C++ Convert the binary data read from a file to a char pointer

Question

I am trying to achieve converting a binary content read from a file to a byte continuously . My file samplefile.bin contains 16bytes and this is in the form of binary . In the Hexed.it it will show like this .

What I needed in this case is I need to extract these binary values in Hex format and store it to a char *

I will post my sample code over here.

#include <iostream>
#include <fstream>
#include <iomanip>
#include <vector>

using namespace std;

int main()
{
   uint8_t *ptr;
   long size;
   ifstream file ("samplefile.bin", ios::in|ios::binary|ios::ate);
   size = file.tellg();
   std::vector<uint8_t> memblock(size);
   file.seekg (0, ios::beg);
   file.read(reinterpret_cast<char*>(&memblock[0]), size);
   file.close();
   ptr = new uint8_t[size];
   int z=0;
   for(int i=0; i < memblock.size(); i++)
   {
     std::cout << memblock.at(i) << ' ';
     z=memblock.at(i)&0xff;
     ptr[i] = z;
   }

   cout<<"Output"<<endl;
   for (int i=0;i<memblock.size();i++)
   {
       cout<<ptr[i];
   }
   delete []ptr;

return 0;

}

I want the ptr to be passed to some other function. But the output of this print cout<<ptr[i]<<endl; is like below which means the conversion is not happening.

╠ ↕ ‼ ¶ ╙ O N ╥ í * : J Z [ \ J

I want something like below

ptr[0] = 0xCC
ptr[1] = 0x12
...
ptr[15] = 0x4A

When I give like this cout<<(int)ptr[i]<<endl; , I am getting the decimal value of 0xCC as 204 and the same thing for other prints.

I already referred to C++ read binary file and convert to hex and Convert binary file to hex notation. I didn't find the exact solution because in these two links they are converting it to string. My intention is not this one.

I referred to this How properly to read data from binary file to char array which looks similar to mine. But it didn't solve my issue.
I want this 0xCC as a single byte and this should be stored to ptr[0] and similarly for the other values

How to workaround this issue ? Am I missing something here ? I am a newbie in C++ and please help me to resolve this issue

Appreciating the efforts.

`cout << "ptr[" << std::dec << i << "]: " << std::hex << (int)ptr[i] << '\n';` — David C. Rankin, Aug 04 '20 at 15:18

score 1 · Answer 1 · answered Aug 04 '20 at 14:19

1

Read file and copy its data to vector of char. You can use iomanip like std::hex or std::uppercase to print integer value into hex.

Allocating and passing raw pointer to another function is not recommended. Simply pass a vector of char or use unique_ptr, they'll prevent you from memory problems.

#include <iostream>
#include <iomanip>

std::ifstream file("samplefile.bin", ios::binary);
auto mem = std::vector<char>(std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>());
for (char c : mem)
{
    std::cout << std::uppercase << std::hex << static_cast<int>(c) << endl;
}

answered Aug 04 '20 at 14:19

John Park

1,644
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17

Pass reference(const if possible) of vector to function. Avoid passing large object by value. – John Park Aug 04 '20 at 14:21
Your 2nd line should be modified to avoid assignment. You can use the range constructor. You can also use CTAD and the default initializer. Modify to --> ````std::vector mem(std::istreambuf_iterator(file),{});```` – A M Aug 04 '20 at 14:26
Thanks for the answer. The function prototype is already defined and we can't able to change it. That's why I asked for copying the content to a `char *` – Vishnu CS Aug 04 '20 at 14:31
Armin, I learn better code thanks to you. Vishun, you can consider std::copy() from istreambuf iterator to your raw pointer. – John Park Aug 04 '20 at 14:44
@Amin The second line does not perform assignment in C++14 (nor even in C++11 with optimisations enabled). Using `auto` is totally fine. – Konrad Rudolph Aug 04 '20 at 14:53
@JohnPark Can you reedit the code by doing this `std::copy()` added – Vishnu CS Aug 04 '20 at 14:58
@Vishnu std::copy(std::istreambuf_iterator(ifs), {}, ptr); would be enough, where ptr is the pointer you allocated. – John Park Aug 05 '20 at 00:17

Konrad Rudolph · Answer 2 · 2020-08-04T15:41:29.160

There are multiple problems in your code, most fundamentally the fact that you seem to expect std::byte8_t to be represented as a hexdump by default when fed to an output stream. This is not the case. You need to explicitly tell the stream to perform the necessary formatting for you. And if you want to store that representation, you’ll need to also store the streamed result.

Many roads lead to Rome, and unfortunately the C++ standard IO stream library doesn’t make working with byte streams very ergonomic. But here’s one approach:

#include <cstdint>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <vector>

using byte = std::uint8_t;

auto read_bytes(std::istream& is, std::vector<byte>& bytes) -> std::istream& {
    return is.read(& reinterpret_cast<char&>(bytes[0]), bytes.size());
}

auto hexdump(std::vector<byte> const& bytes) -> std::string {
    auto ostr = std::ostringstream();
    ostr << std::uppercase << std::hex << std::setw(2) << std::setfill('0');
    for (auto const c : bytes) {
        ostr << int{c};
    }
    return ostr.str();
}

int main(int argc, char** argv) {
    if (argc != 2) return 1;

    auto fs = std::ifstream(argv[1], std::ios_base::binary | std::ios_base::ate);
    auto const size = fs.tellg();
    fs.seekg(0);
    auto buffer = std::vector<byte>(size);
    if (not read_bytes(fs, buffer)) return 1;

    std::cout << hexdump(buffer) << "\n";
}

C++ Convert the binary data read from a file to a char pointer

2 Answers2