Dynamic-link library

Dynamic-link library (also written without the hyphen), or DLL, is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems. These libraries usually have the file extension DLL, OCX (for libraries containing ActiveX controls), or DRV (for legacy system drivers). The file formats for DLLs are the same as for Windows EXE files — that is, Portable Executable (PE) for 32-bit and 64-bit Windows, and New Executable (NE) for 16-bit Windows. As with EXEs, DLLs can contain code, data, and resources, in any combination.

In the broader sense of the term, any data file with the same file format can be called a resource DLL. Examples of such DLLs include icon libraries, sometimes having the extension ICL, and font files, having the extensions FON and FOT.

Background for DLL
The first versions of Microsoft Windows ran every program in a single address space. Every program was meant to co-operate by yielding the CPU to other programs so that the GUI was capable of multitasking and could be as responsive as possible. All Operating-System level operations were provided by the underlying operating system: MS-DOS. All higher level services were provided by Windows Libraries Dynamic Link Libraries. The Drawing API, GDI, was implemented in a DLL called, the user interface in. These extra layers on top of DOS had to be shared across all running windows programs, not just to enable Windows to work in a machine with less than a megabyte of RAM, but to enable the programs to co-operate amongst each other. The Graphics Device Interface code in GDI needed to translate drawing commands to operations on specific devices. On the display, it had to manipulate pixels in the frame buffer. When drawing to a printer, the API calls had to be transformed into requests to a printer. Although it could have been possible to provide hard-coded support for a limited set of devices (like the Color Graphics Adapter display, the HP LaserJet Printer Command Language), Microsoft chose a different approach. GDI would work by loading different pieces of code to work with different output devices—pieces of code called 'Device Drivers'.

The same architectural concept that allowed GDI to load different device drivers is that which allowed the Windows shell to load different windows programs, and for these programs to invoke API calls from the shared USER and GDI libraries. That concept was Dynamic Linking.

In a conventional non-shared, static library, sections of code are simply added to the calling program when its executable is built at the linking phase; if two programs use the same routine, the code has to be included in both. With dynamic linking, shared code is placed into a single, separate file. The programs that call this file are connected to it at run time, with the operating system (or, in the case of early versions of Windows, the OS-extension), performing the binding.

For those early versions of Windows (1.0 to 3.11), the DLLs were the foundation for the entire GUI.
 * Display drivers were merely DLLs with a .DRV extension that provided custom implementations of the same drawing API through a unified Device Driver Interface (DDI).
 * The Drawing (GDI) and GUI (USER) APIs were merely the function calls exported by the GDI and USER, system DLLs with .EXE extension.
 * Windows Screen savers were merely DLLs with a .SCR extension

This notion of building up the operating system from a collection of dynamically loaded libraries is a core concept of Windows that persists even today. DLLs provide the standard benefits of shared libraries, such as modularity. Modularity allows changes to be made to code and data in a single self-contained DLL shared by several applications without any change to the applications themselves.

Another benefit of the modularity is the use of generic interfaces for plug-ins. A single interface may be developed which allows old as well as new modules to be integrated seamlessly at run-time into pre-existing applications, without any modification to the application itself. This concept of dynamic extensibility is taken to the extreme with the Component Object Model, the underpinnings of ActiveX.

In Windows 1.x, 2.x and 3.x, all windows applications shared the same address space, as well as the same memory. A DLL was only loaded once into this address space; from then on all programs using the library accessed it. The library's data was shared across all the programs. This could be used as an indirect form of Inter-process communication, or it could accidentally corrupt the different programs. With Windows 95 and successors every process runs in its own address space. While the DLL code may be shared, the data is private except where shared data is explicitly requested by the library. That said, large swathes of Windows 95, Windows 98 and Windows Me were built from 16-bit libraries, a feature which limited the performance of the Pentium Pro microprocessor when launched, and ultimately limited the stability and scalability of the DOS-based versions of Windows.

While DLLs are the core of the Windows architecture, they have a number of drawbacks, collectively called "DLL hell". Currently, Microsoft promotes Microsoft .NET as one solution to the problems of DLL hell, although they now promote Virtualization based solutions such as Microsoft Virtual PC and Microsoft Application Virtualization, because they offer superior isolation between applications. An alternative mitigating solution to DLL hell has been the implementation of Side-by-Side Assembly.

Memory management
In Win32, the DLL files are organized into sections. Each section has its own set of attributes, such as being writable or read-only, executable (for code) or non-executable (for data), and so on.

The code in a DLL is usually shared among all the processes that use the DLL; that is, they occupy a single place in physical memory, and do not take up space in the page file. If the physical memory occupied by a code section is to be reclaimed, its contents are discarded, and later reloaded directly from the DLL file as necessary.

In contrast to code sections, the data sections of a DLL are usually private; that is, each process using the DLL has its own copy of all the DLL's data. Optionally, data sections can be made shared, allowing inter-process communication via this shared memory area. However, because user restrictions do not apply to the use of shared DLL memory, this creates a security hole; namely, one process can corrupt the shared data, which will likely cause all other sharing processes to behave undesirably. For example, a process running under a guest account can in this way corrupt another process running under a privileged account. This is an important reason to avoid the use of shared sections in DLLs.

If a DLL is compressed by certain executable packers (e.g. UPX), all of its code sections are marked as read-and-write, and will be unshared. Read-and-write code sections, much like private data sections, are private to each process. Thus DLLs with shared data sections should not be compressed if they are intended to be used simultaneously by multiple programs, since each program instance would have to carry its own copy of the DLL, resulting in increased memory consumption.

Import libraries
Linking to dynamic libraries is usually handled by linking to an import library when building or linking to create an executable file. The created executable then contains an import address table (IAT) by which all DLL function calls are referenced (each referenced DLL function contains its own entry in the IAT). At run-time, the IAT is filled with appropriate addresses that point directly to a function in the separately-loaded DLL.

Like static libraries, import libraries for DLLs are noted by the .lib file extension. For example, kernel32.dll, the primary dynamic library for Windows' base functions such as file creation and memory management, is linked via kernel32.lib.

Symbol resolution and binding
Each function exported by a DLL is identified by a numeric ordinal and optionally a name. Likewise, functions can be imported from a DLL either by ordinal or by name. The ordinal represents the position of the function's address pointer in the DLL Export Address table. It is common for internal functions to be exported by ordinal only. For most Windows API functions only the names are preserved across different Windows releases; the ordinals are subject to change. Thus, one cannot reliably import Windows API functions by their ordinals.

Importing functions by ordinal provides only slightly better performance than importing them by name: export tables of DLLs are ordered by name, so a binary search can be used to find a function. The index of the found name is then used to look up the ordinal in the Export Ordinal table. In 16-bit Windows, the name table was not sorted, so the name lookup overhead was much more noticeable.

It is also possible to bind an executable to a specific version of a DLL, that is, to resolve the addresses of imported functions at compile-time. For bound imports, the linker saves the timestamp and checksum of the DLL to which the import is bound. At run-time Windows checks to see if the same version of library is being used, and if so, Windows bypasses processing the imports. Otherwise, if the library is different from the one which was bound to, Windows processes the imports in a normal way.

Bound executables load somewhat faster if they are run in the same environment that they were compiled for, and exactly the same time if they are run in a different environment, so there's no drawback for binding the imports. For example, all the standard Windows applications are bound to the system DLLs of their respective Windows release. A good opportunity to bind an application's imports to its target environment is during the application's installation. This keeps the libraries 'bound' until the next OS update. It does, however, change the checksum of the executable, so is not something that can be done with signed programs, or programs that are managed by a configuration management tool that uses checksums (such as MD5 checksums) to manage file versions. As more recent Windows versions have moved away from having fixed addresses for every loaded library (for security reasons), the opportunity and value of binding an executable is decreasing.

Explicit run-time linking
DLL files may be explicitly loaded at run-time, a process referred to simply as run-time dynamic linking by Microsoft, by using the LoadLibrary (or LoadLibraryEx) API function. The GetProcAddress</tt> API function is used to look up exported symbols by name, and FreeLibrary</tt> — to unload the DLL. These functions are analogous to dlopen</tt>, dlsym</tt>, and dlclose</tt> in the POSIX standard API.

The procedure for explicit run-time linking is the same in any language that supports pointers to functions, since it depends on the Windows API rather than language constructs.

Delayed loading
Normally, an application that was linked against a DLL’s import library will fail to start if the DLL cannot be found, because Windows will not run the application unless it can find all of the DLLs that the application may require. However an application may be linked against an import library to allow delayed loading of the dynamic library. In this case the operating system will not try to find or load the DLL when the application starts; instead, it will only try to find and load the DLL when one of its functions is called. If the DLL cannot be found or loaded, or the called function does not exist, the operating system will generate an exception, which the application can catch and handle appropriately. If the application does not handle the exception, it will be caught by the operating system, which will terminate the program with an error message.

The delay-loading mechanism also provides notification hooks, allowing the application to perform additional processing or error handling when the DLL is loaded and/or any DLL function is called.

Delphi
In the heading of a source file, the keyword library</tt> is used instead of program</tt>. In the end of the file, the functions to be exported are listed in exports</tt> clause.

Delphi does not require LIB</tt> files to import functions from DLLs. To link to a DLL, external</tt> keyword is used in function declaration.

Microsoft Visual Basic
In Visual Basic (VB), only run-time linking is supported; but in addition to using LoadLibrary</tt> and GetProcAddress</tt> API functions, declarations of imported functions are allowed.

When importing DLL functions through declarations, VB will generate a run-time error if the DLL</tt> file cannot be found. The developer can catch the error and handle it appropriately.

When creating DLLs in VB, the IDE will only allow you to create ActiveX DLLs, however methods have been created to allow the user to explicitly tell the linker to include a .DEF file which defines the ordinal position and name of each exported function. This allows the user to create a standard Windows DLL using Visual Basic (Version 6 or lower) which can be referenced through a "Declare" statement.

C and C++
Microsoft Visual C++ (MSVC) provides a number of extensions to standard C++ which allow functions to be specified as imported or exported directly in the C++ code; these have been adopted by other Windows C and C++ compilers, including Windows versions of GCC. These extensions use the attribute __declspec</tt> before a function declaration. When external names follow the C naming conventions, they must also be declared as extern "C"</tt> in C++ code, to prevent them from using C++ naming conventions.

Besides specifying imported or exported functions using __declspec</tt> attributes, they may be listed in IMPORT or EXPORTS section of the <tt>DEF</tt> file used by the project. The <tt>DEF</tt> file is processed by the linker, rather than the compiler, and thus it is not specific to C++.

DLL compilation will produce both <tt>DLL</tt> and <tt>LIB</tt> files. The <tt>LIB</tt> file is used to link against a DLL at compile-time; it is not necessary for run-time linking. Unless your DLL is a COM server, the <tt>DLL</tt> file must be placed in one of the directories listed in the PATH environment variable, in the default system directory, or in the same directory as the program using it. COM server DLLs are registered using regsvr32.exe, which places the DLL's location and its globally unique ID (GUID) in the registry. Programs can then use the DLL by looking up its GUID in the registry to find its location.

Creating DLL exports
The following examples show language-specific bindings for exporting symbols from DLLs.

Delphi

C and C++

Using DLL imports
The following examples show how to use language-specific bindings to import symbols for linking against a DLL at compile-time.

Delphi

C and C++

Make sure you include Example.lib file (assuming that Example.dll is generated) in the project (Add Existing Item option for Project!) before static linking. The file Example.lib is automatically generated by the compiler when compiling the DLL. Not executing the above statement would cause <U>linking error</U> as the linker would not know where to find the definition of AddNumbers. You also need to copy the DLL Example.dll to the location where the .exe file would be generated by the following code.

Using explicit run-time linking
The following examples show how to use the run-time loading and linking facilities using language-specific WIN32 API bindings.

Component Object Model
The Component Object Model (COM) extends the DLL concept to object-oriented programming. Objects can be called from another process or hosted on another machine. COM objects have unique GUIDs and can be used to implement powerful back-ends to simple GUI front ends such as Visual Basic and ASP. They can also be programmed from scripting languages. COM objects are more complex to create and use than DLLs.