How to get the class name of a window with C - c

Question

Objective: Retrieve the class name of the current foreground window with plain C.
I have the following code to retrieve the class name:
PWSTR win_class = NULL;
GetClassNameW(hwnd,&win_class,MAX_PATH);
if(win_class != NULL)
free(win_class);
I am getting the following warnings:
warning C4047: 'function': 'LPWSTR' differs in levels of indirection
from 'PWSTR *' warning C4024: 'GetClassNameW': different types
for formal and actual parameter 2
I have two questions: How to solve those warnings, and how should I create an if condition to validate the result of the function GetClassName and set the value of win_class to "NOT FOUND" in case that the function does not find the class name?

Answer 1

The GetClassNameW() function does not allocate the memory needed for the returned class name - you have to do that (or simply provide automatic storage).
To check for success, simply test the return value of the function: if it succeeds, that will be the length of the class name string (in characters); if it fails, the value will be zero.
Here's a short, runnable program that gets the class name for the console window:
#include <Windows.h>
#include <stdio.h>
int main()
{
HWND hwnd = GetConsoleWindow();
wchar_t win_class[_MAX_PATH];
int status = GetClassNameW(hwnd, win_class, _MAX_PATH);
if (!status) wcscpy(win_class, L"NOT FOUND");
printf("%ls\n", win_class);
return 0;
}

Answer 2

When calling GetClassNameW(), you are passing a PWSTR* (wchar_t**) where a LPWSTR (wchar_t*) is expected. That is what the compiler is complaining about.
GetClassName() requires you to pre-allocate a character buffer and pass in a pointer to it, along with the buffer size. The function will not allocate a buffer and return a pointer back to you, like you are expecting. It will merely fill in your provided buffer as needed.
Try something more like this:
WCHAR win_class[256] = {0};
int win_class_len = 0;
HWND hwnd = GetForegroundWindow();
if (!hwnd)
{
// error handling as needed...
lstrcpyW(win_class, L"WINDOW NOT FOUND");
win_class_len = lstrlenW(win_class);
}
else
{
win_class_len = GetClassNameW(hwnd, win_class, 256);
if (win_class_len == 0)
{
DWORD err = GetLastError();
// error handling as needed...
lstrcpyW(win_class, L"CLASS NOT FOUND");
win_class_len = lstrlenW(win_class);
}
}
// use win_class as needed, up to win_class_len characters...

Answer 3

TCHAR is the universal char type working for Unicode and ANSI. GetClassName is the corresponding function replaced with GetClassNameA or GetClassNameW. If you need static string use TEXT() macro like TCHAR * s = TEXT("abc");
TCHAR win_class[MAX_PATH];
int res = GetClassName( hwnd, win_class, MAX_PATH );
if ( res > 0 )
// success
Your error is that win_class was unallocated (NULL) string pointer. GetClassName needs valid memory address of buffer.

Answer 4

You need to check if you function succseed. To do so, print the return value of the function.
Failiure will be presented by the value 0.