C++ convert std::string that contains binary data to char*

Question

Okay, so I'm having a little bit of a problem here.

What I'm doing is converting a binary file (in this example I used a .exe file) to a Base64 string and then converting this one back to binary data to write it to the disk.

So far so good, this code works:

std::string str = base64_decode(base64str); // base64str is the base64 string made of the actual .exe file
std::ofstream strm("file.exe", std::ios::binary);
strm << str;
strm.close();

The file "file.exe" is being created as expected and I can run it.

Now my problem is that I need the decrypted file as char* instead of std::string, but whenever I call this code

str.c_str();

to either convert it to const char* or char* the contents suddenly no longer equal the binary data contained in str, but rather this:

MZP

So, for instance the following code

std::string str = base64_decode(base64str);
std::ofstream strm("file.exe", std::ios::binary);
char* cstr = new char[str.length()-1];
strcpy(cstr, str.c_str());
strm << cstr;
strm.close();

would create file.exe, but this time it would contain "MZP" instead of the actual binary data

I have no clue on how to fix this. Of course the char* is mandatory.

Can any of you help?

strcpy(cstr, str.c_str()); will stop copying after hitting the first null byte, of which there are probably hundreds in your binary file. — user657267
– user657267, Commented Oct 31, 2014 at 2:04
Don't use strm << cstr it will stop at the first null. Use strm.write(). — Galik
– Galik, Commented Oct 31, 2014 at 3:58

Jonathon Reinhart · Accepted Answer · 2014-10-31 02:15:20Z

2

std::string::c_str() returns a "C string", which is a NUL-terminated array of characters. Your binary data certainly has NUL terminators in it, prior to the end of the data. This is why your data appears truncated. (Look in a hex editor, I bet byte 0x03 is zero.)

For that reason, you should instead use std::basic_string::data to get a pointer to the raw data contained by the string. When copying or writing this data, you'll want to not use strcpy (which stops at a NUL byte), but rather memcpy, or similar. The size of the data contained by the string can be gotten from std::basic_string::size.

answered Oct 31, 2014 at 2:15

Jonathon Reinhart

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4 Comments

xNaXDy Over a year ago

Okay, I now have

std::basic_string<char> dec = base64_decode(base64str);     char* cdec = new char[dec.size()];     memcpy(cdec, dec.data(), dec.size());

Though, it still outputs "MZP" :/

xNaXDy Over a year ago

NOTE: I just tested it, dec.data() also seems to output "MZP"

xNaXDy Over a year ago

NOTE2: dec.data() seems to work fine after changing stm << cdec to strm.write(dec.data(), dec.size()), but strm.write(cdec, sizeof(cdec)) still does not work ("MZP" thing)

Remy Lebeau Over a year ago

cdec is a char*. sizeof(char*) is 4. Use dec.size() instead of sizeof(cdec).

Barry · Accepted Answer · 2014-10-31 02:05:11Z

0

If you want the data inside a std::string as a char*, you can just grab it. Either:

std::string s = ...

char* c1 = &s[0];
char* c2 = const_cast<char*>(s.c_str());
char* c3 = &s.front();

answered Oct 31, 2014 at 2:05

Barry

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