ventana simple en C

#include "pak.h"
#include <Windows.h>
#include <tchar.h>

#define save_instance(ins)\
        do {                    \
                instance = ins; \
        } while(0)

/* prototypes */
static int register_class (void);
static int create_window (HWND *window);
static LRESULT CALLBACK class_proc (HWND window, UINT msg, WPARAM wparam,
                                    LPARAM lparam);
/* global variables */
static HINSTANCE instance = NULL;

int APIENTRY _tWinMain (HINSTANCE ins, HINSTANCE pins, LPTSTR args, int show)
{
        save_instance(ins);

        if (register_class()) {
                MessageBox(NULL, TEXT("Error registrando ventana"),
                           NULL, MB_OK | MB_ICONEXCLAMATION);
                return -1;
        }

        HWND mwindow;
        if (create_window(&mwindow)) {
                MessageBox(NULL, TEXT("Error creando ventana"),
                           NULL, MB_OK | MB_ICONEXCLAMATION);
                return -1;
        }

        ShowWindow(mwindow, show);
        UpdateWindow(mwindow);

        MSG msg;
        while (GetMessage(&msg, NULL, 0, 0) > 0) {
                DispatchMessage(&msg);
        }

        return msg.wParam;
}

static int register_class (void)
{
        WNDCLASS wc = { .lpfnWndProc    =    class_proc,
                        .hInstance      =    instance,
                        .hbrBackground  =    (void *)(COLOR_WINDOW + 1),
                        .lpszClassName  =    TEXT("MainWindow") };

        return (RegisterClass(&wc) == 0);
}

static int create_window (HWND *window)
{
        HWND _window = CreateWindow(TEXT("MainWindow"),
                                    TEXT("Prueba"),
                                    WS_OVERLAPPED,
                                    CW_USEDEFAULT,
                                    CW_USEDEFAULT,
                                    CW_USEDEFAULT,
                                    CW_USEDEFAULT,
                                    NULL,
                                    NULL,
                                    instance,
                                    NULL);
        if (_window == NULL) {
                return -1;
        }

        *window = _window;
        return 0;
}

static LRESULT CALLBACK class_proc (HWND window, UINT msg, WPARAM wparam,
                                    LPARAM lparam)
{
        switch (msg) {
        case WM_CLOSE:
                if (MessageBox(NULL,
                               TEXT("¿Quieres cerrar la ventana?"),
                               TEXT("Cerrar"),
                               MB_YESNO | MB_ICONEXCLAMATION) == IDYES) {
                        DestroyWindow(window);
                        return 0;
                } else {
                        return -1;
                }
        case WM_DESTROY:
                PostQuitMessage(0);
                return 0;
        default:
                return DefWindowProc(window, msg, wparam, lparam);
        }
}