Although the original idea behind the introduction of Dynamic Link Libraries (DLLs) was to make things smoother and less complicated for both software programmers and users, these now ubiquitous files lay at the heart of some of the most difficult problems to afflict computers running under Windows. While DLLs are not something that Windows users will work with directly, knowledge of what they are and how they work within the Windows environment can go a long way toward identifying and troubleshooting many of the common problems that are associated with them.

What are DLLs?

In short, DLLs are collections of small programs, or functions, that are available for shared use by larger programs and by the operating system itself. They are called dynamic because the functions within them can be executed whenever a program calls upon them, and because multiple programs can open and use them simultaneously. Although DLLs are executable files, they cannot run on their own, and are not compiled with the programs that use them. They must be executed from within a program, acting as part of that program after being called and using its own memory space.

In order to better understand just what DLLs are, it is useful to consider what they do. As device drivers, DLLs allow the Windows operating system and installed programs to communicate with system hardware like printers, hard drives, and modems. As function libraries, they provide the information necessary to perform common tasks, like printing or saving files. In fact, DLL files are what make Windows Windows. They contain the source code responsible for the uniformity of look and feel that is characteristic of all Windows software, including the code that creates the standard Graphical User Interface (GUI) of programs in the Windows environment.

DLL files are readily identified by their .dll suffix. They can be stored in the Windows and System directories, or in the directories of individual programs. If you perform a search for files with this .dll suffix on your computer, you will find that it is juggling around a huge number of these DLLs (a search on my own computer found just over 4,000 of them). If you try to open one up, you will find that they are written in machine language - completely unintelligible and supremely uninviting to humans.

DLL Files and Windows

DLL files were introduced because of the tremendous number of functions common to software running under Windows. The idea was to create a central library where all programs can obtain commonly used functions as they are needed. This greatly reduces the amount of code needed for Windows software, speeding things up and making programs more efficient. Hard drive space is minimized through the use of DLLs because the functions contained within them do not have to be written into each program’s code, and RAM usage is minimized because the DLL files are not loaded into RAM memory unless programs call functions within them.

Before DLLs can be used in Windows, they have to be entered into the Registry. Windows can then keep track of the DLLs shared among its programs through a process called reference counting. Whenever you install or uninstall programs on your computer, these programs will either increment or decrement the Registry’s reference count by one for each of the DLLs they use. In this way, a group of programs that share a particular DLL will retain access to its functions after other members of the group have been uninstalled. Similarly, a reference count of zero in the Registry indicates that a DLL is no longer used by any installed programs, and can be safely removed by Windows during the uninstallation of the last program needing it.

"The best laid plans..."

The multitude of problems introduced when the handling of DLLs doesn’t go quite as planned is collectively and affectionately referred to as "DLL Hell". Some of the most common means of descending into DLL Hell are summarized as follows:

• Although program installers are only supposed to replace existing DLLs with newer versions, it sometimes happens that they work in reverse, and older versions are installed over newer ones. Programs using the newer versions will often try to access functions within the DLLs that are significantly different or simply don’t exist in the older versions, resulting in lots of error messages and even system crashes.
  
  
• Another common set of problems related to DLLs has to do with their reference count in the Registry. Bad install utilities sometimes neglect to increment a DLLs count, potentially resulting in the count reaching zero prematurely. Bad uninstallers will sometimes delete a DLL file without even considering the reference count, thereby wreaking havoc within programs trying to reference it afterward.
  
  
• General protection faults (GPFs) can occur if a call or declaration is made to a DLL for a different version, since the call might not exist in that version, or it might require a different set of parameters for its execution. Problems can also arise when the DLL is the wrong version for the operating system or other program that calls it, causing General Page Faults or resulting in system lockups.
  
  
• Since DLLs are used in the same way as .exe files, they are susceptible to the same kinds of Windows afflictions, like viruses, spyware, adware, and malware.