这是5个字

#include
LRESULT CALLBACK WndProc(HWND hwnd, UINT umsg, WPARAM wParam, LPARAM lParam) {
	HWND hwndButton, hwndButton1,hwndButton2;

	switch (umsg) {
	case WM_DESTROY:
		PostQuitMessage(0);
		break;
	case WM_CREATE:
		hwndButton = CreateWindow("button", "anniu_1", WS_CHILD | WS_VISIBLE,
								  0, 0, 60, 30, hwnd, (HMENU)0, ((LPCREATESTRUCT)lParam)->hInstance, NULL);

		hwndButton1 = CreateWindow("button", "anniu_2", WS_CHILD | WS_VISIBLE,
								  70, 0, 60, 30, hwnd, (HMENU)0, ((LPCREATESTRUCT)lParam)->hInstance, NULL);

		hwndButton2 = CreateWindow("button", "anniu_3", WS_CHILD | WS_VISIBLE,
								  140, 0, 60, 30, hwnd, (HMENU)0, ((LPCREATESTRUCT)lParam)->hInstance, NULL);
	}
	return DefWindowProc(hwnd, umsg, wParam, lParam);
}
int WINAPI WinMain(_In_ HINSTANCE hInstance, _In_opt_ HINSTANCE hPrevInstance,
					_In_ LPSTR lpCmdLine, _In_ int nCmdShow) {
	WNDCLASS wc = { 0 };
	wc.cbClsExtra = 0;
	wc.cbWndExtra = 0;
	wc.hbrBackground = (HBRUSH)(NULL_BRUSH);
	wc.hCursor = NULL;
	wc.hIcon = NULL;
	wc.hInstance = hInstance;
	wc.lpfnWndProc = WndProc;
	wc.lpszClassName = "Main";
	wc.lpszMenuName = NULL;
	wc.style = CS_HREDRAW | CS_VREDRAW;//当窗口水平或垂直大小发生变化时，重绘窗口
	RegisterClass(&wc);//将窗口类函数写入系统内核

	HWND hwnd = CreateWindow("Main", "123", WS_OVERLAPPEDWINDOW, 0, 0, 500, 400, NULL, NULL, hInstance, NULL);

	ShowWindow(hwnd, SW_SHOW);
	UpdateWindow(hwnd);

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

1.在C语言中，函数的定义顺序是有讲究的：默认情况下，只有后面定义的函数才可以调用前面定义过的函数

第5行定义的main函数调用了第1行的sum函数，这是合法的。如果调换sum函数和main函数的顺序，在标准的C编译器环境下是不合法的（在GCC编译器环境下只是一个警告）