DLLs in Visual C++

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A dynamic-link library (DLL) is an executable file that acts as a shared library of functions and resources. Dynamic linking enables an executable to call functions or use resources stored in a separate file. These functions and resources can be compiled and deployed separately from the executables that use them. The operating system can load the DLL into the executable's memory space when the executable is loaded, or on demand at runtime. DLLs also make it easy to share functions and resources across executables. Multiple applications can access the contents of a single copy of a DLL in memory at the same time.

Static linking copies all the object code in a .lib file into an executable. Dynamic linking includes only the information needed at run time to locate and load the DLL that contains a data item or function. When you create a DLL, you also create a .lib file that contains this information. When you build an executable that calls the DLL, the linker uses the exported symbols in the .lib file to store this information for the loader. When the loader loads a DLL, the DLL is mapped into the memory space of your executable. A special function in the DLL, DllMain, is called to perform any initialization the DLL requires.

Using dynamic linking instead of static linking offers several advantages. When you use DLLs, you can save memory space and reduce swapping. When multiple applications can use a single copy of a DLL, you can save disk space and download bandwidth. DLLs can be deployed and updated separately, which lets you provide after-market support and software updates without having to rebuild and ship all your code. DLLs are a convenient way to supply locale-specific resources, which can support multi-language programs, and ease creating international versions of your applications.

The following topics provide detailed information about how to program DLLs.

In This Section

Walkthrough: Creating and Using a Dynamic Link Library (C++)
Describes how to create and use a DLL using Visual Studio.

Differences Between Applications and DLLs
Describes the fundamental differences between applications and DLLs.

Advantages of Using DLLs
Describes the advantages of dynamic linking.

Kinds of DLLs
Provides information about the different kinds of DLLs that can be built.

DLL Frequently Asked Questions
Provides answers to frequently asked questions about DLLs.

Linking an Executable to a DLL
Describes explicit and implicit linking to a DLL.

Initializing a DLL
Discusses DLL initialization code (such as allocating memory) that must execute when your DLL loads.

Run-Time Library Behavior
Describes how the run-time library performs the DLL startup sequence.

LoadLibrary and AfxLoadLibrary
Discusses using LoadLibrary and AfxLoadLibrary to explicitly link to a DLL at runtime.

GetProcAddress
Discusses using GetProcAddress to obtain the address of an exported function in the DLL.

FreeLibrary and AfxFreeLibrary
Discusses using FreeLibrary and AfxFreeLibrary when the DLL module is no longer needed.

Search Path Used by Windows to Locate a DLL
Describes the search path that the Windows operating system uses to locate a DLL on the system.

Module States of a Regular DLL Dynamically Linked to MFC
Describes the module states of a regular DLL dynamically linked to MFC.

Extension DLLs
Explains DLLs that typically implements reusable classes derived from the existing Microsoft Foundation Class Library classes.

Creating a Resource-Only DLL
Discusses a resource-only DLL, which contains nothing but resources, such as icons, bitmaps, strings, and dialog boxes.

Localized Resources in MFC Applications: Satellite DLLs
Provides enhanced support for satellite DLLs, a feature that helps in creating applications localized for multiple languages.

Importing and Exporting
Describes importing public symbols into an application or exporting functions from a DLL

Active Technology and DLLs
Allows object servers to be implemented inside a DLL.

Automation in a DLL
Describes what the Automation option in the MFC DLL Wizard supplies.

Naming Conventions for MFC DLLs
Discusses how the DLLs and libraries included in MFC follow a structured naming convention.

Calling DLL functions from Visual Basic Applications
Describes how to call DLL functions from Visual Basic applications.

Using MFC as Part of a DLL
Describes regular DLLs, which let you use the MFC library as part of a Windows dynamic-link library.

DLL Version of MFC
Describes how you can use the MFCxx.dll and MFCxxD.dll (where x is the MFC version number) shared dynamic-link libraries with MFC applications and extension DLLs.

(NOTINBUILD)Visual C++ Programming Methodologies
Provides links to topics that describe conceptual information about the Visual C++ libraries and topics that discuss various coding technologies and techniques.