Thursday, April 7, 2011

What are DLL files in windows?

Dynamic Link Library (DLL) files are an essential part of the Windows operating system. Although they are ubiquitous, most PC users neither know nor care what these files do. A little understanding of DLL files can make the computer a little less of a mystery box. Only programmers and computer technicians need to know any of the gory details of the structure and function of a DLL, but these files are so important that all of us should know a few simple facts about them.

What Do DLL Files Do?

A DLL file is indicated by the extension DLL in its name. 
  • "A Dynamic Link Library (DLL) is a file of code containing functions that can be called from other executable code (either an application or another DLL). Programmers use DLLs to provide code that they can reuse and to parcel out distinct jobs. Unlike an executable (EXE) file, a DLL cannot be directly run. DLLs must be called from other code that is already executing."

Another way of putting it is that DLL files are like modules that can be plugged into different programs whenever a program needs the particular features that come with the DLL. The original concept behind DLL files was to simplify things. It was recognized that there were many functions common to a lot of software. For example, most programs need to create the graphical interface that appears on the screen. Instead of having to contain the code to create the interface themselves, programs call on a DLL for that function. The idea is to have a central library where everyone can obtain the commonly used functions, as they are needed. This cuts down on code, speeds things up, is more efficient, etc. They are called dynamic links because they are put to use only when a program calls on them and they run in the program’s own memory space. More than one program can use the functions of a particular DLL at the same time.

  • Parenthetically, I have to say that the software developers (not least of all, Microsoft) have strayed from the path of keeping things simple. A computer today may contain a thousand or more different DLL files. Microsoft maintains a database with information about various DLLs to help with version conflicts.
  • There are several very important DLLs that contain a large number of the basic Windows functions.

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