请问回调函数跟winmain什么关系？如何使用？...

WinMain函数是Windows应用程序入口点。
回调函数用CALLBACK标识，用于系统调用。
典型的应用是处理消息，当系统接受到消息后，会调用标识指定的CALLBACK函数来处理消息
#include <windows.h>
#include <stdio.h>
LRESULT CALLBACK WinProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{
WNDCLASS wndClass;
wndClass.cbClsExtra = 0;
wndClass.cbWndExtra = 0;
wndClass.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);
wndClass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndClass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndClass.hInstance = hInstance;
wndClass.lpfnWndProc = WinProc;
wndClass.lpszClassName = "MyFirstWindow";
wndClass.lpszMenuName = NULL;
wndClass.style = CS_VREDRAW | CS_HREDRAW;
RegisterClass(&wndClass);
HWND hwnd;
hwnd = CreateWindow("MyFirstWindow", "我的第一个C++窗口程序", WS_OVERLAPPEDWINDOW,
   CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, hInstance, NULL);
ShowWindow(hwnd, SW_SHOWNORMAL);
MSG msg;
while(GetMessage(&msg, hwnd, 0, 0))
{
   TranslateMessage(&msg);
   DispatchMessage(&msg);
}
return 0;
}
LRESULT CALLBACK WinProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
HDC hdc;
switch(uMsg)
{
   case WM_LBUTTONDOWN:
    MessageBox(hwnd, "按下了鼠标左键", "提示", MB_OK);
    break;
   case WM_CLOSE:
    if (IDYES == MessageBox(hwnd, "确定要关闭窗口吗？", "提示", MB_YESNO))
    {
     DestroyWindow(hwnd);
    }
    break;
   case WM_DESTROY:
    PostQuitMessage(0);
    break;
   default:
    return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
return 0;
}