linking, instead of static library linking, share executable libraries of machine instructions across packages and applications. In these systems, conflicting relationships between different packages requiring different versions of libraries results in a challenge colloquially known as "dependency hell". On Microsoft Windows systems, this is also called "DLL hell" when working with dynamically linked libraries. Modern package managers have mostly solved these problems, by allowing parallel installation of multiple versions of a library (e.g. OPENSTEP's Framework system), a dependency of any kind (e.g. slots in Gentoo Portage), and even of packages compiled with different compiler versions (e.g. dynamic libraries built by the Glasgow Haskell Compiler, where a stable ABI does not exist), in order to enable other packages to specify which version they were linked or even installed against. Front-ends for locally compiled packages System administrators may install and maintain software using tools other than package management software. For example, a local administrator may download unpackaged source code, compile it, and install it. This may cause the state of the local system to fall out of synchronization with the state of the package manager's database. The local administrator will be required to take additional measures, such as manually managing some dependencies or integrating the changes into the package manager. There are tools available to ensure that locally compiled packages are integrated with the package management. For distributions based on .deb and .rpm files as well as Slackware Linux, there is CheckInstall, and for recipe-based systems such as Gentoo Linux and hybrid systems such as Arch Linux, it is possible to write a recipe first, which then ensures that the package fits into the local package database. Maintenance of configuration Particularly troublesome with software upgrades are upgrades of configuration files. Since package managers, at least on Unix systems, originated as extensions of file archiving utilities, they can usually only either overwrite or retain configuration files, rather than applying rules to them. There are exceptions to this that usually apply to kernel configuration (which, if broken, will render the computer unusable after a restart). Problems can be caused if the format of configuration files changes; for instance, if the old configuration file does not explicitly disable new options that should be disabled. Some package managers, such as Debian's dpkg, allow configuration during installation. In other situations, it is desirable to install packages with the default configuration and then overwrite this configuration, for instance,