Grand Central Dispatch

GCD works by allowing specific tasks in a program that can be run in parallel to be queued up for execution and, depending on availability of processing resources, scheduling them to execute on any of the available processor cores (referred to as "routing" by Apple). A task can be expressed either as a function or as a "block." Blocks are an extension to the syntax of C, C++, and Objective-C programming languages that encapsulate code and data into a single object in a way similar to a closure. Grand Central Dispatch still uses threads at the low level but abstracts them away from the programmer, who will not need to be concerned with as many details. Tasks in GCD are lightweight to create and queue; Apple states that 15 instructions are required to queue up a work unit in GCD, while creating a traditional thread could easily require several hundred instructions. ==Features==

The dispatch framework declares several data types and functions to create and manipulate them: • Dispatch Queues are objects that maintain a queue of tasks, either anonymous code blocks or functions, and execute these tasks in their turn. The library automatically creates several queues with different priority levels that execute several tasks concurrently, selecting the optimal number of tasks to run based on the operating environment. A client to the library may also create any number of serial queues, which execute tasks in the order they are submitted, one at a time. == Applications ==

GCD is used throughout macOS (beginning with 10.6 Snow Leopard), and Apple has encouraged its adoption by macOS application developers. FreeBSD developer Robert Watson announced the first adaptation of a major open source application, the Apache HTTP Server, to use GCD via the Apache GCD MPM (Multi-Processing Module) on May 11, 2010, in order to illustrate the programming model and how to integrate GCD into existing, large-scale multi-threaded, applications. His announcement observed that the GCD MPM had one third to half the number of lines as other threaded MPMs. == Internals ==

GCD is implemented by libdispatch, with support from pthreads non-POSIX extensions developed by Apple. Apple has changed the interface since its inception (in OS X 10.5) through the official launch of GCD (10.6), Mountain Lion (10.8) and Mavericks (10.9). The latest changes involve making the code supporting pthreads, both in user mode and kernel, private (with kernel pthread support reduced to shims only, and the actual workqueue implementation moved to a separate kernel extension). == See also ==