Dynamic-link library

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Dynamic-link library
File extension: .dll
MIME type: application/x-msdownload
Uniform Type Identifier: com.microsoft.windows-​dynamic-link-library
Developed by: Microsoft
Container for: shared library
This article is about dynamic libraries in Microsoft Windows. For other uses of similar terms, see dynamic library (disambiguation).

Dynamic-link library (DLL), also known as dynamic link library (without the hyphen), 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 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.

Contents

[edit] Background

The original purpose for DLLs was saving both disk space and memory required for applications by storing it locally on the hard drive. In a conventional non-shared library, sections of code are simply added to the calling program. If two programs call the same routine, that code would be duplicated. Instead, any code which many applications share could be separated into a DLL which only exists as a single disk file and a single instance in memory. Extensive use of DLLs allowed early versions of Windows to work under tight memory conditions.

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. This basic form of modularity allows for relatively compact patches and service packs for large applications, such as Microsoft Office, Microsoft Visual Studio, and even Microsoft Windows itself.

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 ActiveX.

With these many benefits comes a significant drawback, termed "DLL hell", when several applications conflict on which version of a shared DLL library is to be used. Such conflicts can usually be resolved by placing the different versions of the problem DLL into the applications' folders, rather than a system-wide folder; however, this also nullifies the savings provided by using shared DLLs. Currently, Microsoft .NET is targeted as a solution to the problem of DLL hell by allowing side-by-side coexistence of different versions of a same shared library. With modern computers which have plenty of disk space and memory, it can be a reasonable approach.

[edit] Features

[edit] 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 an executable packer, such as 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 and backed up by the page file. Thus, compressing DLLs increases both their memory and disk space consumption, and should be generally avoided for shared DLLs.

[edit] 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. 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. So, one cannot reliably import Windows API functions by their ordinals.

Importing functions by ordinal does not necessarily provide better performance than importing them by name: export tables of DLLs are ordered by name, so binary search can be used to find a function in this table by its name. On the other hand, only linear search can be used to find a function by its ordinal.

It is also possible to bind an executable to a specific version of 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.

[edit] 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 lookup exported symbols by name, and FreeLibrary — to unload the DLL. These functions are analogous to dlopen, dlsym, and dlclose in the POSIX standard API.

Note that with implicit run-time linking, referred to simply as load-time dynamic linking by Microsoft, if the linked DLL file cannot be found, Windows will display an error message and fail to load the application. The application developer cannot handle the absence of DLL files linked implicitly by the compile-time linker. On the other hand, with explicit run-time linking, developers have the opportunity to provide a graceful fall-back facility.

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

[edit] Compiler and language considerations

[edit] Delphi

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

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

[edit] Microsoft Visual Basic

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

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.

[edit] 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 before a function declaration. When external names follow the C naming conventions, they must also be declared as extern "C" in C++ code, in order to prevent it from using C++ naming conventions.

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 COM server, the DLL file must be placed in one of the directories listed in the PATH environment variable, or 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.

[edit] Programming examples

[edit] Creating DLL exports

The following examples show language Specific bindings for exporting symbols from DLLs.

Delphi

library Example;

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

// Export this function
exports
AddNumbers;

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

C and C++

#include <windows.h>

// Export this function
extern "C" __declspec(dllexport) double AddNumbers(double a, double b);

// DLL initialization function
BOOL APIENTRY
DllMain(HANDLE hModule, DWORD dwReason, LPVOID lpReserved)
{
        return TRUE;
}


// Function that adds two numbers
double AddNumbers(double a, double b)
{
        return a + b;
}

[edit] 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

program Example;
{$APPTYPE CONSOLE}

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

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

C and C++

#include <windows.h>
#include <stdio.h>

// 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;
}

[edit] 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.

Microsoft Visual Basic

Option Explicit
Declare Function AddNumbers Lib "Example.dll" _ 
(ByVal a As Double, ByVal b As Double) As Double

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

C and C++

#include <windows.h>
#include <stdio.h>

// DLL function signature
typedef double (*importFunction)(double, double);

int main(int argc, char **argv)
{
        importFunction addNumbers;
        double result;

        // Load DLL file
        HINSTANCE hinstLib = LoadLibrary("Example.dll");
        if (hinstLib == NULL) {
                printf("ERROR: unable to load DLL\n");
                return 1;
        }

        // Get function pointer
        addNumbers = (importFunction)GetProcAddress(hinstLib, "AddNumbers");
        if (addNumbers == NULL) {
                printf("ERROR: unable to find DLL function\n");
               FreeLibrary(hinstLib);
                return 1;
        }

        // Call function.
        result = addNumbers(1, 2);

        // Unload DLL file
        FreeLibrary(hinstLib);

        // Display result
        printf("The result was: %f\n", result);

        return 0;
}

[edit] 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.

[edit] See also

[edit] External links

[edit] References

  • Hart, Johnson. Windows System Programming Third Edition. Addison-Wesley, 2005. ISBN 0-321-25619-0
  • Rector, Brent et al. Win32 Programming. Addison-Wesley Developers Press, 1997. ISBN 0-201-63492-9.
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