WHAT IS DLL FILE?

DLL is a library that contains code and data that can be used by more than one program at the same time. For example, in Windows operating systems, the Comdlg32 DLL performs common dialog box related functions. Each program can use the functionality that is contained in this DLL to implement an Open dialog box. It helps promote code reuse and efficient memory usage.

By using a DLL, a program can be modularized into separate components. For example, an accounting program may be sold by module. Each module can be loaded into the main program at run time if that module is installed. Because the modules are separate, the load time of the program is faster. And a module is only loaded when that functionality is requested.

Additionally, updates are easier to apply to each module without affecting other parts of the program. For example, you may have a payroll program, and the tax rates change each year. When these changes are isolated to a DLL, you can apply an update without needing to build or install the whole program again.

The following list describes some of the files that are implemented as DLLs in Windows operating systems:

• ActiveX Controls (.ocx) filesAn example of an ActiveX control is a calendar control that lets you select a date from a calendar.
• Control Panel (.cpl) filesAn example of a .cpl file is an item that is located in Control Panel. Each item is a specialized DLL.
• Device driver (.drv) filesAn example of a device driver is a printer driver that controls the printing to a printer.

Types of DLLs

When you load a DLL in an application, two methods of linking let you call the exported DLL functions. The two methods of linking are load-time dynamic linking and run-time dynamic linking.

Load-time dynamic linking

In load-time dynamic linking, an application makes explicit calls to exported DLL functions like local functions. To use load-time dynamic linking, provide a header (.h) file and an import library (.lib) file when you compile and link the application. When you do this, the linker will provide the system with the information that is required to load the DLL and resolve the exported DLL function locations at load time.

Run-time dynamic linking

In run-time dynamic linking, an application calls either the LoadLibrary function or the LoadLibraryEx function to load the DLL at run time. After the DLL is successfully loaded, you use the GetProcAddress function to obtain the address of the exported DLL function that you want to call. When you use run-time dynamic linking, you do not need an import library file.

The following list describes the application criteria for when to use load-time dynamic linking and when to use run-time dynamic linking:

• Startup performanceIf the initial startup performance of the application is important, you should use run-time dynamic linking.
• Ease of useIn load-time dynamic linking, the exported DLL functions are like local functions. This makes it easy for you to call these functions.
• Application logicIn run-time dynamic linking, an application can branch to load different modules as required. It is important when you develop multiple-language versions.

DLL dependencies

When a program or a DLL uses a DLL function in another DLL, a dependency is created. The program is no longer self-contained, and the program may experience problems if the dependency is broken. For example, the program may not run if one of the following actions occurs:

• A dependent DLL is upgraded to a new version.
• A dependent DLL is fixed.
• A dependent DLL is overwritten with an earlier version.
• A dependent DLL is removed from the computer.

These actions are known as DLL conflicts. If backward compatibility is not enforced, the program may not successfully run.

The following list describes the changes that have been introduced in Windows 2000 and in later Windows operating systems to help minimize dependency issues:

• Windows File ProtectionIn Windows File Protection, the operating system prevents system DLLs from being updated or deleted by an unauthorized agent. When a program installation tries to remove or update a DLL that is defined as a system DLL, Windows File Protection will look for a valid digital signature.
• Private DLLsPrivate DLLs let you isolate a program from changes that are made to shared DLLs. Private DLLs use version-specific information or an empty .local file to enforce the version of the DLL that is used by the program. To use private DLLs, locate your DLLs in the program root folder. Then, for new programs, add version-specific information to the DLL. For old programs, use an empty .local file. Each method tells the operating system to use the private DLLs that are located in the program root folder.

DLL advantages

The following list describes some of the advantages that are provided when a program uses a DLL:

• Uses fewer resourcesWhen multiple programs use the same library of functions, a DLL can reduce the duplication of code that is loaded on the disk and in physical memory. It can greatly influence the performance of not just the program that is running in the foreground, but also other programs that are running on the Windows operating system.
• Promotes modular architectureA DLL helps promote developing modular programs. It helps you develop large programs that require multiple language versions or a program that requires modular architecture. An example of a modular program is an accounting program that has many modules that can be dynamically loaded at run time.
• Eases deployment and installationWhen a function within a DLL needs an update or a fix, the deployment and installation of the DLL does not require the program to be relinked with the DLL. Additionally, if multiple programs use the same DLL, the multiple programs will all benefit from the update or the fix. This issue may more frequently occur when you use a third-party DLL that is regularly updated or fixed.

DLL troubleshooting tools

Several tools are available to help you troubleshoot DLL problems. The following tools are some of these tools.

Dependency Walker

The Dependency Walker tool can recursively scan for all dependent DLLs that are used by a program. When you open a program in Dependency Walker, Dependency Walker does the following checks:

• Dependency Walker checks for missing DLLs.
• Dependency Walker checks for program files or DLLs that are not valid.
• Dependency Walker checks that import functions and export functions match.
• Dependency Walker checks for circular dependency errors.
• Dependency Walker checks for modules that are not valid because the modules are for a different operating system.

By using Dependency Walker, you can document all the DLLs that a program uses. It may help prevent and correct DLL problems that may occur in the future. Dependency Walker is located in the following directory when you install Visual Studio 6.0:

drive\Program Files\Microsoft Visual Studio\Common\Tools

DLL Universal Problem Solver

The DLL Universal Problem Solver (DUPS) tool is used to audit, compare, document, and display DLL information. The following list describes the utilities that make up the DUPS tool:

• Dlister.exeThis utility enumerates all the DLLs on the computer and logs the information to a text file or to a database file.
• Dcomp.exeThis utility compares the DLLs that are listed in two text files and produces a third text file that contains the differences.
• Dtxt2DB.exeThis utility loads the text files that are created by using the Dlister.exe utility and the Dcomp.exe utility into the dllHell database.
• DlgDtxt2DB.exeThis utility provides a graphical user interface (GUI) version of the Dtxt2DB.exe utility.

DLL Help database

The DLL Help database helps you locate specific versions of DLLs that are installed by Microsoft software products.

DLL development

This section describes the issues and the requirements that you should consider when you develop your own DLLs.

WHAT IS VISUAL STUDIO?

Microsoft Visual Studio is an integrated development environment (IDE) from Microsoft. It is used to develop computer programs, as well as websites, web apps, web services and mobile apps.

40

SHARES

Share on Facebook

Post on X

Follow us