lpData not get value after RegQueryValueEx - c++

Question

I'm not sure where problem is occur.
I have demo code get value from an registry:
using namespace std;
typedef struct REGKEYS
{
HKEY hKey;
LPCTSTR subKey;
LPCTSTR value;
LPCTSTR name;
LPCTSTR file;
LPCTSTR tag;
} REGKEYS;
REGKEYS regkeys = {HKEY_LOCAL_MACHINE,"Software\\Internet Download Manager", "Serial", "IDM", NULL, NULL};
HKEY hKey;
LPBYTE szDataBuf;
DWORD dwSize;
int _tmain(int argc, _TCHAR* argv[])
{
RegOpenKeyEx(regkeys.hKey, regkeys.subKey, 0, KEY_READ, &hKey);
RegQueryValueEx(hKey, regkeys.value, NULL, NULL, szDataBuf, &dwSize);
int lerr = GetLastError();
system("pause");
return 0;
}
When debug, I see regkeys.hKey not point to anything (it need point to "HKEY_LOCAL_MACHINE", is it?).
And szDataBuf is empty.
(GetLastError() not help me - it return 0)
Anyone tell me where is problem and how to fix it?
EDIT2:
I edited my code to :
int _tmain(int argc, _TCHAR* argv[])
{
DWORD dwSize = 0;
RegOpenKeyEx(regkeys.hKey, regkeys.subKey, 0, KEY_READ, &hKey);
RegQueryValueEx(hKey, regkeys.value, NULL, NULL, NULL, &dwSize);
char *szDataBuf = new char(dwSize);
RegQueryValueEx(hKey, regkeys.value, NULL, NULL, (LPBYTE)szDataBuf, &dwSize);
delete[] szDataBuf;
system("pause");
return 0;
}
I debug and see szDataBuf get right value, but my program break and get warning: warning C4244: 'initializing' : conversion from 'DWORD' to 'char', possible loss of data.

Answer 1

First of all, your error checking code is not correct. The registry functions all return Win32 error codes. You need to take notice of them and check for errors.
To be clear, GetLastError is no use because these functions do not call SetLastError. They return the error code directly. You must read the documentation carefully. There are often subtle differences in the way different parts of Win32 do error handling.
The code in your updated question is better, but it still fails to check for errors in the call to RegOpenKeyEx.
Now, beyond that you are not actually calling RegQueryValueEx correctly. You have not initialized dwSize, and you have not allocated szDataBuf. Since these are global variables, they will be zero initialised. So your call to RegQueryValueEx will indeed succeed if the value exists. But you cannot expect any value to arrive in the buffer since you did not allocate a buffer. You are passing NULL to the lpData parameter.
So, allocate a buffer. For instance:
char szDataBuf[128];
Initialise dwSize:
DWORD dwSize = sizeof(szDataBuf);
Then your call to RegQueryValueEx should be
int lerr = RegQueryValueEx(hKey, regkeys.value, NULL, NULL,
(LPBYTE)szDataBuf, &dwSize);
Although you may wish to allocate the buffer dynamically:
dwSize = 0;
int lerr = RegQueryValueEx(hKey, regkeys.value, NULL, NULL, NULL, &dwSize);
// check lerr here
vector<char> szDataBuf(dwSize);
int lerr = RegQueryValueEx(hKey, regkeys.value, NULL, NULL,
(LPBYTE)&szDataBuf[0], &dwSize);
// check lerr here
You might also consider avoiding the use of global variables here. All of the variables you use would be make more sense as locals.