目录
1 概述
Win32程序的开头都可看到：
#include <windows.h>
WINDOWS.H是一个最重要的头文件，它包含了其他Windows头文件，这些头文件的某些也包含了其他头文件。这些头文件中最重要的和最基本的是：
WINDEF.H 基本数据类型定义。
WINNT.H 支持Unicode的类型定义。
WINBASE.H Kernel(内核)函数。
WINUSER.H 用户界面函数。
WINGDI.H 图形设备接口函数。
这些头文件定义了Windows的所有资料型态、函数调用、资料结构和常数识别字，它们是Windows文件中的一个重要部分。
文件路径 Windows 64位系统：
C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include[Windows 7 SDK]
C:\Program Files (x86)\Windows Kits\8.0\Include\um [Windows 8 SDK]
C:\Program Files (x86)\Windows Kits\8.1\Include\um [Windows 8.1 SDK]
不同版本的SDK位置不一样，x86的在C:\Program Files\ 相应的SDK下。
2 文件内容
/**
* @file windows.h
* Copyright 2012, 2013 MinGW.org project
*
* Permission is hereby granted, free of charge, to any person obtaining a
* copy of this software and associated documentation files (the "Software"),
* to deal in the Software without restriction, including without limitation
* the rights to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice (including the next
* paragraph) shall be included in all copies or substantial portions of the
* Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
* DEALINGS IN THE SOFTWARE.
*/
/* Written by Anders Norlander <anorland@hem2.passagen.se> */
#ifndef _WINDOWS_H
#define _WINDOWS_H
#pragma GCC system_header
#include <_mingw.h>
/* translate GCC target defines to MS equivalents. Keep this synchronized
with winnt.h. */
#if defined(__i686__) && !defined(_M_IX86)
#define _M_IX86 600
#elif defined(__i586__) && !defined(_M_IX86)
#define _M_IX86 500
#elif defined(__i486__) && !defined(_M_IX86)
#define _M_IX86 400
#elif defined(__i386__) && !defined(_M_IX86)
#define _M_IX86 300
#endif
#if defined(_M_IX86) && !defined(_X86_)
#define _X86_
#elif defined(_M_ALPHA) && !defined(_ALPHA_)
#define _ALPHA_
#elif defined(_M_PPC) && !defined(_PPC_)
#define _PPC_
#elif defined(_M_MRX000) && !defined(_MIPS_)
#define _MIPS_
#elif defined(_M_M68K) && !defined(_68K_)
#define _68K_
#endif
#ifdef RC_INVOKED
/* winresrc.h includes the necessary headers */
#include <winresrc.h>
#else
#include <stdarg.h>
#include <windef.h>
#include <wincon.h>
#include <winbase.h>
#ifndef NOGDI
#include <wingdi.h>
#endif
#include <winuser.h>
#include <winnls.h>
#include <winver.h>
#include <winnetwk.h>
#include <winreg.h>
#include <winsvc.h>
#ifndef WIN32_LEAN_AND_MEAN
#include <cderr.h>
#include <dde.h>
#include <ddeml.h>
#include <dlgs.h>
#include <imm.h>
#include <lzexpand.h>
#include <mmsystem.h>
#include <nb30.h>
#include <rpc.h>
#include <shellapi.h>
#include <winperf.h>
#ifndef NOGDI
#include <commdlg.h>
#include <winspool.h>
#endif
/* __USE_W32_SOCKETS is a __CYGWIN__ guard */
#if defined(__USE_W32_SOCKETS) || !(defined(__CYGWIN__) || defined(__MSYS__))
#include <winsock.h>
#endif
#ifndef NOGDI
/* In older versions we disallowed COM declarations in __OBJC__
because of conflicts with @interface directive. Define _OBJC_NO_COM
to keep this behaviour. */
#if !defined (_OBJC_NO_COM)
#include <ole2.h>
#endif /* _OBJC_NO_COM */
#endif
#endif /* WIN32_LEAN_AND_MEAN */
#endif /* RC_INVOKED */
#ifdef __OBJC__
/* FIXME: Not undefining BOOL here causes all BOOLs to be WINBOOL (int),
but undefining it causes trouble as well if a file is included after
windows.h
*/
#undef BOOL
#endif
#endif
3 用法
C/C++ 程序在源文件前面写 #include <windows.h>即可
4 作用
头文件封装了库函数以及一些类，将一些复杂的工作由库函数处理，而用户不必把精力放在这些地方。比如说cout<<，为标准输出流，其实说到底还是函数调用，不过这个函数有些特殊，用的是运算符重载，确切地说是重载了“<<”运算符，作用是将键盘输入的在屏幕上打印出来，这个功能要是由我们去写，估计也得学个两三年才有可能。所以就简化了操作。
而这一系列的函数都在头文件中包含（是一个函数库）。在调用时包涵后便可直接用。