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DLL-hell is a colorful term given to any problem based on a difficulty in managing Dynamic Link Libraries (DLLs) installed on a particular copy of an operating system. This includes conflicts between various versions of these libraries, difficulty in obtaining a large number of such libraries, and/or having many unnecessary copies of different versions (which can cause both disk space and performance problems).

Generally, the concept of DLLs means that many applications can share the same DLL file. However, in many cases, applications may introduce a changed version of a particular DLL which is already present on a system, either overwriting the old copy (which can, in turn, break compatibility with other applications), or install another copy, wasting disk space, memory space and slowing program load times because it takes more time to locate the right DLL among many.

As time goes by, the DLL-hell problem can become only worse, since software that installed the unnecessary DLLs is unlikely to remove them when uninstalled. This could eventually cause a chaos of thousands of mysterious DLL-files, some of which are necessary for the system to function normally, while others are just wasting space, and with no way to distinguish between them.

DLL-hell as described above is a very common phenomenon on Microsoft Windows systems, as they have limited facilities for system file management and versioning of libraries (and existing programs often disrespect the few facilities that do exist). Many "DLL-hell" problems are less likely on Unix-like systems, because there are a large number of standards and conventions that enable having multiple different DLL versions available simultaneously, Unix-like systems' search systems for libraries tend to be efficient (so load times are not strongly affected), and most tend to have package managers to support automated management of DLLs (including automated update of a library, and removal once a version of a library is no longer needed).

However, even with systems with standard locations for dynamic libraries and careful versioning schemes, problems may be encountered with the use of such libraries. For example, the GNOME desktop environment, common on Linux and other Unix-like systems, is notorious for bundling an enormous number of libraries. Even though the versioned dynamic linking largely solves the runtime issues, the sheer number of libraries (which must be retrieved and installed separately on some systems) is in itself a burden. Additionally, whilst the system provides facilities for installing side-by-side versions of the runtime libraries, it doesn't provide much assistance for side-by-side installs of the header files required for compiling the programs. However, these issues mainly inconvenience developers and people who compile their software from source, rather than end users who usually download precompiled software from their distribution provider. Distributions tend to use package managers that significantly reduce these problems for end-users.

There are several measures known to avoid DLL-hell, which have to be used simultaneously, for optimal results:

  • Ship the operating system with a capable package management system, that would be able to track the DLL dependencies. Declare using the package manager good style and using manual installs bad style.
  • Have a central authority for distributing the library. Changes to the library can be submitted to this authority; this, it can make sure compatibility is preserved in the developed branches. If some older software is incompatible with the current library, the authority can provide a compatibility interface for it, or bundle the old version as a distinct package.
  • If software developers need to customize a library, and if the main library release is unlikely to incorporate the changes that they need, they can use static linking against their own version, or create a new package (with a different name) for the library.
  • Proper software design is paramount. DLLs are best for modularizing the system's components and as third-party libraries; their usage is not imperative in all cases. For example, if an application needs a library that won't be used anywhere else, they can be linked statically, with no space penalty and with a speed gain.

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