Base64 image file encoding with C++ - c++

Question

I am writing some simple code to encode files to base64. I have a short c++ code that reads a file into a vector and converts it to unsigned char*. I do this so I can properly use the encoding function I got.
The problem: It works with text files (of different sizes), but it won't work with image files. And I can't figure it out why. What gives?
For an simple text.txt containing the text abcd, the output for both my code and a bash $( base64 text.txt ) is the same.
On the other hand, when I input an image the output is something like iVBORwOKGgoAAAAAAA......AAA== or sometimes it ends with an corrupted size vs prev_size Aborted (core dumped), the first few bytes are correct.
The code:
static std::vector<char> readBytes(char const* filename)
{
std::ifstream ifs(filename, std::ios::binary|std::ios::ate);
std::ifstream::pos_type pos = ifs.tellg();
std::vector<char> result(pos);
ifs.seekg(0, std::ios::beg);
ifs.read(&result[0], pos);
return result;
}
static char Base64Digits[] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
int ToBase64Simple( const BYTE* pSrc, int nLenSrc, char* pDst, int nLenDst )
{
int nLenOut= 0;
while ( nLenSrc > 0 ) {
if (nLenOut+4 > nLenDst) {
cout << "error\n";
return(0); // error
}
// read three source bytes (24 bits)
BYTE s1= pSrc[0]; // (but avoid reading past the end)
BYTE s2= 0; if (nLenSrc>1) s2=pSrc[1]; //------ corrected, thanks to jprichey
BYTE s3= 0; if (nLenSrc>2) s3=pSrc[2];
DWORD n;
n = s1; // xxx1
n <<= 8; // xx1x
n |= s2; // xx12
n <<= 8; // x12x
n |= s3; // x123
//-------------- get four 6-bit values for lookups
BYTE m4= n & 0x3f; n >>= 6;
BYTE m3= n & 0x3f; n >>= 6;
BYTE m2= n & 0x3f; n >>= 6;
BYTE m1= n & 0x3f;
//------------------ lookup the right digits for output
BYTE b1 = Base64Digits[m1];
BYTE b2 = Base64Digits[m2];
BYTE b3 = Base64Digits[m3];
BYTE b4 = Base64Digits[m4];
//--------- end of input handling
*pDst++ = b1;
*pDst++ = b2;
if ( nLenSrc >= 3 ) { // 24 src bits left to encode, output xxxx
*pDst++ = b3;
*pDst++ = b4;
}
if ( nLenSrc == 2 ) { // 16 src bits left to encode, output xxx=
*pDst++ = b3;
*pDst++ = '=';
}
if ( nLenSrc == 1 ) { // 8 src bits left to encode, output xx==
*pDst++ = '=';
*pDst++ = '=';
}
pSrc += 3;
nLenSrc -= 3;
nLenOut += 4;
}
// Could optionally append a NULL byte like so:
*pDst++= 0; nLenOut++;
return( nLenOut );
}
int main(int argc, char* argv[])
{
std::vector<char> mymsg;
mymsg = readBytes(argv[1]);
char* arr = &mymsg[0];
int len = mymsg.size();
int lendst = ((len+2)/3)*4;
unsigned char* uarr = (unsigned char *) malloc(len*sizeof(unsigned char));
char* dst = (char *) malloc(lendst*sizeof(char));;
mymsg.clear(); //free()
// convert to unsigned char
strncpy((char*)uarr, arr, len);
int lenOut = ToBase64Simple(uarr, len, dst, lendst);
free(uarr);
int cont = 0;
while (cont < lenOut) //(dst[cont] != 0)
cout << dst[cont++];
cout << "\n";
}
Any insight is welcomed.

Answer 1

I see two problems.
First, you are clearing your mymsg vector before you're done using it. This leaves the arr pointer dangling (pointing at memory that is no longer allocated). When you access arr to get the data out, you end up with Undefined Behavior.
Then you use strncpy to copy (potentially) binary data. This copy will stop when it reaches the first nul (0) byte within the file, so not all of your data will be copied. You should use memcpy instead.