What is a DLL File?

DLL Files: What They Are & Why They're Important

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A DLL file, short for Dynamic Link Library, is a type of file that contains instructions that other programs can call upon to do certain things. This way, multiple programs can share the abilities programmed into a single file, and even do so simultaneously.

For example, several different programs might all call upon the veryuseful.dll file (I made that up, of course) to find the free space on a hard drive, locate a file in a particular directory, and print a test page to the default printer.

Unlike executable programs, like those with the EXE file extension, DLL files can't be run directly but instead must be called upon by other code that is already running. However, DLLs are in the same format as EXEs and some may even use the .EXE file extension. While most Dynamic Link Libraries end in the file extension .DLL, others may use .OCX, .CPL, or .DRV.

Fixing DLL Errors

DLL files, because of how many there are and how often they're used, tend to be the focus of a large percentage of the errors seen when starting, using, and shutting down Windows.

While it might be easy to just download that missing or not found DLL file, that's rarely the best way to go. See my Important Reasons NOT to Download DLL Files for more on that.

If you get a DLL error, your best bet is to find troubleshooting information specific to that DLL problem so you're sure to solve it the right way and for good. I may even have a specific fix-it guide for the one you have.

I have a list of the most common DLL errors and how to fix them.

Otherwise, see my How to Fix DLL Errors for some general advice.

More About DLL Files

The word "dynamic" in Dynamic Link Library is used because the data is only put to use in a program when the program actively calls for it instead of having the data always be available in memory.

Lots of DLL files are available from Windows by default but third-party programs can install them too. However, it's uncommon to open a DLL file because there's never really a need to edit one, plus doing so is likely to cause problems with programs and other DLLs.

DLL files are useful because they can allow a program to separate its different components into unique modules which can then be added or removed to include or exclude certain functionalities. When the software works this way with DLLs, the program can use less memory because it doesn't need to load everything at once.

Also, DLLs provide a way for parts of a program to be updated without having to rebuild or reinstall the entire program all over. The benefit is even amplified when more than program uses the DLL because all the applications can then take advantage of the update from that single DLL file.

ActiveX Controls, Control Panel files, and device drivers are some of the files that Windows uses as Dynamic Link Libraries. Respectively, these files use the OCX, CPL, and DRV file extension.

When a DLL uses instructions from a different DLL, that first DLL is now dependent on the second one. This makes it easier for the DLLs functionalities to break because instead of there being a chance for just the first DLL to malfunction, it now depends on the second as well, which would affect the first if it were to experience issues.

If a dependent DLL is upgraded to a newer version, overwritten with an older version, or removed from the computer, the program relying on the DLL file may no longer work as it should.

Resource DLLs are data files that are in the same file format as DLLs but use the ICL, FON, and FOT file extensions. ICL files are icon libraries while FONT and FOT files are font files.