DLL Hell

In computing, DLL Hell is a term for the complications that arise when working with dynamic link libraries (DLLs) used with Microsoft Windows operating systems, particularly legacy 16-bit editions which all run in a single memory space.

DLL Hell can manifest itself in many different ways in which applications do not launch or work correctly.

DLL Hell is the Windows ecosystem specific form of the general concept dependency hell.

DLLs are Microsoft's implementation of shared libraries. Shared libraries allow common code to be bundled into a wrapper, the DLL, and used by any application software on the system without loading multiple copies into memory. A simple example might be the GUI text editor, which is widely used by many programs. By placing this code in a DLL, all the applications on the system can use it without using more memory. This contrasts with static libraries, which are functionally similar but copy the code directly into the application. In this case, every application grows by the size of all the libraries it uses, and this can be quite large for modern programs.

The problem arises when the version of the DLL on the computer is different than the version that was used when the program was being created. DLLs have no in-built mechanism for backward compatibility, and even minor changes to the DLL render its internal structure so different than previous versions that attempting to use them will generally cause the application to crash. Static libraries avoid this problem because the version that was used to build the application is included inside it, so even if a newer version exists elsewhere on the system this does not effect the application.

A key reason for the version incompatibility is the structure of the DLL file. The file contains a directory of the individual methods (procedures, routines, etc.) contained within the DLL and the types of data they take and return. Even minor changes to the DLL code can cause this directory to be re-arranged, in which case an application that calls a particular method believing it to be the 4th item in the director might end up calling an entirely different and incompatible routine, which would normally cause the application to crash.

There are a number of problems commonly encountered with DLLs, especially after numerous applications have been installed and uninstalled on a system. The difficulties include conflicts between DLL versions, difficulty in obtaining required DLLs, and having many unnecessary DLL copies.

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