Windows RegQueryValueEx odd return results - windows

Question

I am getting odd results when using RegQueryValueEx and I cannot figure out why.
This is what I had set up before making the RegQueryValueEx
DWORD dataSize;
TCHAR data[256];
The first time I call
LONG ret = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
ret is equal to 234 (ERROR_MORE_DATA)
But when I call the same thing on the next line
LONG ret2 = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
ret2 is equal to 0 (ERROR_SUCCESS)
Why would this function return ERROR_MORE_DATA the first time I call it, then return ERROR_SUCESS on the same call on the very next line?
I attempted to change TCHAR data[1024] but I got the exact same results. Any ideas?
Complete code:
for( int i=0; i<NUM_HISTORY; i++){
CString dataKey = getDataKey(i);
DWORD dataSize = 1024;
TCHAR data[1024];
LONG ret = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
LONG ret2 = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
// Breakpoint to see what ret and ret2 are equal to
int j = 0;
}

Answer 1

This is by design. The first call failed because you specified as size that was too small. But what you didn't count on is that it also updated your dataSize variable. To tell you how much memory to allocate so the call can succeed.
So the second call succeeded since you now specify a size that's exactly correct. But without doing the other thing you needed to do, actually make the buffer bigger. Nothing much good can happen when the call then causes a buffer overflow and corrupt your stack frame, be sure to use the /RTC compile option so you'll get a runtime error from that.
You avoided this problem by increasing the buffer size from 256 to 1024. But your code is still incorrect, your program will fail miserably if the registry value ever gets larger than 1024 bytes. Don't use a local array, use the new operator or malloc() to allocate the buffer so it can never fail like this. Or simply fail the call and declare "bad data".
Also note another bug, the dataSize is in bytes but the buffer is TCHAR, not a byte. Which is probably why you didn't corrupt the stack frame, the buffer was big enough by accident. You don't want to rely on accidents like this. Consider a helper class like CRegKey to avoid these kind of mistakes.