Libraries and Headers
Kernel-mode drivers use the native system services routines by calling the Nt and Zw entry points in the Ntoskrnl.exe dynamic link library (DLL). This DLL contains the actual implementations of these routines. To access these entry points, a driver statically links to the Ntoskrnl.lib library, which is available in the Windows Driver Kit (WDK). The routines that are implemented in Ntoskrnl.lib are stubs that dynamically link to the entry points in Ntoskrnl.exe at run time.
The WDK documentation describes some, but not all, of the Zw entry points in Ntoskrnl.exe. For descriptions of the Zw routines that can be called by drivers, see ZwXxx Routines.
Most of the documented Zw routines are defined in the Wdm.h header file in the WDK, but a few are defined in other header files, such as Ntddk.h and Ntifs.h.
Typically, user-mode applications do not call the Nt and Zw routines. Instead, an application might call a Win32 routine, such as CreateFile, which then calls a native system services routine, such as NtCreateFile or ZwCreateFile, to perform the requested operation. However, a user-mode application might directly call an Nt or Zw routine to perform an operation that is not supported by the Win32 routines.
User-mode applications use the native system services routines by calling the entry points in the Ntdll.dll dynamic link library. These entry points convert calls to Nt and Zw routines into system calls that are trapped to kernel mode. To access these entry points, a user-mode application statically links to the Ntdll.lib library, which is available in the WDK. The routines that are implemented in Ntdll.lib are stubs that dynamically link to the entry points in Ntdll.dll at run time.
The Windows SDK documentation describes some, but not all, of the Nt entry points in Ntdll.lib. Most of the documented Nt routines are defined in the Winternl.h header file in the Windows SDK. This documentation makes little mention of the Zw entry points, and no header files in the Windows SDK contain definitions of Zw routines.
With a couple of minor exceptions, each entry point in Ntdll.dll for an Nt routine has a matching entry point for a Zw routine. The documentation for the WDK and Windows SDK recommends that application developers avoid calling undocumented Nt entry points, and warns that the Zw entry points might disappear from Ntdll.dll in a future version of Windows. Application developers who call the Zw routines from user mode should be prepared for this occurrence.
For descriptions of the Nt routines that can be called by applications, see Winternl, Files, and Miscellaneous Low-Level Client Support. Some reference pages for Nt routines in the Windows SDK documentation label the routines as "deprecated" and advise readers to use the equivalent Win32 routines instead of the deprecated Nt routines.
A user-mode application cannot call the entry points in Ntoskrnl.exe, and a kernel-mode driver cannot call the entry points in Ntdll.dll.