Weird problem using ifstream / rdbuf

Hi,

I'm trying to read in large amounts of data using the rdbuf member of ifstream, it works fine as long as the bufferSize I pass into rdbuf as a parametre is an even number, but if its an odd number I get a completely wrong set of data from the file. Ill show the code and the results i'm getting.

Code:

...

unsigned int bufferSize = 20;
size_t searching = bufferSize;
char *bufferPrimary = new char[bufferSize + 1];	
std::ifstream fin(filename->c_str(), std::ifstream::in);

if(!fin.is_open())
{
	fin.close();
	return false;
}
else
{
	fin.rdbuf()->pubsetbuf(bufferPrimary, bufferSize);
	searching = fin.rdbuf()->_Sgetn_s(bufferPrimary, bufferSize, bufferSize);
	bufferPrimary[bufferSize] = fin.widen('\0');

	std::cout << "Using set: " << bufferPrimary << std::endl;
}

...

Data in file is: LDHGSJDHFKJASTEVEDHSFDLHSFDHSFAHDSVICTORKFHDSAKJFH (file goes on but this is the start)

If bufferSize is 20 I get: LDHGSJDHFKJASTEVEDHS (this seems correct)
If bufferSize is 21 I get: FDLHSFDHSFAHDSVICTORF (this is not the start of the file... and I dont know where the F on the end comes from)

Is this because I may be using pubsetbuf wrong? I used it because I wanted to limit the buffer size read in.
If anyone could tell me what i'm doing wrong I would be very greatful, thanks.

_Sgetn_s is a non-standard member. Who knows what it does, or if it will change in the future. Call sgetn() instead.

You shouldn't call pubsetbuf() on a stream buffer you didn't implement. The effects of calling this are implementation defined - except that pubsetbuf(0,0) will cause the file stream to become unbuffered. Just let the stream do its buffering.

gg