BSD Facilities

The facilities that are available to a user process are logically divided into two parts: kernel facilities and system facilities implemented by or in cooperation with a server process.

The facilities implemented in the kernel define the virtual machine in which each process runs. Like many real machines, this virtual machine has memory management, an interrupt facility, timers, and counters.

The virtual machine also allows access to files and other objects through a set of descriptors. Each descriptor resembles a device controller and supports a set of operations. Like devices on real machines, some of which are internal to the machine and some of which are external, parts of the descriptor machinery are built into the operating system, while other parts are often implemented in server processes.

The BSD component provides the following kernel facilities:

• processes and protection

  • host and process identifiers

  • process creation and termination

  • user and group IDs

  • process groups

• memory management

  • text, data, stack, and dynamic shared libraries

  • mapping pages

  • page protection control

• POSIX synchronization primitives

• POSIX shared memory

• signals

  • signal types

  • signal handlers

  • sending signals

• timing and statistics

  • real time

  • interval time

• descriptors

  • files

  • pipes

  • sockets

• resource controls

  • process priorities

  • resource utilization and resource limits

  • quotas

• system operation support

  • bootstrap operations

  • shut-down operations

  • accounting

BSD system facilities (facilities that may interact with user space) include

• generic input/output operations such as read and write, nonblocking, and asynchronous operations

• file-system operations

• interprocess communication

• handling of terminals and other devices

• process control

• networking operations

Differences between OS X and BSD

Although the BSD portion of OS X is primarily derived from FreeBSD, some changes have been made:

• The sbrk() system call for memory management is deprecated. Its use is not recommended in OS X.

• The OS X runtime model uses a different object file format for executables and shared objects, and a different mechanism for executing some of those executables.

  The primary native format is Mach-O. This format is supported by the dynamic link editor (dyld).

  The PEF binary file format is supported by the Code Fragment Manager (CFM).

  The kernel supports execve() with Mach-O binaries. Mapping and management of Mach-O dynamic shared libraries, as well as launching of PEF-based applications, are performed by user-space code.

• OS X does not support memory-mapped devices through the mmap() function. (Graphic device support and other subsystems provide similar functionality, but using different APIs.) In OS X, this interface should be done through user clients. See the Apple I/O Kit documents for additional information.

• The swapon() call is not supported; macx_swapon() is the equivalent call from the Mach pager.

• The Unified Buffer Cache implementation in OS X differs from that found in FreeBSD.

• Mach provides a number of IPC primitives that are not traditionally found in UNIX. See Boundary Crossings for more information on Mach IPC. Some System V primitives are supported, but their use is discouraged in favor of POSIX equivalents.

• Several changes have been made to the BSD security model to support single-user and multiple-administrator configurations, including the ability to disable ownership and permissions on a volume-by-volume basis.

• The locking mechanism used throughout the kernel differs substantially from the mechanism used in FreeBSD.

• The kernel extension mechanism used by OS X is completely different. The OS X driver layer, the I/O Kit, is an object-oriented driver stack written in C++. The general kernel programming interfaces, or KPIs, are used to write non-driver kernel extensions. These mechanisms are described more in I/O Kit Overview and KPI Reference, respectively.

In addition, several new features have been added that are specific to the OS X (Darwin) implementation of BSD. These features are not found in FreeBSD.

• enhancements to file-system buffer cache and file I/O clustering

  • adaptive and speculative read ahead

  • user-process controlled read ahead

  • time aging of the file-system buffer cache

• enhancements to file-system support

  • implementation of Apple extensions for ISO-9660 file systems

  • multithreaded asynchronous I/O for NFS

  • addition of system calls to support semantics of Mac OS Extended (HFS+) file systems

  • additions to naming conventions for pathnames, as required for accessing multiple forks in Mac OS Extended file systems

For Further Reading

The BSD component of the OS X kernel is complex. A complete description is beyond the scope of this document. However, many excellent references exist for this component. If you are interested in BSD, be sure to refer to the bibliography for further information.

Although the BSD layer of OS X is derived from 4.4BSD, keep in mind that it is not identical to 4.4BSD. Some functionality of 4.4 BSD has not been included in OS X. Some new functionality has been added. The cited reference materials are recommended for additional reading. However, they should not be presumed as forming a definitive description of OS X.