Write vector of unsigned char to binary file c++

Question

I am reading binary file cmd.exe into unsigned chars array. Total bytes read into bytes_read are 153. I converted it to base64 string and then decode this string back (code from 2nd answer base64 decode snippet in c++) into vector<'BYTE>. Here BYTE is unsigned char. decodedData.size() is also 153. But when I write this vector to file in binary mode to get my cmd.exe file again I get only 1 KB file. What thing I missed?

// Reading size of file
    FILE * file = fopen("cmd.exe", "r+");
    if (file == NULL) return 1;
    fseek(file, 0, SEEK_END);
    long int size = ftell(file);
    fclose(file);
// Reading data to array of unsigned chars
    file = fopen("cmd.exe", "r+");
    unsigned char * myData = (unsigned char *)malloc(size);
    int bytes_read = fread(myData, sizeof(unsigned char), size, file);
    fclose(file);

    std::string encodedData = base64_encode(&myData[0], bytes_read);
    std::vector<BYTE> decodedData = base64_decode(encodedData);

////write data to file
    ofstream outfile("cmd.exe", ios::out | ios::binary);
    outfile.write((const char *)decodedData.data(), decodedData.size());

Update: Thanks @chux for suggesting "r+" --> "rb+" Problem resolved.

1 kb may be the smallest possible non-empty file... have you looked at the file? — Grady Player
– Grady Player, Commented Jul 31, 2017 at 18:19
If cmd.exe is binary, it should also be read in binary mode. — Sam Varshavchik
– Sam Varshavchik, Commented Jul 31, 2017 at 18:20
Even worse than using fopen and fread in C++, why are you using malloc in C++ code? — Fred Larson
– Fred Larson, Commented Jul 31, 2017 at 18:23

2785528 · Accepted Answer · 2017-07-31 19:52:14Z

You marked this as C++.

This is one C++ approach using fstream to read a binary file. To simplify for this example, I created a somewhat bigger m_buff than needed. From the comments, it sounds like your fopen("cmd.exe", "r+") was in error, so I'm only providing a C++ binary read.

Method tReader() a) opens a file in binary mode, b) reads the data into m_buff, and c) captures gCount for display.

It also demonstrates one possible use of chrono to measure duration.

#include <chrono>
// 'compressed' chrono access --------------vvvvvvv
typedef std::chrono::high_resolution_clock  HRClk_t; 
typedef HRClk_t::time_point                 Time_t;  
typedef std::chrono::microseconds           US_t;    
using   namespace std::chrono_literals;  // suffixes 100ms, 2s, 30us

#include <iostream>
#include <fstream>
#include <cassert>

class T516_t
{
   enum BuffConstraints : uint32_t {
      Meg           = (1024 * 1024),
      END_BuffConstraints
   };

   char*   m_buff;
   int64_t m_gCount;

public:

   T516_t()
      : m_buff (nullptr)
      , m_gCount (0)
      {
         m_buff = new char[Meg];
      }

   ~T516_t() = default;

   int exec()
      {
         tReader();
         return(0);
      }

private: // methods

   void tReader()
      {
         std::string pfn = "/home/dmoen/.wine/drive_c/windows/system32/cmd.exe";
         // open file in binary mode
         std::ifstream sIn (pfn, std::ios_base::binary);

         if (!sIn.is_open()) {
            std::cerr << "UNREACHABLE:  unable to open sIn " << pfn
                      << " priviledges? media offline?";
            return;
         }

         Time_t start_us = HRClk_t::now();
         do
         {
            // perform read
            sIn.read (m_buff, Meg);
            // If the input sequence runs out of characters to extract (i.e., the
            // end-of-file is reached) before n characters have been successfully
            // read, buff contains all the characters read until that point, and
            // both eofbit and failbit flags are set

            m_gCount = sIn.gcount();

            if(sIn.eof()) { break; } // exit when no more data

            if(sIn.failbit ) {
               std::cerr << "sIn.faileBit() set" << std::endl;
            }

         }while(1);
         auto  duration_us = std::chrono::duration_cast<US_t>(HRClk_t::now() - start_us);

         sIn.close();

         std::cout << "\n  " << pfn
                   << "   " << m_gCount << " bytes"
                   << "   " << duration_us.count() << " us"
                   << std::endl;

      } // int64_t tReader()

}; // class T516_t

int main(int , char**)
{
   Time_t start_us = HRClk_t::now();

   int retVal = -1;
   {
      T516_t   t516;
      retVal = t516.exec();
   }
   auto  duration_us = std::chrono::duration_cast<US_t>(HRClk_t::now() - start_us);

   std::cout << "  FINI   " << duration_us.count() << " us" << std::endl;
   return(retVal);
}

One typical output on my system looks like:

/home/dmoen/.wine/drive_c/windows/system32/cmd.exe   722260 bytes  1180 us   
FINI   1417 us

Your results will vary.

Your ofstream use looks good (so did not replicate).

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