C++ read binary file and convert to hex

Question

I'm having some problems reading a binary file and converting it's bytes to hex representation.

What I've tried so far:

ifstream::pos_type size;
char * memblock;

ifstream file (toread, ios::in|ios::binary|ios::ate);
  if (file.is_open())
  {
    size = file.tellg();
    memblock = new char [size];
    file.seekg (0, ios::beg);
    file.read (memblock, size);
    file.close();

    cout << "the complete file content is in memory" << endl;

std::string tohexed = ToHex(memblock, true);


    std::cout << tohexed << std::endl;

   }

Converting to hex:

string ToHex(const string& s, bool upper_case)
{
    ostringstream ret;

    for (string::size_type i = 0; i < s.length(); ++i)
        ret << std::hex << std::setfill('0') << std::setw(2) << (upper_case ? std::uppercase : std::nouppercase) << (int)s[i];

    return ret.str();
}

Result: 53514C69746520666F726D61742033.

When I open the original file with a hex editor, this is what it shows:

53 51 4C 69 74 65 20 66 6F 72 6D 61 74 20 33 00
04 00 01 01 00 40 20 20 00 00 05 A3 00 00 00 47
00 00 00 2E 00 00 00 3B 00 00 00 04 00 00 00 01
00 00 00 00 00 00 00 00 00 00 00 01 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 05 A3
00 2D E2 1E 0D 03 FC 00 06 01 80 00 03 6C 03 D3

Is there a way to get the same desired output using C++?

Working solution (by Rob):

...

std::string tohexed = ToHex(std::string(memblock, size), true);

...
string ToHex(const string& s, bool upper_case)
{
    ostringstream ret;

    for (string::size_type i = 0; i < s.length(); ++i)
    {
        int z = s[i]&0xff;
        ret << std::hex << std::setfill('0') << std::setw(2) << (upper_case ? std::uppercase : std::nouppercase) << z;
    }

    return ret.str();
}

"memblock contains only first 15 bytes, stopping at the null byte (16th)" What makes you say that? I don't see where you are printing out the contents of memblock. I suspect that memblock contains the entire file, but that the code you aren't showing us misinterprets its contents. Please reduce your program to the smallest complete program that demonstrates the error, and post that program in the question. sscce.org — Robᵩ
– Robᵩ, Commented Mar 8, 2012 at 17:20
@Rob okay shall i repost the first 15 bytes for you to be more clear? — user562854
– user562854, Commented Mar 8, 2012 at 17:21
Assuming that this is a homework or a learning assignment of some sort, here's a couple of hints: (1) you are missing a while loop, (2) calling tellg() on a stream that you have just opened is premature. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Mar 8, 2012 at 17:22
@develroot - No, please just tell us how you concluded that it only contained the first fifteen bytes. — Robᵩ
– Robᵩ, Commented Mar 8, 2012 at 17:22

Robᵩ · Accepted Answer · 2012-03-08 17:30:23Z

8

char *memblock;
… 
std::string tohexed = ToHex(memblock, true);
…

string ToHex(const string& s, bool upper_case)

There's your problem, right there. The constructor std::string::string(const char*) interprets its input as a nul-terminated string. So, only the characters leading up to '\0' are even passed to ToHex. Try one of these instead:

std::string tohexed = ToHex(std::string(memblock, memblock+size), true);
std::string tohexed = ToHex(std::string(memblock, size), true);

answered Mar 8, 2012 at 17:30

Robᵩ

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3 Comments

user562854 Over a year ago

okay...that worked for the hex conversion...but i still have a problem: the result isn't exactly the same. 47 is now 00, or 2D is now 05..and so are all the non-ASCII characters.

Robᵩ Over a year ago

@develroot - if it helps, you have a sign-extension bug in your ToHex routine. Try (s[i]&0xff) instead of (int)s[i].

ReinstateMonica3167040 Over a year ago

This is amazing! :)

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