << "Element sizes: " << data_out.size() << "  " << data_in.size() <<  
            "\n";
   //*/
   return 0;
}

The problem is that when I try to write and read (and then print) both the header and the data it fails (I confirmed that it doesn't read the data then, but displays the header correctly). But when I comment out one of the write sections (header and/or data), it displays that part correctly indicating the read worked. I am sure I am not doing the read properly. Perhaps I am missing the usage of seekg somewhere.

Works for me. I get "Read header: Header\n Read data size: 100\n" then 100 Testing input/output... — Naddiseo
– Naddiseo, Commented Jan 10, 2012 at 3:20
That's odd. It does not work for me. The code as is, prints that reads data of size 0 and so it segfaults in the print section following that. I tested on Cygwin and Linux (RHEL5). Haven't yet tested on Windows. — squashed.bugaboo
– squashed.bugaboo, Commented Jan 10, 2012 at 4:02

David Brown · Accepted Answer · 2012-01-10 04:09:57Z

1

The code runs fine for me. However you never check if the file is successfully opened for writing, so it could be silently failing on your system. After you open ofs you should add

if (!ofs) {
    std::cout << "Could not open file for writing" << std::endl;
    return 1;
}

And the same thing after you open ifs

if (!ifs) {
    std::cout << "Could not open file for reading" << std::endl;
    return 1;
}

Or something along those lines. Also I do not understand why you check if the file exists first since you do the same whether it exists or not.

edited Jan 10, 2012 at 4:09

answered Jan 10, 2012 at 3:34

David Brown

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3 Comments

squashed.bugaboo Over a year ago

Thanks for the comment.. but this is a relatively minor issue not resolving my actual problem of reading the data in its entirety, i.e. header and doubles (I think).

David Brown Over a year ago

Have you tried it though? The code you've posted works fine for me unless I am unable to open the file data.bin in which case it crashes.

squashed.bugaboo Over a year ago

Thanks. You are right. It works when it opens and crashes when if(ofs) fails (fail bit). I fixed this. That gives me some ideas.

Curious · Accepted Answer · 2016-02-11 04:53:53Z

0

This should work

#include <iostream>
using std::cout;
using std::cerr;
using std::cin;
using std::endl;
#include <fstream>
using std::ifstream;
#include <cstdint>

int main() {

    ifstream fin;
    fin.open("input.dat", std::ios::binary | std::ios::in);
    if (!fin) {
        cerr << "Cannot open file " << "input.dat" << endl;
        exit(1);
    }

    uint8_t input_byte;
    while (fin >> input_byte) {
        cout << "got byte " << input_byte << endl;
    }

    return 0;
}

answered Feb 11, 2016 at 4:53

Curious

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