求助！为什么我的消息处理函数会出错？

一个在窗体上绘制正弦曲线的程序，不知道为什么消息处理函数总是不对，因为是自学，也不知道是哪里的问题~请大家指点迷津~
程序如下：
// a.cpp : Defines the entry point for the application.
//

LRESULT _stdcall WndProc(HWND,UNIT,WPARAM,LPARAM)   //消息处理函数声明

#include "stdafx.h"
#include "math.h"

int APIENTRY WinMain(HINSTANCE hInstance,         //构建窗体
              HINSTANCE hPrevInstance,
              LPSTR   lpCmdLine,
              int     nCmdShow)
{
     // TODO: Place code here.
           char szClassName[]="MainWClass";
     WNDCLASSEX wndclass;
     wndclass.cbSize=sizeof(wndclass);
     wndclass.style=CS_HREDRAW|CS_VREDRAW;
     wndclass.lpfnWndProc=MainWndProc;
     wndclass.cbClsExtra=0;
     wndclass.cbWndExtra=0;
     wndclass.hInstance=hInstance;
     wndclass.hIcon=::LoadIcon(NULL,IDI_APPLICATION);
     wndclass.hCursor=::LoadCursor(NULL,IDC_ARROW);
     wndclass.hbrbackground=(HBRUSH)::GetStockObject(WHITE_BRUSH);
     wndclass.lpszMenuName=NULL;
     wndclass.lpszClassName=szClassName;
     wndclass.hIconSm=NULL;
     ::RegisterClassEx(&wndclass);
     HWND hwnd=::CreateWindowEx(0,szClassName,"My first Window!",WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,NULL,NULL,hInstance,NULL);
     if(hwnd==NULL)
     {
           ::MessageBox(NULL,"创建窗口出错！","error",MB_OK);
           return -1;
     }
     ::ShowWindow(hwnd,nCmdShow);
     ::UpdateWindow(hwnd);
     MSG msg;
     while(::GetMessage(&msg,NULL,0,0))
     {
           ::TranslateMessage(&msg);
           ::DispatchMessage(&msg);
     }
     return msg.wParam;
}

//以下为消息处理函数：

LRESULT _stdcall WndProc(HWND hWnd,UNIT message,WPARAM wParam,LPARAM lParam)
{
#define SEGMENTS 500
#define PI 3.1415926
     HDC hdc;
     PAINTSTRUCT ps;
     RECT rt;
     int cxClient,cyClient;
     POINT pt[SEGMENTS];
     int i;
     switch(message)
     {
     case WM_PAINT:
           hdc=::BeginPaint(hWnd,&ps);
           ::GetClientRect(hWnd,&rt);
           cxClient=rt.right-rt.left;
           cyClient=rt.bottom-rt.top;
           ::MoveToEx(hdc,0,cyClient/2,NULL);
           ::LineTo(hdc,cxClient,cyClient/2);
           for(i=0;i<SEGMENTS;i++)
           {
                 pt.x=cxClient*i/SEGMENTS;
                 pt.y=(int)((cyClient/2)*(1-sin(2*PI*i/SEGMENTS)));
           }
           ::Polyline(hdc,pt,SEGMENTS);
           ::EndPaint(hWnd,&ps);
           break;
     case WM_DESTROY:
           ::PostQuitMessage(0);
           break;
     }
     return ::DefWindowProc(hWnd,message,wParam,lParam);
}
===========================
以下为调试信息：
--------------------Configuration: a - Win32 Debug--------------------
Compiling...
a.cpp
C:\my works\a\a.cpp(4) : error C2143: syntax error : missing ';' before '__stdcall'
C:\my works\a\a.cpp(4) : error C2501: 'LRESULT' : missing storage-class or type specifiers
C:\my works\a\a.cpp(4) : error C2065: 'HWND' : undeclared identifier
C:\my works\a\a.cpp(4) : error C2065: 'UNIT' : undeclared identifier
C:\my works\a\a.cpp(4) : error C2065: 'WPARAM' : undeclared identifier
C:\my works\a\a.cpp(4) : error C2065: 'LPARAM' : undeclared identifier
d:\program files\microsoft visual studio\vc98\include\excpt.h(36) : error C2448: '<Unknown>' : function-style initializer appears to be a function definition
d:\program files\microsoft visual studio\vc98\include\excpt.h(36) : fatal error C1004: unexpected end of file found
Error executing cl.exe.

a.obj - 8 error(s), 0 warning(s)