Mac OS X

Table of Contents

1. Introduction

MacOS X (pronounced "Mac Oh Ess Ten") is the name of Apple’s latest version
of the Macintosh operating system. Though it shares a similar name, it
shares very little in common with previous releases of MacOS.

MacOS X is a UNIX-like operating system with the look and feel of a Macintosh
computer. In 1996, Apple acquired NeXT, Inc. (the acquisition that brought
Steve Jobs back to Apple). It was the NeXT codebase (NEXTSTEP/OPENSTEP) that
Apple used as the foundation for what eventually became MacOS X.

The following outline aims to point some of the interesting aspects of
MacOS X, both from a user and developer perspective.

2. The Graphical User Interface

The graphical user interface of any
operating system is the part that is often evaluated in magazine reviews or
is touted in discussions about which OS is better. True, the GUI is an
important part of the operating system, but not the only part. The MacOS X
GUI is composed of several key components.

2.1. Dock

The Dock was an idea brought over from NEXTSTEP and
OPENSTEP. Dock-like interfaces as also quite popular among UNIX window
managers, such as Window Maker and AfterStep. The MacOS X dock is
virtually a complete reworking of the classic NEXTSTEP dock, but does
have some things in common. To place items on the dock, a user just drags
the object to the dock. To remove an item, you drag it off the dock. The
dock can be positioned along any edge of the screen and also offers some
special effects to make it that much more interesting. The Dock under
MacOS X replaces the Apple menu from previous releases of MacOS.

The Dock

2.2. Finder

Finder is the name of the GUI "shell" that runs when
you log in to MacOS X. Finder is responsible for putting the menu bar
at the top of the screen and also placing icons on the desktop. Finder
can be stopped and restarted without rebooting, a feature that previous
releases lacked (Well, this isn’t totally true. You could restart the
Finder in previous MacOS releases, but it wasn’t an officially
supported feature and was rather flaky.). Finder is also responsible for
the file manager windows that pop up when you open a folder or drive icon.
The views available in the MacOS X Finder are similar to NEXTSTEP
(horizontal scrolling view).

2.3. Login Window

Since MacOS X is a UNIX-like operating system,
it requires one to log in at boot time. Users of xdm and other X11-based
login managers may notice a similarity to the MacOS X login screen. You
can get a console login from the MacOS X graphical login window. Just
type ">console" as the username and click Login.

3. The UNIX part

The "UNIX part" of MacOS X is called Darwin. This
is the part that’s open source and available to anyone. It runs on both
PowerPC Macintosh computers as well as Intel-based computers. Darwin
consists of the kernel, userland and system tools, and networking code.
Basically it’s everything except the graphical user interface.

3.1. Technologies

Apple loves to associate fancy names with every
software project they currently have going. These names are thrown around
and since they usually bear no meaning towards the actual project, it can
sometimes get confusing. Here are the common ones you’ll hear when
discussing MacOS X:

  1. DisplayPDF: The underlying concept behind
    graphics primitives in MacOS X. All 2D graphics are stored as PDF
    primitives, which expands on the NEXTSTEP usage of DisplayPostScript
  2. Aqua: The name of the theme present in a
    default MacOS X install. This is what people try to clone for KDE and
    other environments.
  3. Quartz: Roughly equivalent to the X server
    on a UNIX system. Quartz draws stuff on the screen.

Several existing and new technologies came
together to create Darwin.

  1. FreeBSD: The TCP/IP stack as well as some
    of the userland tools from FreeBSD 3.2 were incorporated into the
    Darwin source tree.
  2. NEXTSTEP/OPENSTEP: The Mach+BSD kernel
    architecture from NEXTSTEP provided the foundation on which the
    Darwin kernel was built. Filesystem layout, packaging system, and
    object concepts were incorporated from NEXTSTEP.
  3. GNU: Many GNU programs were incorporated
    into the Darwin source tree, such as gcc, GNU make, and grep. For
    the most part, userland tools in Darwin and MacOS X are the same as
    what you would find on a Linux distribution.
  4. NetBSD: Very portable code. Whenever a
    FreeBSD tool couldn’t easily be ported, the Darwin team went to
    the NetBSD source tree. This is also a trick used by many Linux

3.2. Booting

The system boot procedure currently resembles that
of NEXTSTEP. It’s almost a pure BSD-style boot system, with init
running /etc/rc. The change under OS X is the addition of
SystemStarter. The rc scripts handle basic system init stuff, such as
loading kernel modules and mounting filesystems. After the critical
steps, SystemStarter takes over and brings up services, networking, and
the GUI stuff. The SystemStarter works like a SysV init system. Each
"service" can be found in /System/Library/StartupItems. Each service
is a subdirectory containing resource files and a script that actually
does the work. Apple’s goal is to eventually move to SystemStarter for
all boot time stuff.

3.3. Filesystem Layout

The filesystem layout may look a bit
unfamiliar if you are a Linux user, but it’s not too difficult to

  1. Standard Directories: You’ll find the
    /bin, /dev, /sbin, and /usr directories. Those pretty much follow
    a standard layout. The /etc, /tmp, and /var directories are
    actually symlinks into directories of the same name under
  2. /private: The /private directory holds
    files specific to the local machine (configuration files, temporary
    data, swap data, and logs).
  3. Capital Letter Directories: All of the
    directories beginning with a capital letter are fairly self
    explanatory. /System is for system-specific services and libraries,
    /Users is home directories, /Library is like /lib, developer tools
    and documentation is in /Developer, bundled applications are in
    /Applications. You can add more directories like this, but don’t
    remove the ones created by the installer.

3.4. Things Missing

If you’re familiar with another UNIX-like
operating system, you may be surprised to not find some fairly common

  1. mount/umount: That’s right, no mount or
    umount command. So how do filesystems get mounted? Just like under
    MacOS. Everything found is mounted at boot time to the location
    specified in the volume header.
  2. NFS: Support for NFS is lacking, and
    without a mount or umount command, it’s even more difficult to
  3. virtual terminals: Missing on many
    commercial UNIX-like operating systems as well, but still a feature
    liked by many. But, with the GUI, you can run multiple terminal
    windows on the same screen, a la X.

3.5. Kernel

The Darwin kernel, called xnu, is a combined
source tree starting with NEXTSTEP and merging in FreeBSD and NetBSD
stuff, and adding some new things (like the object oriented device
driver layer called IOKit). It can be compiled for PowerPC or Intel
machines. On MacOS X, you’ll find the main kernel file in / and named

4. The Programmer Part

MacOS X offers many things specifically
for the developer. There are tools for bringing classic MacOS
applications to MacOS X, as well as tools for bringing UNIX applications
to MacOS X. (much of the information below was gathered from

4.1. ProjectBuilder IDE vs. GNU development tools

Seasoned MacOS
developers will prefer the ProjectBuilder IDE that comes with the
developer CD. It’s a nice GUI for editing code and designing UI
components. Seasoned UNIX developers will enjoy the standard set of
UNIX development tools available (cc, make, bison, yacc, etc.).

4.2. Mach-O binary format

The binary format used by MacOS X is
the Mach-O format. It’s not ELF and should not be confused with ELF
(even though it offers roughly the same functionality). One cool
feature of Mach-O is "fat" binaries, that is, having multiple
architectures supported by a single binary.

4.3. Compiler and Preprocessor

The compiler (/usr/bin/cc) is
based on gcc 2.95.2 with Apple modifications. The code is actively
being prepared for merging into the mainline gcc source tree, but
it’s difficult. Among the additions by Apple is the support for
AltiVec registers. The preprocessor (cpp) is a custom Apple creation
that works nothing like GNU cpp. Most notably is the precomp problem
experienced by people trying to compile open source software on
MacOS X. Precomps are precompiled headers. The special cpp can read
precompiled headers (binary data files consisting of all the tokens
and dependency information) and regular headers. Problem is, it does
not always work with regular headers. The -no-cpp-precomp flag is
usually preferred when compiling software on MacOS X. (precomp
information gathered from

4.4. Libtool

GNU libtool has problems on MacOS X. There are
patches for both the 1.3 and 1.4 versions to correct shared library
generation. MacOS X ships with GNU libtool 1.3.5 patched for MacOS X,
but it’s not completely fixed. The patch can be found at

4.5. Libraries

  1. .a and .dylib: Traditional static
    libraries are offered through the .a files. Dynamic libraries end
    with the name .dylib and do not function the same as .so’s on
  2. dyld: The equivalent on MacOS
  3. dlcompat: To help port software from UNIX
    to MacOS X, the dlcompat library was created to translate dlopen()
    calls to the appropriate dyld action.
  4. Versioning and Naming: The dynamic linker
    checks major and minor version numbers, unlike Linux. Naming also
    differs slightly. The version is part of the library name, with
    .dylib being at the very end of the filename. This makes it a bit
    easier to specify certain library versions for certain
  5. Modules, Libs, and Bundles: In Linux, a
    shared library and loadable module for a program ("plug-in") are the
    same. Under MacOS X, a shared library is a .dylib file. A loadable
    module is a bundle ending with .bundle (but sometimes .so).
    Loadable modules are loaded and unloaded through dyld, which is why
    the dlcompat interface exists for ported software.
  6. Compiler Flags: Common symbols are not
    allowed in shared libraries, so you need to use -fno-common.
    Position independent code is default, so there’s no need to specify
    a PIC flag. To build a loadable module, use the -bundle,
    -flat_namespace and "-undefined suppress" compiler flags.

4.6. Linker and Assembler

The assembler is GNU-derived, but the
linker is not GNU at all, which presents problems for GNU libtool and
other open source software.

4.7. KEXTS

Kernel extensions are similar to modules in the Linux
kernel. They can be added to the kernel without recompiling it. A
KEXT is a bundle, so it is a directory containing the actual loadable
module and any control files or other resources used by the driver.
The driver configuration file is an XML document called Info-macos.xml.

4.8. Packaging

A MacOS X package is a special directory
(Bundle) that ends with .pkg. In this directory are icons, text files
displayed by the installer, scripts, and the actual package contents.

  1. PackageMaker & /usr/bin/package: MacOS X
    currently offers two built-in methods for generating software
    packages. The one that receives the most attention is the
    PackageMaker application. It’s a graphical, fill-in-the-blanks
    program that creates the package for you. /usr/bin/package is the
    NEXTSTEP packaging tool, updated for MacOS X. It should be noted
    that both programs generate packages compatible with,
    but that have different internal formats.
  2. Bill of Materials: The package manifest
    is a binary data file called the bill of materials, or bom for
    short. The bom lists the contents of the package (symlinks, files,
    directories), the permissions and ownerships for each item, and a
    32-bit checksum of the file.
  3. This program is
    responsible for adding packages to the system. It reads in the
    contents of a pkg bundle and walks you through the installation
    process. A record of the installed package is retained in the
    /Library/Receipts directory. NOTE: Currently, there is no way to
    remove, query, or upgrade packages. You can only install and make
    packages at this time.
  4. How to distribute pkg Bundles: Since

    MacOS X packages are directories, it’s rather difficult to
    distribute them online. You can use standard tools, such as tar to
    make the package a single file, but the end user will have to untar
    the package before being able to install it. The preferred solution
    is to use Disk Copy to make a disk image file the size you need and
    drop the package in there. Disk image files are self mounting on
    MacOS X.

4.9. Cocoa API

Cocoa is the Objective-C API native to MacOS X.
It originated in OPENSTEP.

4.10. Carbon API

Carbon is the API that works on MacOS 8.6 or
higher and MacOS X.

5. Other Sources of Information

    – Official Apple web site for MacOS X
  2. – Nice
    site with writeups on how to do things and reviews on applications.
    Somewhat wordy and difficult to navigate, but useful.
    Independent web site devoted to information about Darwin, the open source
    operating system on which MacOS X is based. FAQs and HOWTO documents,
    plus news about Darwin developments.
    Think Wisely
    Nice site with hints and such related to MacOS X
  6. – Installing
    and using XFree86 4.x on Darwin and MacOS X.

6. Copyright

Copyright 2001 David L. Cantrell Jr., Atlanta, GA, USA

by: David L. Cantrell Jr. (
Permission to reprint this information granted provided the above copyright
notice remains attached.