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Darwin

Beneath the appealing, easy-to-use interface of Mac OS X is a rock-solid, UNIX-based foundation called Darwin that is engineered for stability, reliability, and performance. Darwin integrates a number of technologies, the most important of which are Mach 3.0, operating-system services based on FreeBSD 5, high-performance networking facilities, and support for multiple, integrated file systems. Because the design of Darwin is highly modular, you can dynamically add such things as device drivers, networking extensions, and new file systems.

The following sections describe some of the key features of Darwin. For pointers to more information, see Getting Started with Darwin.

In this section:

Mach
Device-Driver Support
BSD
File-System Support
Network Support
Scripting Support
Threading Support
X11

Mach

Mach is at the heart of Darwin because it provides some of the most critical functions of the operating system. Much of what Mach provides is transparent to applications. It manages processor resources such as CPU usage and memory, handles scheduling, enforces memory protection, and implements a messaging-centered infrastructure for untyped interprocess communication, both local and remote. Mach provides many important advantages to Macintosh computing:

• Protected memory. The stability of an operating system should not depend on all executing applications being good citizens. Even a well-behaved process can accidentally write data into the address space of the system or another process, which can result in the loss or corruption of data or even precipitate system crashes. Mach ensures that an application cannot write in another application’s memory or in the operating system’s memory. By walling off applications from each other and from system processes, Mach makes it virtually impossible for a single poorly behaved application to damage the rest of the system. Best of all, if an application crashes as the result of its own misbehavior, the crash affects only that application and not the rest of the system.

• Preemptive multitasking. With Mach, processes share the CPU efficiently. Mach watches over the computer’s processor, prioritizing tasks, making sure activity levels are at the maximum, and ensuring that every task gets the resources it needs. It uses certain criteria to decide how important a task is and therefore how much time to allocate to it before giving another task its turn. Your process is not dependent on another process yielding its processing time.

• Advanced virtual memory. In Mac OS X, virtual memory is “on” all the time. The Mach virtual memory system gives each process its own private virtual address space. For 32-bit applications, this virtual address space is 4 GB. For 64-bit applications, the theoretical maximum is approximately 18 exabytes, or 18 billion billion bytes. Mach maintains address maps that control the translation of a task’s virtual addresses into physical memory. Typically only a portion of the data or code contained in a task’s virtual address space resides in physical memory at any given time. As pages are needed, they are loaded into physical memory from storage. Mach augments these semantics with the abstraction of memory objects. Named memory objects enable one task (at a sufficiently low level) to map a range of memory, unmap it, and send it to another task. This capability is essential for implementing separate execution environments on the same system.

• Real-time support. This feature guarantees low-latency access to processor resources for time-sensitive media applications.

Mach also enables cooperative multitasking, preemptive threading, and cooperative threading.

Device-Driver Support

Darwin offers an object-oriented framework for developing device drivers called the I/O Kit framework. This framework facilitates the creation of drivers for Mac OS X and provides much of the infrastructure that they need. It is written in a restricted subset of C++. Designed to support a range of device families, the I/O Kit is both modular and extensible.

Device drivers created with the I/O Kit acquire several important features:

• True plug and play

• Dynamic device management (“hot plugging”)

• Power management (for both desktops and portables)

If your device conforms to standard specifications, such as those for mice, keyboards, audio input devices, modern MIDI devices, and so on, it should just work when you plug it in. If your device doesn’t conform to a published standard, you can use the I/O Kit resources to create a custom driver to meet your needs. Devices such as AGP cards, PCI and PCIe cards, scanners, and printers usually require custom drivers or other support software in order to work with Mac OS X.

For information on creating device drivers, see I/O Kit Device Driver Design Guidelines.

BSD

Integrated with Mach is a customized version of the Berkeley Software Distribution (BSD) operating system (currently FreeBSD 5). Darwin’s implementation of BSD includes much of the POSIX API, which higher-level applications can also use to implement basic application features. BSD serves as the basis for the file systems and networking facilities of Mac OS X. In addition, it provides several programming interfaces and services, including:

• The process model (process IDs, signals, and so on)

• Basic security policies such as file permissions and user and group IDs

• Threading support (POSIX threads)

• Networking support (BSD sockets)

For more information about the FreeBSD operating system, go to http://www.freebsd.org/. For more information about the boot process of Mac OS X, including how it launches the daemons used to implement key BSD services, see System Startup Programming Topics.

File-System Support

The file-system component of Darwin is based on extensions to BSD and an enhanced Virtual File System (VFS) design. The file-system component includes the following features:

• Permissions on removable media. This feature is based on a globally unique ID registered for each connected removable device (including USB and FireWire devices) in the system.

• Access control lists (available in Mac OS X version 10.4 and later)

• URL-based volume mount, which enables users (via a Finder command) to mount such things as AppleShare and web servers

• Unified buffer cache, which consolidates the buffer cache with the virtual-memory cache

• Long filenames (255 characters or 755 bytes, based on UTF-8)

• Support for hiding filename extensions on a per-file basis

• Journaling of all file-system types to aid in data recovery after a crash

Because of its multiple application environments and the various kinds of devices it supports, Mac OS X handles file data in many standard volume formats. Table 2-1 lists the supported formats.

HFS+ volumes support aliases, symbolic links, and hard links, whereas UFS volumes support symbolic links and hard links but not aliases. Although an alias and a symbolic link are both lightweight references to a file or directory elsewhere in the file system, they are semantically different in significant ways. For more information, see “Aliases and Symbolic Links” in File System Overview.

Because Mac OS X is intended to be deployed in heterogeneous networks, it also supports several network file-sharing protocols. Table 2-2 lists these protocols.

Network Support

Mac OS X is one of the premier platforms for computing in an interconnected world. It supports the dominant media types, protocols, and services in the industry as well as differentiated and innovative services from Apple.

The Mac OS X network protocol stack is based on BSD. The extensible architecture provided by network kernel extensions, summarized in “Networking Extensions,” facilitates the creation of modules implementing new or existing protocols that can be added to this stack.

Standard Network Protocols

Mac OS X provides built-in support for a large number of network protocols that are standard in the computing industry. Table 2-3 summarizes these protocols.

Apple also implements a number of file-sharing protocols; see Table 2-2 for a summary of these protocols.

Legacy Network Services and Protocols

Apple includes the following legacy network products in Mac OS X to ease the transition from earlier versions of the Mac OS.

• AppleTalk is a suite of network protocols that is standard on the Macintosh and can be integrated with other network systems. Mac OS X includes minimal support for compatibility with legacy AppleTalk environments and solutions.

• Open Transport implements industry-standard communications and network protocols as part of the I/O system. It helps developers incorporate networking services in their applications without having to worry about communication details specific to any one network.

These protocols are provided to support legacy applications, such as those running in the Classic environment. You should never use these protocols for any active development. Instead, you should use newer networking technologies such as CFNetwork.

Network Technologies

Mac OS X supports the network technologies listed in Table 2-4.

Routing and Multihoming

Mac OS X is a powerful and easy-to-use desktop operating system but can also serve as the basis for powerful server solutions. Some businesses or organizations have small networks that could benefit from the services of a router, and Mac OS X offers IP routing support for just these occasions. With IP routing, a Mac OS X computer can act as a router or even as a gateway to the Internet. The Routing Information Protocol (RIP) is used in the implementation of this feature.

Mac OS X also allows multihoming and IP aliasing. With multihoming, a computer host is physically connected to multiple data links that can be on the same or different networks. IP aliasing allows a network administrator to assign multiple IP addresses to a single network interface. Thus one computer running Mac OS X can serve multiple websites by acting as if it were multiple servers.

Zero-Configuration Networking

Introduced in Mac OS X version 10.2, Bonjour is Apple’s implementation of zero-configuration networking. Bonjour enables the dynamic discovery of computer services over TCP/IP networks without the need for any complex user configuration of the associated hardware. Bonjour helps to connect computers and other electronic devices by providing a mechanism for them to advertise and browse for network-based services. See “Bonjour” for more information.

NetBoot

NetBoot is most often used in school or lab environments where the system administrator needs to manage the configuration of multiple computers. NetBoot computers share a single System folder, which is installed on a centralized server that the system administrator controls. Users store their data in home directories on the server and have access to a common Applications folder, both of which are also commonly installed on the server.

To support NetBoot, applications must be able to run from a shared, locked volume and write a user’s personal data to a different volume. Preferences and user-specific data should always be stored in the Preferences folder of the user’s home directory. Users should also be asked where they want to save their data, with the user’s Documents folder being the default location. Applications must also remember that multiple users may run the application simultaneously.

See Technical Note TN1151, “Creating NetBoot Server–Friendly Applications,” for additional information. For information on how to write applications that support multiple simultaneous users, see Multiple User Environments.

Personal Web Sharing

Personal Web Sharing allows users to share information with other users on an intranet, no matter what type of computer or browser they are using. Basically, it lets users set up their own intranet site. Apache, the most popular web server on the Internet, is integrated as the system’s HTTP service. The host computer on which the Personal Web Sharing server is running must be connected to a TCP/IP network.

Networking Extensions

Darwin offers kernel developers a technology for adding networking capabilities to the operating system: network kernel extensions (NKEs). The NKE facility allows you to create networking modules and even entire protocol stacks that can be dynamically loaded into the kernel and unloaded from it. NKEs also make it possible to configure protocol stacks automatically.

NKE modules have built-in capabilities for monitoring and modifying network traffic. At the data-link and network layers, they can also receive notifications of asynchronous events from device drivers, such as when there is a change in the status of a network interface.

For information on how to write an NKE, see Network Kernel Extensions Programming Guide.

Network Diagnostics

Introduced in Mac OS X version 10.4, network diagnostics is a way of helping the user solve network problems. Although modern networks are generally reliable, there are still times when network services may fail. Sometimes the cause of the failure is beyond the ability of the desktop user to fix, but sometimes the problem is in the way the user’s computer is configured. The network diagnostics feature provides a diagnostic application to help the user locate problems and correct them.

If your application encounters a network error, you can use the new diagnostic interfaces of CFNetwork to launch the diagnostic application and attempt to solve the problem interactively. You can also choose to report diagnostic problems to the user without attempting to solve them.

For more information on using this feature, see the header files of CFNetwork.

Scripting Support

Darwin includes all of the scripting languages commonly found in UNIX-based operating systems. In addition to the scripting languages associated with command-line shells (such as bash and csh), Darwin also includes support for Perl, Python, Ruby, and others.

In Mac OS X v10.5, Darwin added support for several new scripting features. In addition to adding support for Ruby on Rails, Mac OS X also added scripting bridges to the Objective-C classes of Cocoa. These bridges let you use Cocoa classes from within your Python and Ruby scripts. For information about using these bridges, see Ruby and Python Programming Topics for Mac OS X.

For information about scripting tools, see “Scripting Tools.” For information on using command-line shells, see “Command Line Primer.”

Threading Support

Mac OS X provides full support for creating multiple preemptive threads of execution inside a single process. Threads let your program perform multiple tasks in parallel. For example, you might create a thread to perform some lengthy calculations in the background while a separate thread responds to user events and updates the windows in your application. Using multiple threads can often lead to significant performance improvements in your application, especially on computers with multiple CPU cores. Multithreaded programming is not without its dangers though and requires careful coordination to ensure your application’s state does not get corrupted.

All user-level threads in Mac OS X are based on POSIX threads (also known as pthreads). A pthread is a lightweight wrapper around a Mach thread, which is the kernel implementation of a thread. You can use the pthreads API directly or use any of the threading packages offered by Cocoa, Carbon, or Java, all of which are implemented using pthreads. Each threading package offers a different combination of flexibility versus ease-of-use. All offer roughly the same performance, however.

For more information about threading support and guidelines on how to use threads safely, see Threading Programming Guide.

X11

In Mac OS X v10.3 and later, the X11 windowing system is provided as an optional installation component for the system. This windowing system is used by many UNIX applications to draw windows, controls, and other elements of graphical user interfaces. The Mac OS X implementation of X11 uses the Quartz drawing environment to give X11 windows a native Mac OS X feel. This integration also makes it possible to display X11 windows alongside windows from native applications written in Carbon and Cocoa.

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