Darling is a free and open-sourcemacOScompatibility layer for GNU/Linux. It duplicates functions of macOS by providing alternative implementations of the libraries and frameworks that macOS programs call. This method of duplication differs from other methods that might also be considered emulation, where macOS programs run in a virtual machine. Darling has been called the counterpart to WINE for running OS Xapps. The project started in Summer 2012 and builds on a previous project, named maloader, which was discontinued due to a lack of time. The developer is testing applications, such as Midnight Commander or The Unarchiver on the layer. So far, the layer has been shown to work with many console apps, but does not currently support graphical applications. Darling does have the ability to extract an Apple Disk Image. The project may also support iOS applications in the future.
Architecture
At the entry of the Darling system is a loader for Mach-O binaries, the executable format for Apple's operating systems. Darling's predecessor, maloader, presented a maximalist approach to the problem by trying to replicate everything that Apple's dynamic library loader dyld does. This proved to be hard, and since an 2017 "Mach-O transition" Darling has been using a lightweight loader just enough to launch the open-source Apple dyld instead. To provide the macOS binaries with a kernel, Darling uses a modified XNU kernel wrapped into a Linux kernel module. This module handles the typical job of a Mach kernel, mainly Ports IPC handling. Some licensing issues exist in the darling-mach module, as the team are adding GNU GPL modifications to the APSL kernel. Higher than the kernel is the root environment. Darling, like wine, supports chroot prefixes, implemented using the Linux overlayfs. PID, IPC, and UTS namespaces are used to create a container for the Darwin system inside. The frameworks and system libraries in Darling are, to the best possible extent, based on source codereleased by Apple. The Mach-O transition allows these frameworks to be built more easily, because they are now built as the Mach-O format they were intended for. To fill in the gaps for many higher-level frameworks like Cocoa, Darling uses code from Cocotron, ApportableFoundation, and GNUstep.
