Like most modern operating systems, Microsoft Windows supports shared libraries, collections of code which can be used by multiple processes while only being loaded once into memory. Windows terms its shared libraries Dynamic-link libraries (DLLs).
Most core Windows functionality is contained within Native Applications and a set of DLLs, which together implement the various subsystems under which code can run (Win32, Microsoft Windows Services for UNIX (SUA), Virtual DOS machine, etc.). Each subsystem defines and implements a large number of functions, or Application programming interfaces (APIs), which programs running under the subsystems can call. These APIs are exported by, and in many cases implemented in, one or more DLLs specific to the subsystem.
Many other DLLs exist within Windows as components of specific programs or services that are provided with the operating system, and third-party programs or services added to the system can come with DLLs of their own as well. These are not considered library files, as they are not (in general) callable by other applications, and are not discussed here.
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The library files in this section are not used directly by most programs; however, they do help implement functions in other libraries that are so used.
Hal.dll is Windows' Hardware Abstraction Layer, or HAL. The HAL implements a number of functions that are implemented in different ways by different hardware platforms, which in this context, refers mostly to the Chipset. Other components in the operating system can then call these functions in the same way on all platforms, without regard for the actual implementation. For example, responding to an interrupt is quite different on a machine with an Advanced Programmable Interrupt Controller (APIC) than on one without. The HAL abstracts such differences so that nothing outside the HAL need be concerned with them.
The HAL is loaded into kernel address space and runs in kernel mode, so routines in the HAL cannot be called directly by applications, and no user mode APIs correspond directly to HAL routines. Instead the HAL provides services primarily to the Windows executive and kernel and to kernel mode device drivers. Although drivers for most hardware are contained in other files, commonly of file type .sys, a few core drivers are compiled into Hal.dll.
Kernel mode device drivers for devices on buses such as PCI and PCI Express directly call routines in the HAL to access I/O ports and registers of their devices. The drivers use HAL routines because different platforms may require different implementations of these operations. The HAL implements the operations appropriately for each platform, so the same driver executable file can be used on all platforms using the same CPU architecture, and the driver source file can be portable across all architectures.
On x86 systems there are several different HAL files on the installation media. The Windows installation procedure determines which ones are appropriate for the current platform and copies it to the hard drive, renaming it to Hal.dll if necessary. Among the criteria for this selection are the presence of an ACPI-compatible BIOS, the presence of an APIC, and whether or not multiple processors are present and enabled. (The multiple cores of a multi-core CPU, and even the "logical processors" implemented by a hyperthreading CPU, all count as "processors" for this purpose.) On x86-64 and Itanium platforms there is just one possible Hal.dll for each CPU architecture.
Ntdll.dll exports the Windows Native API. The Native API is the interface used by user-mode components of the operating system that must run without support from Win32 or other API subsystems.[1] Most of this API is implemented in ntdll.dll and at the upper edge of ntoskrnl.exe (and its variants); the majority of exported symbols within these libraries are prefixed Nt, e.g., NtDisplayString. Native APIs are also used to implement many of the "kernel APIs" or "base APIs" exported by Kernel32.dll.
Applications that are linked directly against this library are known as native applications; the primary reason for their existence is to perform tasks that must run early in the system startup sequence before the Win32 subsystem is available. An obvious but important example is the creation of the Win32 subsystem process, csrss.exe. Before the csrss.exe process exists, no Win32 processes may be created, therefore the process that creates it (Smss.exe, the "session manager") must be a native application. csrss.exe is itself a native application.
Despite having a ".exe" file extension, native applications cannot be executed by the user (or any program in the Win32 or other subsystems). An example is the autochk.exe binary that runs chkdsk during the system initialization "Blue Screen". Other prominent examples are the services that implement the various subsystems, such as csrss.exe.
Unlike Win32 applications, native applications instantiate within the Kernel runtime code (ntoskrnl.exe) and so they must have a different entry point (NtProcessStartup, rather than (w)(Win)MainCRTStartup as is found in a Win32 application),[1] obtain their command-line arguments via a pointer to an in-memory structure, manage their own memory using the Rtl heap API, and return execution with a call to NtTerminateProcess (as opposed to ExitProcess). A common library linked with Native applications is nt.lib, which contains startup code for Native applications, similar to how the C runtime provides startup code for Win32 apps.
Though most of the API is undocumented, Native Applications can be built using the Windows Driver Development Kit; many AntiVirus and other utility software vendors incorporate Native Applications within their products, usually to perform some boot-time task that cannot be carried out in userspace.
It is possible, though both unsupported and unusual, for ordinary Windows programs to call functions from Ntdll.dll directly, bypassing the Win32 client libraries. Occasionally this is done to use a native API function that has no Win32 equivalent.
The libraries in this section each implement various subsets of the Win32 API.
Kernel32.dll exposes to applications most of the Win32 base APIs, such as memory management, input/output operations, (process and thread) creation, and synchronization functions. Many of these are implemented within Kernel32.dll by calling corresponding functions in the native API, exposed by Ntdll.dll.[2]
Gdi32.dll exports Graphics Device Interface (GDI) functions that perform primitive drawing functions for output to video displays and printers. Applications call GDI functions directly to perform low-level drawing, text output, font management, and similar functions.[2]
user32.dll implements the Windows USER component that creates and manipulates the standard elements of the Windows user interface, such as the desktop, windows, and menus. It thus enables programs to implement a graphical user interface that matches the Windows look and feel. Programs call functions from Windows USER to perform operations such as creating and managing windows, receiving window messages (which are mostly user input such as mouse and keyboard events, but also notifications from the operating system), displaying text in a window, and displaying message boxes.
Many of the functions in User32.dll call upon GDI functions exported by Gdi32.dll to do the actual rendering of the various elements of the user interface. Some types of programs will also call GDI functions directly to perform lower-level drawing operations within a window previously created via User32 functions.
comctl32.dll implements a wide variety of standard Windows controls, such as File Open, Save, and Save As dialogs, progress bars, and list views. It calls functions from both User32.dll and Gdi32.dll to create and manage the windows for these UI elements, place various graphic elements within them, and collect user input.
Msvcrt.dll is the Microsoft Visual C++ Run-Time for Visual C++ version 4.2 to 6.0. It provides programs compiled with these versions of Visual C++ a typical set of library functions required by C and C++ programs. These include string manipulation, memory allocation, C-style input/output calls, etc.
It has also shipped with Windows versions since Windows 2000 for use by other Windows components. In older versions of Windows, programs which linked against Msvcrt.dll were expected to install a compatible copy in the System32 folder, but this contributed to DLL Hell.
Versions of Visual C++ before 4.0 and since 7.0 have used differently named DLLs for each version (msvcr20.dll, msvcr70.dll, msvcr71.dll, etc.). Applications are required to install the appropriate version.[3]
shscrap.dll is part of the Object Linking and Embedding (OLE) mechanism. It implements support for shell scrap files. These are automatically created when you drag selected content from an OLE-capable application into an Explorer window (or onto the Desktop),[4] but you can also use the Object Packager to create them. They can then be dragged into another OLE-capable application.
This functionality was removed from Windows Vista (and therefore later versions) to improve security and rid the operating system of generally unused functionality.[5] Scrap (.shs) files have been used by viruses because they can contain a wide variety of files (including executable code), and the file extension is not shown even when "Hide file extensions from known file types" is disabled.[6] The functionality can be restored by copying registry entries and the DLL from a Windows XP system.[7]