windows编程之内核对象

      学好windows编程，理解内核对象还是至关重要的(●'◡'●)。闲话不多说，下面先来了解一下关于内核对象的知识：

      内核对象（kernel object）：内核对象是用于管理进程、线程和文件等诸多种类的大量资源。

      内核对象的分类：进程对象，线程对象，互斥量（mutex）对象，信号量（semaphore）对象，事件对象，作业对象，文件对象，文件映射对象，管道（pipe）对象，邮件槽（mailslot）对象，I/O完成端口对象，线程池工厂（thread pool  worker  factory）对象，令牌（access token）对象、可等待的计时器（waitable timer）对象等。

      内核对象的特性：

      每个内核对象都只是一个内存块，它由操作系统分配，并只能由操作系统内核访问，这个内存块是一个数据结构，其成员维护着与对象相关的信息。少数成员（安全描述符和使用计数等）是所有对象都有的，但其他大多数成员都是不同类型的对象特有的。例如，进程对象有一个ID，一个基本的优先级和一个退出代码；而文件对象有一个字节偏移量（byte offset）、一个共享模式和一个打开模式。

      由于内核对象的数据结构只能被操作系统访问，故应用程序不能在内存中定位这些数据结构并更改其内容；

     当调用一个可以创建内核对象的函数后，该函数会返回一个用于标识该对象的句柄。如果将该句柄值传递给另一个进程中的一个线程，那么这另一个进程使用你的进程的句柄值所作的调用就会失败。如果想在多个进程中共享内核对象，要通过一定的机制（如对象句柄的继承性，命名对象，复制对象句柄）。

   

      下图所示是一个可以查看一个包含所有内核对象类型的列表的软件（WinObj）界面。有兴趣的可以下载学习一下，加深一下对内核对象的认识。

     

内核对象的安全性：内核对象可以用一个安全描述符（security descriptor，SD）来保护。用于创建内核对象的所有函数几乎都指向一个SECURITY_ATTRIBUTES结构的指针作为参数：如下面的

                                                CreateFileMapping（

                                                                              HHANDLE hFile，

                                                                              PSECURITY_ATTRIBUTES  psa，

                                                                              DWORD flProtect，

                                                                              DWORD dwMaximumSizeHigh，  

                                                                              DWORD dwMaximumSiizelow，

                                                                              PCTSTR pszName              

                                                                           ）；

大多数应用程序只是为这个参数传入NULL，这样创建的内核对象具有默认的安全性----具体包括哪些默认的安全性，要取决于当前进程的安全令牌（security token）。但是也可以分配一个SECURITY_ATTRIBUTES结构，并对它进行初始化，再将它的地址传给这个参数。SECURITY_ATTRIBUTES结构体如下所示：

                                                typedef struct   _SECURITY_ATTRIBUTES {

                                                                              DWORD nLength;

                                                                              LPVOID lpSecurityDescriptor;

                                                                              BOOL  bInheritHandle;

                                                                              } SECURITY_ATTRIBUTES;

虽然这个结构称为SECURITY_ATTRIBUTES,但它实际上只包含一个和安全性有关的成员，即lpSecurityDescriptor。如果相对创建的内核对象加以访问限制，就必须创建一个安全描述符，然后按下面所示初始化其结构：

                                                                             SECURITY_ATTRIBUTES  sa；

                                                                             sa.nLength = sizeof(sa);

                                                                             sa.lpSecrityDeccriptor = pSD;

                                                                             sa.bInheritHandle = FALSE;

                                                                             HANDLE hFileMApping = CreateFileMapping(INVALID_HANDLE_VALUE,

                                                                                                                                         &sa,

                                                                                                                                         P0AGE_READWRITE,

                                                                                                                                         0,

                                                                                                                                         1024,

                                                                                                                                         TEXT(“MyFileMapping”));

若想访问一个现有的文件映射对象，以便从中读取数据，可以如下下述方式调用：

                                                                             HANDLE hFileMapping = OpenFileMapping(

                                                                                                                                         FILE_MAP_READ,FALSE,

                                                                                                                                         FALSE,

                                                                                                                                         TEXT(“MyFileMapping”));

若想判断一个对象是不是内核对象，最简单的方式是查看创建这个对象的函数。几乎所有创建内核对象的函数都有一个指定安全属性信息的参数，就像上述的CreateFileMapping函数一样。相反，用于创建用户对象或GDI对象的函数都没有PSECURITY_ATTRIBUTES参数。如下所示的CreateIcon函数

                                                                       HICON CreateIcon（

                                                                                                             HINSTANCE  hinst，

                                                                                                             int  nWidth,

                                                                                                             int nHight,

                                                                                                             BYTE cPlanes,

                                                                                                             BYTE cBitsPixel,

                                                                                                             CONST BYTE *pbANDbits,

                                                                                                             CONST BYTE *pbXORbits）;

ps：像菜单、窗口、鼠标光标、画刷和字体这样的对象属于用户对象或GDI（Graphical Device  Interface）对象。 

       部分创建内核对象的函数：

                                                               HANDLE CraeateThread (

                                                                                                              PSECURITY_ATTRIBUTES  psa,

                                                                                                              size_t  dwStackSize,

                                                                                                              LPTHREAD_START_ROUTINE pfnstartAddress,

                                                                                                              PVOID pvParam,

                                                                                                              DWORD dwCreatinFlags,

                                                                                                              PDWORD pdwThread );

                                                                 HANDLE CraeateFile    (

                                                                                                              PCTSTR pszFileName

                                                                                                              DWORD  dwDesiredAccess,

                                                                                                              DWORD  dwShareMode,

                                                                                                              PSECURITY_ATTRIBUTES  psa,

                                                                                                              DWORD   daCreationDisposition,

                                                                                                              DWORD   dwFlagsAndAttributes,

                                                                                                              HANDLE   hTemplateFile );

                                                           HANDLE CreateSemaphore (

                                                                                                              PSECURITY_ATTRIBUTES  psa,

                                                                                                              LONG lInitialCount,

                                                                                                              LONG lMaxiumumCount,

                                                                                                              PCTSTR pszName);

                                                           ……

结束内核对象，需调用：BOOL CloseHandle(HANDLE  hobject);