How to use the GetLastError function in a Win32 Console Application? - winapi

Question

I am trying to open a Registry Key using the RegOpenKeyEx function from the Windows API, and have this code:
#include <windows.h>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace std;
int wmain(int argc, wchar_t*argv [])
{
HKEY hKey = HKEY_CURRENT_USER;
LPCTSTR lpSubKey = L"Demo";
DWORD ulOptions = 0;
REGSAM samDesired = KEY_ALL_ACCESS;
HKEY phkResult;
long R = RegOpenKeyEx(hKey, lpSubKey, ulOptions, samDesired, &phkResult);
if (R == ERROR_SUCCESS)
{
cout << "The registry key has been opened." << endl;
}
else //How can I retrieve the standard error message using GetLastError() here?
{
}
}
How do I use the GetLastError() function to show a generic error message instead of valid any Error Message ID into the else?
Edit: I know there is a FormatMessage function but have the same problem, I don't know how to use it on my code.

Answer 1

The Registry functions do not use GetLastError(). They return the actual error codes directly:
long R = RegOpenKeyEx(hKey, lpSubKey, ulOptions, samDesired, &phkResult);
if (R == ERROR_SUCCESS)
{
cout << "The registry key has been created." << endl;
}
else
{
cout << "The registry key has not been created. Error: " << R << endl;
}
If you want to display a system error message, use FormatMessage() for that:
long R = RegOpenKeyEx(hKey, lpSubKey, ulOptions, samDesired, &phkResult);
if (R == ERROR_SUCCESS)
{
cout << "The registry key has been created." << endl;
}
else
{
char *pMsg = NULL;
FormatMessageA(
FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_ARGUMENT_ARRAY | FORMAT_MESSAGE_ALLOCATE_BUFFER,
NULL,
R,
0,
(LPSTR)&pMsg,
0,
NULL
);
cout << "The registry key has not been created. Error: (" << R << ") " << pMsg << endl;
LocalFree(pMsg);
}

Answer 2

Try this
HKEY hKey = HKEY_CURRENT_USER;
LPCTSTR lpSubKey = L"Demo";
DWORD ulOptions = 0;
REGSAM samDesired = KEY_ALL_ACCESS;
HKEY phkResult;
char *ErrorMsg= NULL;
long R = RegOpenKeyEx(hKey, lpSubKey, ulOptions, samDesired, &phkResult);
if (R == ERROR_SUCCESS)
{
printf("The registry key has been opened.");
}
else //How can I retrieve the standard error message using GetLastError() here?
{
FormatMessageA(
FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_ARGUMENT_ARRAY | FORMAT_MESSAGE_ALLOCATE_BUFFER,
NULL,
R,
0,
(LPSTR)&ErrorMsg,
0,
NULL
);
printf("Error while creating Reg key.");
}