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 call the same routine, the routine is included in both the programs during the linking stage of the two. 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.

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 that limited the performance of the Pentium Pro microprocessor when launched, and ultimately limited the stability and scalability of the DOS-based versions of Windows.

Although DLLs are the core of the Windows architecture, they have several drawbacks, collectively called "DLL hell". Microsoft currently promotes .NET Framework 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 implementing side-by-side assembly.

Features of DLL

It is not possible to directly execute a DLL, since it requires an EXE for the operating system to load it through an entry point, hence the existence of utilities of rundll which provide the entry point and minimal framework for DLLs that contain enough functionality to execute without much support.

DLLs provide a mechanism for shared code and data, allowing a developer of shared code/data to upgrade functionality without requiring applications to be re-linked or re-compiled. From the application development point of view Windows and OS/2 can be thought of as a collection of DLLs that are upgraded, allowing applications for one version of the O/S to work in a later one, provided that the O/S vendor has ensured that the interfaces and functionality are compatible.

DLLs execute in the memory space of the calling process and with the same access permissions which means there is little overhead in their use but also that there is no protection for the calling EXE if the DLL has any sort of bug.

Memory management

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

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

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 it 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 API function is used to look up exported symbols by name, and FreeLibrary — to unload the DLL. These functions are analogous to dlopen, dlsym, and dlclose in the POSIX standard API.

// LSPaper draw using OLE2 function if available on client

HINSTANCE hOle2Dll ;

hOle2Dll = LoadLibrary ( "OLE2.DLL" ) ;

if ( hOle2Dll != NULL ) { FARPROC lpOleDraw ;

lpOleDraw = GetProcAddress ( hOle2Dll , "OleDraw" ) ;

if ( lpOleDraw != (FARPROC)NULL ) { (*lpOleDraw) (pUnknown , dwAspect , hdcDraw , lprcBounds ) ; } FreeLibrary ( hOle2Dll ) ; }

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

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.

Compiler and language considerations

In the basest application package, you would find at least a single EXE file that may or may not be accompanied with one or more DLL files. An EXE file contains the entry point or the part in the code where the operating system is supposed to begin the execution of the application. DLL files do not have this entry point and cannot be executed on their own.

The most major advantage of DLL files is in its reusability. A DLL file can be used in other applications as long as the coder knows the names and parameters of the functions and procedures in the DLL file. Because of this capability, DLL files are ideal for distributing device drivers. The DLL would facilitate the communication between the hardware and the application that wishes to use it. The application would not need to know the intricacies of accessing the hardware just as long as it is capable of calling the functions on the DLL.

Launching an EXE would mean creating a process for it to run on and a memory space. This is necessary in order for the program to run properly. Since a DLL is not launched by itself and is called by another application, it does not have its own memory space and process. It simply shares the process and memory space of the application that is calling it. Because of this, a DLL might have limited access to resources as it might be taken up by the application itself or by other DLLs.

Delphi

Delphi does not need LIB files to import functions from DLLs; to link to a DLL, the external keyword is used in the function declaration.

Microsoft Visual Basic

When importing DLL functions through declarations, VB will generate a run-time error if the DLL 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++

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

DLL compilation will produce both DLL and LIB files. The LIB file is used to link against a DLL at compile-time; it is not necessary for run-time linking. Unless your DLL is a Component Object Model (COM) server, the DLL 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.

Programming examples

Creating DLL exports

Delphi

library Example;

// function that adds two numbers function AddNumbers(a, b : Double): Double; cdecl; begin Result := a + b; end;

// export this function exports AddNumbers;

// DLL initialization code: no special handling needed begin end.

C and C++ #include

// DLL entry function (called on load, unload, ...) BOOL APIENTRY DllMain(HANDLE hModule, DWORD dwReason, LPVOID lpReserved) { return TRUE; }

// Exported function - adds two numbers extern "C" __declspec(dllexport) double AddNumbers(double a, double b) { return a + b; }

Using DLL imports

Delphi

{$APPTYPE CONSOLE}

program Example;

// import function that adds two numbers function AddNumbers(a, b : Double): Double; cdecl; external 'Example.dll';

// main program var R: Double;

begin R := AddNumbers(1, 2); Writeln('The result was: ', R); end.

MS Visual 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 linking error 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. #include #include

// Import function that adds two numbers extern "C" __declspec(dllimport) double AddNumbers(double a, double b);

int main(int argc, char *argv[]) { double result = AddNumbers(1, 2); printf("The result was: %f\n", result); return 0; }

Using explicit run-time linking

Microsoft Visual Basic

Sub Main() Dim Result As Double Result = AddNumbers(1, 2) Debug.Print "The result was: " & Result End Sub

Delphi

{$APPTYPE CONSOLE}

uses Windows;

var AddNumbers : function (a, b: Double): Double; cdecl; LibHandle : HMODULE;

begin LibHandle := LoadLibrary('example.dll');

if LibHandle = 0 then Exit;

AddNumbers := GetProcAddress(LibHandle, 'AddNumbers');

if Assigned( AddNumbers ) then Writeln( '1 + 2 = ', AddNumbers( 1, 2 ) ); else Writeln('Error: unable to find DLL function');

FreeLibrary(LibHandle); end.

C and C++